[issue7160] Crash when returning a 64-bit char pointer in Python 2.6.3 ctypes
Michael J. Fromberger michael.j.fromber...@gmail.com added the comment: Thank you for setting me straight. I see now that I misunderstood the scope of `CFUNCTYPE`, as I was using it as a general wrapper when in fact it's only needed for callbacks. Mistakenly, I inferred from reading section 16.15.2.4 of the ctypes manual [1] that it would be necessary to create prototype wrappers for calls into the foreign library as well. Obviously that is not the case, since your described solution works fine. [1] http://www.python.org/doc/2.6.3/library/ctypes.html#function- prototypes -- status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue7160 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue7160] Crash when returning a 64-bit char pointer in Python 2.6.3 ctypes
New submission from Michael J. Fromberger michael.j.fromber...@gmail.com:
A segmentation fault is generated in _ctypes.so when calling a function that
returns a char pointer on a system
with 64-bit pointer types. The attached crash dump is from a Python 2.6.3
built from MacPorts (port install
python26 +no_tkinter), but the same behaviour occurs with the Python 2.6.1
installed by Apple.
To reproduce, build the attached sample program (testlib.c):
% gcc -Wall -c testlib.o
% ld -dylib -o testlib.so testlib.o
Then, in Python:
# Common setup for each of the cases below.
from ctypes import *
lib = CDLL('testlib.so')
# Case 1: Integer return value (no crash).
get_value = CFUNCTYPE(c_int)(lib.get_value)
get_value()
12345
# Case 2: Pointer argument value (no crash).
buf = create_string_buffer(256)
copy_message = CFUNCTYPE(None, c_char_p)(lib.copy_message)
copy_message(buf)
# Case 3: Pointer return value (crash).
get_message = CFUNCTYPE(c_char_p)(lib.get_message)
get_message()
Segmentation fault
-- System information:
% uname -a
MacOS 10.6.1
Darwin gorion.local 10.0.0 Darwin Kernel Version 10.0.0: Fri Jul 31 22:47:34
PDT 2009; root:xnu-
1456.1.25~1/RELEASE_I386 i386
% python
Python 2.6.3 (r263:75183, Oct 17 2009, 01:49:30)
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
% gcc --version
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5646) (dot 1)
--
assignee: theller
components: ctypes
files: testlib.c
messages: 94181
nosy: creachadair, theller
severity: normal
status: open
title: Crash when returning a 64-bit char pointer in Python 2.6.3 ctypes
type: crash
versions: Python 2.6
Added file: http://bugs.python.org/file15154/testlib.c
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[issue7160] Crash when returning a 64-bit char pointer in Python 2.6.3 ctypes
Changes by Michael J. Fromberger michael.j.fromber...@gmail.com: Added file: http://bugs.python.org/file15155/crash-report.txt ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue7160 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue7160] Crash when returning a 64-bit char pointer in Python 2.6.3 ctypes
Michael J. Fromberger michael.j.fromber...@gmail.com added the comment: I believe this error occurs because a pointer value is being truncated to 32 bits. The exception code is KERN_INVALID_ADDRESS at 0x002fe020 If you add a diagnostic printout to the body of get_message(), you will see that its return value is 0x1002fe020, so in other words, the high- order word 0x0001 is being discarded somewhere. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue7160 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue5809] No such file or directory with framework build under MacOS 10.4.11
Michael J. Fromberger michael.j.fromber...@gmail.com added the comment: Ned Deily writes: --enable-framework and --enable-shared are mutually exclusive options. Aha, I did not realize that, though I suppose in retrospect it should have been obvious. Removing --enable-shared from my build configuration does enable me to build with --enable-framework selected. I agree that this should be either better documented, or checked for by the configurator. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue5809 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue5809] No such file or directory with framework build under MacOS 10.4.11
New submission from Michael J. Fromberger michael.j.fromber...@gmail.com: Checkout: http://svn.python.org/projects/python/tags/r262 Configure: env CFLAGS=-I/opt/local/include LDFLAGS=-L/opt/local/lib sh ./configure --enable-framework --enable-shared --enable-readline Build: make The configuration step completes successfully, and the build proceeds; however, the following error occurs: gcc -L/opt/local/lib -u _PyMac_Error Python.framework/Versions/2.6/Python -o python.exe \ Modules/python.o \ -L. -lpython2.6 -ldl This appears to be a problem in the definition of the LINKFORSHARED variable in the Makefile, but I am not sure what the intended meaning of that variable is, so I'm not sure what the fix should be. A complete make trace is attached. -- components: Build files: makedump.txt messages: 86245 nosy: creachadair severity: normal status: open title: No such file or directory with framework build under MacOS 10.4.11 type: compile error versions: Python 2.6 Added file: http://bugs.python.org/file13728/makedump.txt ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue5809 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue5809] No such file or directory with framework build under MacOS 10.4.11
Michael J. Fromberger michael.j.fromber...@gmail.com added the comment: Sorry, I managed to not copy the error message. It is: i686-apple-darwin8-gcc-4.0.1: Python.framework/Versions/2.6/Python: No such file or directory make: *** [python.exe] Error 1 -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue5809 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
Re: Python Exponent Question
In article [EMAIL PROTECTED], databyss [EMAIL PROTECTED] wrote: I have a simple program and the output isn't what I expect. Could somebody please explain why? Here's the code: #simple program print v = 2 v = 2 print v**v = 2**2 =, v**v print v**v**v = 2**2**2 =, v**v**v print v**v**v**v = 2**2**2**2 =, v**v**v**v #end program Here's the output: v = 2 v**v = 2**2 = 4 v**v**v = 2**2**2 = 16 v**v**v**v = 2**2**2**2 = 65536 I would expect 2**2**2**2 to be 256 Python's ** operator associates to the right, not to the left; thus, 2 ** 2 ** 2 ** 2 ... really means 2 ** (2 ** (2 ** 2)) ... and not ((2 ** 2) ** 2) ** 2 ... as you seem to expect. As usual, you can enforce different associations by explicitly including the parentheses. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop three lists at the same time?
In article [EMAIL PROTECTED], Davy [EMAIL PROTECTED] wrote: Hi all, I have three lists with the same length. Is there any method to loop the three lists without a loop counter? Best regards, Davy Davy, Look at the zip function, http://docs.python.org/lib/built-in-funcs.html Look near the bottom of the page. Example: zip([1,2,3], [4,5,6], [7,8,9]) == [(1, 4, 7), (2, 5, 8), (3, 6, 9)] Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Iteration for Factorials
In article [EMAIL PROTECTED], Py-Fun [EMAIL PROTECTED] wrote: I'm stuck trying to write a function that generates a factorial of a number using iteration and not recursion. Any simple ideas would be appreciated. Well, first think about the recursive implementation: def fact(n): if n 0: return n * fact(n - 1) else: return 1 To turn this into an iterative computation, you must first get rid of the deferred operations -- in this case, the multiplication step after the recursive call to fact(n - 1). Since multiplication commutes, we can re-write this recursion to keep an accumulating parameter instead of deferring the operation: def fact2(n, acc = 1): if n 0: return fact2(n - 1, acc * n) else: return acc This is a little bit better, because now the recursive call is in tail position, and so in principle no state needs to be saved across recursive calls: Once the inner call to fact2 is complete, its value is simply returned. But we're not done yet, because Python _does_ save state across recursive calls, even in this construction. By a gentle twist of perspective, the inner call to fact2(n - 1, acc * n) is really just a kind of jump back to the beginning of the function. In another (hypothetical) language, you might write it like this: # Not legal Python code. def fact3(n, acc = 1): TOP: if n 0 n = n - 1 acc = acc * n goto TOP else: return acc Naturally, of course, Python does not provide a goto statement. But it does have one that's similar: while TEST: BODY is equivalent in meaning to the pseudo-code: X: if TEST: BODY goto X Can you now see how you would re-write fact3 into legal Python code, using this equivalence? Once you have done so, you will also be able to get rid of the extra accumulating parameter, and then you will have what you wanted. I hope this helps. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Last iteration?
additional setup. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: sorting a list numbers stored as strings
In article [EMAIL PROTECTED], [EMAIL PROTECTED] wrote: hi, I have the following list - [1, 11, 2, 22] how do I sort it like this - [1, 2, 11, 22] thanks, aine Try: lst.sort(key = lambda s: int(s)) Assuming, of course, that lst is your original list; this will sort it in place. Also, you should probably read about the sorted function here: http://docs.python.org/lib/built-in-funcs.html Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: frange() question
In article [EMAIL PROTECTED], George Trojan [EMAIL PROTECTED] wrote: A while ago I found somewhere the following implementation of frange(): def frange(limit1, limit2 = None, increment = 1.): Range function that accepts floats (and integers). Usage: frange(-2, 2, 0.1) frange(10) frange(10, increment = 0.5) The returned value is an iterator. Use list(frange) for a list. if limit2 is None: limit2, limit1 = limit1, 0. else: limit1 = float(limit1) count = int(math.ceil(limit2 - limit1)/increment) return (limit1 + n*increment for n in range(count)) I am puzzled by the parentheses in the last line. Somehow they make frange to be a generator: print type(frange(1.0, increment=0.5)) type 'generator' But I always thought that generators need a keyword yield. What is going on here? Hi, George, The expression returned is a generator expression, return (limit1 + n*increment for n in range(count)) Thus, although frange itself is not written as a generator, it does return a generator as its result. The syntax is like that of list comprehensions; see: http://docs.python.org/ref/genexpr.html Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Parsing problems: A journey from a text file to a directory tree
Hi, John, Your comments below are all reasonable. However, I would like to point out that the purpose of my example was to provide a demonstration of an algorithm, not an industrial-grade solution to every aspect of the original poster's problem. I am confident the original poster can deal with these aspects of his problem space on his own. In article [EMAIL PROTECTED], John Machin [EMAIL PROTECTED] wrote: [...] .while new_level len(state): .state.pop() Hmmm ... consider rewriting that as the slightly less obfuscatory while len(state) new_level: state.pop() This seems to me to be an aesthetic consideration only; I'm not sure I understand your rationale for reversing the sense of the comparison. Since it does not change the functionality, it's hardly worthy of complaint, but I don't see any improvement, either. A warning message if there are too many - characters might be a good idea: [foo] |-bar |-zot |---plugh Perhaps so. Again, the original poster will have to decide what should be the correct response to input of this sort; at present, the implementation is tolerant of such variations, without loss of generality. And if the input line matches neither regex? I believe it should be clear that such lines are ignored. Again, this is an opportunity for the original poster to determine an alternative response -- perhaps an exception could be raised, if that is his desire. The problem specification did not constrain this case. To call this, pass a file-like object to parse_folders(), e.g.: test1 = ''' [New client]. Won't work with the dot on the end. My mistake. The period was a copy-and-paste artifact, which I missed. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Parsing problems: A journey from a text file to a directory tree
In article [EMAIL PROTECTED],
Martin M. [EMAIL PROTECTED] wrote:
Hi everybody,
Some of my colleagues want me to write a script for easy folder and
subfolder creation on the Mac.
The script is supposed to scan a text file containing directory trees
in the following format:
[New client]
|-Invoices
|-Offers
|--Denied
|--Accepted
|-Delivery notes
As you can see, the folder hierarchy is expressed by the amounts of
minuses, each section header framed by brackets (like in Windows
config files).
After the scan process, the script is supposed to show a dialog, where
the user can choose from the different sections (e.g. 'Alphabet',
'Months', 'New client' etc.). Then the script will create the
corresponding folder hierarchy in the currently selected folder (done
via AppleScript).
But currently I simply don't know how to parse these folder lists and
how to save them in an array accordingly.
First I thought of an array like this:
dirtreedb = {'New client': {'Invoices': {}, 'Offers': {'Denied': {},
'Accpeted': {}}, 'Delivery notes': {}}}
But this doesn't do the trick, as I also have to save the hierarchy
level of the current folder as well...
Argh, I really don't get my head around this problem and I need your
help. I have the feeling, that the answer is not that complicated, but
I just don't get it right now...
Hello, Martin,
A good way to approach this problem is to recognize that each section of
your proposed configuration represents a kind of depth-first traversal
of the tree structure you propose to create. Thus, you can reconstruct
the tree by keeping track at all times of the path from the root of
the tree to the current location in the tree.
Below is one possible implementation of this idea in Python. In short,
the function keeps track of a stack of dictionaries, each of which
represents the contents of some directory in your hierarchy. As you
encounter |-- lines, entries are pushed to or popped from the stack
according to whether the nesting level has increased or decreased.
This code is not heavily tested, but hopefully it should be clear:
.import re
.
.def parse_folders(input):
.Read input from a file-like object that describes directory
.structures to be created. The input format is:
.
.[Top-level name]
.|-Subdirectory1
.|--SubSubDirectory1
.|--SubSubDirectory2
.|---SubSubSubDirectory1
.|-Subdirectory2
.|-Subdirectory3
.
.The input may consist of any number of such groups. The result is
.a dictionary structure in which each key names a directory, and
.the corresponding value is a dictionary structure showing the
.contents of that directory, possibly empty.
.
.
.# This expression matches header lines, defining a new section.
.new_re = re.compile(r'\[([\w ]+)\]\s*$')
.
.# This expression matches nesting lines, defining subdirectories.
.more_re = re.compile(r'(\|-+)([\w ]+)$')
.
.out = {}# Root: Maps section names to subtrees.
.state = [out] # Stack of dictionaries, current path.
.
.for line in input:
.m = new_re.match(line)
.if m: # New section begins here...
.key = m.group(1).strip()
.out[key] = {}
.state = [out, out[key]]
.continue
.
.m = more_re.match(line)
.if m: # Add a directory to an existing section
.assert state
.
.new_level = len(m.group(1))
.key = m.group(2).strip()
.
.while new_level len(state):
.state.pop()
.
.state[-1][key] = {}
.state.append(state[-1][key])
.
.return out
To call this, pass a file-like object to parse_folders(), e.g.:
test1 = '''
[New client].
|-Invoices
|-Offers
|--Denied
|--Accepted
|---Reasons
|---Rhymes
|-Delivery notes
'''
from StringIO import StringIO
result = parse_folders(StringIO(test1))
As the documentation suggests, the result is a nested dictionary
structure, representing the folder structure you encoded. I hope this
helps.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: fcntl problems
In article [EMAIL PROTECTED], mhearne808[insert-at-sign-here]gmail[insert-dot-here]com [EMAIL PROTECTED] wrote: Looking at my flock(3) man page, I'm guessing that 35 is the error code for EWOULDBLOCK. Which system header file am I supposed to look in to figure that magic number out? On a MacOS system, you can find them in /usr/include/sys/errno.h On a Linux system, try /usr/include/asm-generic/errno.h However, if you're writing in Python, you will probably have an easier time using the errno module, e.g., ] import errno ] errno.errorcode[35] 'EDEADLOCK' Note that some codes have multiple names (e.g., EAGAIN and EWOULDBLOCK) so that this lookup may not return exactly the name you're expecting. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: tempfile.mkstemp and os.fdopen
In article [EMAIL PROTECTED], billiejoex [EMAIL PROTECTED] wrote: Hi there. I'm trying to generate a brand new file with a unique name by using tempfile.mkstemp(). In conjunction I used os.fdopen() to get a wrapper around file properties (write read methods, and so on...) but 'name' attribute does not contain the correct file name. Why? import os import tempfile fileno, name = tempfile.mkstemp(prefix='ftpd.', dir=os.getcwd()) fd = os.fdopen(fileno, 'wb') fd.name fdopen Moreover, I'd like to know if I'm doing fine. Does this approach avoid race conditions or other types of security problems? Thanks in advance In brief, since os.fdopen() only has access to the file descriptor, it does not have a convenient way to obtain the file's name. However, you might also want to look at the TemporaryFile and NamedTemporaryFile classes in the tempfile module -- these expose a file-like API, including a .name attribute. Assuming tempfile.mkstemp() is implemented properly, I think what you are doing should be sufficient to avoid the obvious file-creation race condition. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Image manipulation
In article [EMAIL PROTECTED], Slava [EMAIL PROTECTED] wrote: I need to add dynamic text to animated GIF images. What is a best way to do it? Thanks. One possibility is the GD library, which supports animated GIF. There is a Python binding: http://newcenturycomputers.net/projects/gdmodule.html I do not know, however, whether or not the Python wrapper supports the animated GIF portions of the library. You'll probably have to do some digging. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Regular Expression Grouping
In article [EMAIL PROTECTED],
[EMAIL PROTECTED] wrote:
Fairly new to this regex thing, so this might be very juvenile but
important.
I cannot understand and why 'c' constitutes a group here without being
surrounded by ( ,) ?
import re
m = re.match(([abc])+, abc)
m.groups()
('c',)
Grateful for any clarity.
Hello!
I believe your confusion arises from the placement of the + operator
in your expression. You wrote:
'([abc])+'
This means, in plain language, one or more groups in which each group
contains a string of one character from the set {a, b, c}.
Contrast this with what you probably intended, to wit:
'([abc]+)'
The latter means, in plain language, a single group containing a string
of one or more characters from the set {a, b, c}.
In the former case, the greedy property of matching attempts to maximize
the number of times the quantified expression is matched -- thus, you
match the group three times, once for each character of abc, and the
result shows you only the last occurrence of the matching.
Compare this with the following:
] import re
] m = re.match('([abc]+)', 'abc')
] m.groups()
= ('abc',)
I suspect the latter is what you are after.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re��: ��get wikipedia source failed�� (��urrlib2��)
In articleâ [EMAIL PROTECTED],â¬
â [EMAIL PROTECTED] wroteâ:â¬
â â¬Hiâ,â¬
â â¬I'm trying to get wikipedia page source with urllib2â:â¬
â â¬usockâ =
â¬urllib2â.â¬urlopenâ(â¬httpâ://â¬en.wikipedia.org/wikiâ/â¬
â â¬Albert_Einsteinâ)â¬
â â¬dataâ = â¬usock.readâ();â¬
â â¬usock.closeâ();â¬
â â¬return data
â â¬I got exception because HTTP 403â â¬errorâ. â¬whyâ? â¬with my
browser i can't
â â¬access it without any problemâ?â¬
â â¬
â â¬Thanksâ,â¬
â â¬Shaharâ.â¬
It appears that Wikipedia may inspect the contents of the User-Agentâ â¬
HTTP headerâ, â¬and that it does not particularly like the string itâ â¬
receives from Python's urllibâ. â¬I was able to make it work with urllibâ
â¬
via the following codeâ:â¬
import urllib
class CustomURLopenerâ (â¬urllib.FancyURLopenerâ):â¬
â â¬versionâ = 'â¬Mozilla/5.0â'â¬
urllibâ.â¬_urlopenerâ = â¬CustomURLopenerâ()â¬
uâ =
â¬urllib.urlopenâ('â¬httpâ://â¬en.wikipedia.org/wiki/Albert_Einsteinâ')â¬
dataâ = â¬u.readâ()â¬
I'm assuming a similar trick could be used with urllib2â, â¬though I
didn'tâ â¬
actually try itâ. â¬Another thing to watch out forâ, â¬is that some
sitesâ â¬
will redirect a public URL X to an internal URL Yâ, â¬and will check thatâ
â¬
access to Y is only permitted if the Referer field indicates coming fromâ â¬
somewhere internal to the siteâ. â¬I have seen both of these techniquesâ
â¬
used to foil screen-scrapingâ.â¬
Cheersâ,â¬
â-â¬M
â-- â¬
Michael Jâ. â¬Frombergerâ | â¬Lecturerâ, â¬Deptâ. â¬of
Computer Science
httpâ://â¬www.dartmouth.eduâ/â¬~stingâ/ | â¬Dartmouth Collegeâ,
â¬Hanoverâ, â¬NHâ, â¬USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: boolean operations on sets
In article [EMAIL PROTECTED], Diez B. Roggisch [EMAIL PROTECTED] wrote: Flavio wrote: Hi, I have been playing with set operations lately and came across a kind of surprising result given that it is not mentioned in the standard Python tutorial: with python sets, intersections and unions are supposed to be done like this: [...] The results are not what you would expect from an AND or OR operation, from the mathematical point of view! aparently the and operation is returning the the second set, and the or operation is returning the first. [...] It has nothing to do with sets - it stems from the fact that certain values in python are considered false, and all others true. And these semantics were introduced at a point where there was no explicit True/False, so the operators were defined in exact the way you observed. Consider this: foo or bar - foo So - nothing to do with sets. In addition to what Diez wrote above, it is worth noting that the practise of returning the value of the determining expression turns out to be convenient for the programmer in some cases. Consider the following example: x = some_function(a, b, c) or another_function(d, e) This is a rather nice shorthand notation for the following behaviour: t = some_function(a, b, c) if t: x = t else: x = another_function(d, e) In other words, the short-circuit behaviour of the logical operators gives you a compact notation for evaluating certain types of conditional expressions and capturing their values. If the or operator converted the result to True or False, you could not use it this way. Similarly, x = some_function(a, b, c) and another_function(d, e) ... behaves as if you had written: x = some_function(a, b, c) if x: x = another_function(d, e) Again, as above, if the results were forcibly converted to Boolean values, you could not use the shorthand. Now that Python provides an expression variety of if, this is perhaps not as useful as it once was; however, it still has a role. Suppose, for example, that a call to some_function() is very time-consuming; you would not want to write: x = some_function(a, b, c) \ if some_function(a, b, c) else another_function(d, e) ... because then some_function would get evaluated twice. Python does not permit assignment within an expression, so you can't get rid of the second call without changing the syntax. Also, it is a common behaviour in many programming languages for logical connectives to both short-circuit and yield their values, so I'd argue that most programmers are proabably accustomed to it. The and || operators of C and its descendants also behave in this manner, as do the AND and OR of Lisp or Scheme. It is possible that beginners may find it a little bit confusing at first, but I believe such confusion is minor and easily remedied. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: a dict trick
In article [EMAIL PROTECTED],
james_027 [EMAIL PROTECTED] wrote:
hi
for example I have this dictionary
dict = {'name':'james', 'language':'english'}
value = 'sex' in dict and dict['sex'] or 'unknown'
is a right pythonic of doing this one? I am trying to get a value from
the dict, but if the key doesn't exist I will provide one.
Hi, James,
You might prefer:
d = {'name': 'James', 'language': 'English'}
value = d.get('sex', 'unknown')
This accomplishes what your above code does, using a method of the
built-in dict object.
If you also wish to ADD the new value to the dictionary, you may also
use the following:
value = d.setdefault('sex', 'unknown')
This returns the same value as the above, but also adds the key 'sex' to
the dictionary as a side-effect, with value 'unknown'.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: How to initialize a table of months.
In article [EMAIL PROTECTED],
Steven W. Orr [EMAIL PROTECTED] wrote:
I'm reading a logfile with a timestamp at the begging of each line, e.g.,
Mar 29 08:29:00
I want to call datetime.datetim() whose arg2 is a number between 1-12 so I
have to convert the month to an integer.
I wrote this, but I have a sneaky suspicion there's a better way to do it.
mons = {'Jan':1, 'Feb':2, 'Mar':3, 'Apr':4, 'May':5, 'Jun':6,
'Jul':7, 'Aug':8, 'Sep':9, 'Oct':10, 'Nov':11, 'Dec':12 }
def mon2int( mon ):
global mons
return mons[mon]
Is there a generator expression or a list comprehension thingy that would
be *betterer*? (I realize it's probably not that important but I find lots
of value in learning all the idioms.)
There's no harm in your method as written, though it were probably wise
to tolerate variations in capitalization. But if you want to be cute
about it, you could write something like this to set up your table:
from datetime import date
months = dict((date(1900, x+1, 1).strftime('%b').lower(), x+1)
for x in xrange(12))
def month2int(mName):
return months[mName.lower()]
If you don't like the lookup table, you can get a nicely portable result
without it by using time.strptime(), e.g.,
import time
def month2int(mName):
return time.strptime(mName, '%b').tm_mon # parses short names
Without knowing anything further about your needs, I would probably
suggest the latter simply because it makes the hard work be somebody
else's problem.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: how to remove multiple occurrences of a string within a list?
In article [EMAIL PROTECTED],
bahoo [EMAIL PROTECTED] wrote:
Hi,
I have a list like ['0024', 'haha', '0024']
and as output I want ['haha']
If I
myList.remove('0024')
then only the first instance of '0024' is removed.
It seems like regular expressions is the rescue, but I couldn't find
the right tool.
If you know in advance which items are duplicated, then there have been
several simple solutions already proposed. Here's another way to tackle
the problem of removing ANY duplicated item from the list (i.e., any
string that appears 1 time).
def killdups(lst):
Filter duplicated elements from the input list, and return the
remaining (unique) items in their original order.
count = {}
for elt in lst:
count[elt] = count.get(elt, 0) + 1
return [elt for elt in lst if count[elt] == 1]
This solution is not particularly tricky, but it has the nice properties
that:
1. It works on lists of any hashable type, not just strings,
2. It preserves the order of the unfiltered items,
3. It makes only two passes over the input list.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: How to sort list
In article [EMAIL PROTECTED],
Lad [EMAIL PROTECTED] wrote:
I have a list of emails and I would like to sorted that list by domains
E.g.
If the list is
Emails=['[EMAIL PROTECTED]','[EMAIL PROTECTED]','[EMAIL PROTECTED]','[EMAIL
PROTECTED]',]
after sorting I would like to have
Emails=['[EMAIL PROTECTED]','[EMAIL PROTECTED]','[EMAIL PROTECTED]','[EMAIL
PROTECTED]',]
What is the best/easiest way?
One reasonable option is to use the .sort() method of a list:
Emails.sort(key = lambda s: list(reversed(s.split('@'
The key parameter specifies how to obtain a sort key from each element
in the source list.
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: +1 QOTW
In article [EMAIL PROTECTED], Terry Reedy [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] Did anyone else crack up when Larry Wall described python with the statement: Python, as the anti-Perl, is heavily invested in maintaining Order. In the state of the onion address? http://www.perl.com/pub/a/2006/09/21/onion.html There is also this: 'But I think the basic Perl paradigm is Whatever-oriented programming.' But what this really means, in practise, is dis-oriented programming. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Python 2.5 and Unicode on MacOS
I'm not sure whether this is a bug, or a feature that I do not fully understand. I would therefore appreciate some advice, if you have any, on the following problem: I have just installed Python 2.5 (release) under MacOS 10.4.7 (Tiger). Unfortunately, the unit test test_unicodefile.py (in Lib/tests/) exhibits a warning when invoked, as shown here: [Lib/test] % python2.5 test_unicode_file.py test_directories (__main__.TestUnicodeFiles) ... ok test_unicode_file.py:103: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal filename1==filename2 test_equivalent_files (__main__.TestUnicodeFiles) ... ok test_single_files (__main__.TestUnicodeFiles) ... ok -- Ran 3 tests in 0.252s As you can see, the unit tests appear to pass, but I suspect there is a hidden problem, as a subsequent build of the latest version of BeautifulSoup now fails its unit tests, citing the same warning. I can't tell whether this is a problem with the way BeautifulSoup is doing things, or whether there is a more general problem with Unicode support in the MacOS build of Python 2.5. For reference, I have done a framework build of Python, and it seems to work fine for everything else I have tried. Do you have any idea what might be causing this trouble? I'd be grateful for your insights. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Semantics of propagated exceptions
In article [EMAIL PROTECTED], Thomas Lotze [EMAIL PROTECTED] wrote: Suppose you have a function f which, as part of its protocol, raises some standard exception E under certain, well-defined circumstances. Suppose further that f calls other functions which may also raise E. How to best distinguish whether an exception E raised by f has the meaning defined by the protocol or just comes from details of the implementation? My recommendation is to define your own exception type(s), and have f raise it (them) for errors that are truly generated by f. Or, if you really want to use one of the existing exception types, then perhaps you can catch the exceptions from functions called by f, within f itself, and raise some other error (or suppress the errors, if appropriate). Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Dispatch with multiple inheritance
In article [EMAIL PROTECTED], Michele Simionato [EMAIL PROTECTED] wrote: Michael J. Fromberger ha scritto: Consider the following class hierarchy in Python: snip Is there a better (i.e., more elegant) way to handle the case marked (**) above? Curious, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA Look at http://www.python.org/download/releases/2.3/mro Hi, Michele, I understand what Python's method resolution order is, and how it is constructed. Is there something else in that article, that you hoped would address my original concern? In particular, I am not asking WHY super(A, self).__init()__ yields the next method in order -- that is quite clear. Rather, I want to know if there is a better way to invoke the ancestry of class E from D, than to explicitly denote the ancestry of C in the code. The latter is an ugly violation of abstraction, and renders the class hierarchy brittle. Of course, I could just bypass super, and explicitly invoke them as: C.__init__(self, ...) D.__init__(self, ...) ... but that seems to me to defeat the purpose of having super in the first place. It seems as if perhaps there is no better solution at present. That is a pity, albeit a mild one. Thank you for taking the time to respond. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Dispatch with multiple inheritance
In article [EMAIL PROTECTED], Nick Vatamaniuc [EMAIL PROTECTED] wrote: Michael, You only need to call the __init__ method of the superclass if you need to do something special during initialization. Hi, Nick, Thank you for responding. I understand the purpose in invoking the superclasses' __init__ methods. Let us take it as a given that I require this behaviour; the simple example does not show it, but I am using the same technique in a much more elaborate program, where in fact the superclass initialization is required. In general I just use the SuperClass.__init__(self,...) way of calling the super class constructors. Yes, and that certainly works just fine. But it obviates the point of having super(). But perhaps that is the take-home message. I do not want A's constructor to dispatch further along in the MRO for E for two reasons: One, because the constructors along the (E D B) chain take different argument lists (in my real code) than the (E C A) path; and two, because I happen to care about the order in which the methods are invoked. In general note that __init__ is NOT a constuctor it is an initializer Yes, you're right; I apologize for the imprecision. However, for the purposes of this example, the distinction is irrelevant. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Dispatch with multiple inheritance
In article [EMAIL PROTECTED], Michael Spencer [EMAIL PROTECTED] wrote: As others have pointed out, super, is designed to do something different from what you want. See http://www.python.org/download/releases/2.2.3/descrintro/#cooperation for GvR's explanation of super's intent, limitations, and a pure-python implementation that you might alter for your purposes. Indeed so. The documentation for super() led me to an erroneous conclusion. Given what you assert about your inheritance graph (two independent ancestor chains, that do not propagate super calls from their respective base classes), what is objectionable about: C.__init__(self, ...) D.__init__(self, ...) Well, since super() were much better labelled call_next_method, I must agree that the Class.__init__(self, ...) form is probably the best solution! Thanks for your feedback. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Dispatch with multiple inheritance
Consider the following class hierarchy in Python: class A (object): def __init__(self): print cons A class B (object): def __init__(self): print cons B class C (A): def __init__(self): super(C, self).__init__() print cons C class D (B): def __init__(self): super(D, self).__init__() print cons D Now, suppose I would like to define a new class as follows: class E (C, D): ... In the constructor for E, I would like to invoke the constructors of both parent classes. The correct way to do this seems to be exemplified by the following: class E (C, D): def __init__(self): super(E, self).__init__() # calls C constructor super(A, self).__init__() # calls D constructor (**) print cons E This works, but I find it somewhat troubling. It seems to me that one should not have to know (i.e., write down the names of) the ancestors of C in order to dispatch to superclass methods in D, since C and D share no common ancestors south of object. Is there a better (i.e., more elegant) way to handle the case marked (**) above? Curious, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Arrghh... Problems building Python from source on OS X --
In article [EMAIL PROTECTED], J. Jeffrey Close [EMAIL PROTECTED] wrote: Hi all, I have been trying for some time to build Python 2.4.x from source on OS X 10.4.6. I've found *numerous* postings on various mailing lists and web pages documenting the apparently well-known problems of doing so. Various problems arise either in the ./configure step, with configure arguments that don't work, or in the compile, or in my case in the link step with libtool. The configure options I'm using are the following: --enable-framework --with-pydebug --with-debug=yes --prefix=/usr --with-dyld --program-suffix=.exe --enable-universalsdk I've managed to get past configure and can compile everything, but in the link I get the error Undefined symbols: ___eprintf . This appears to have something to do with dynamic library loading not properly pulling in libgcc. I've tried with -lgcc in the LD options, but that produces a configure error cannot compute sizeof If I remove --enable-framework the complete build works, but unfortunately that is the one critical element that I need. The web pages I've found referring to this range from 2001 to present -- still apparently everybody is having problems with this. Does *anybody* here have Python built from source on this OS? Hi, Jeffrey, Yes, I use Python 2.4.3 built this way. I did not have any significant troubles building Python on my 10.4 system. My configuration step was a little different from yours, but basically I just checked out the 2.4.3 source from Subversion, and this is how I configured it: env CFLAGS=-I/usr/local/include -I/Library/Frameworks/Tk.framework/Headers \ LDFLAGS=-L/usr/local/lib \ configure --enable-framework --enable-shared I included /usr/local/include and /usr/local/lib so that the build could use the version of GNU readline I installed via Darwin ports. The Tk headers allow pythonw to build properly. Having configured, I built and installed via: make sudo make frameworkinstall I hope this may be helpful to you. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Finding yesterday's date with datetime
In article [EMAIL PROTECTED], Thierry Lam [EMAIL PROTECTED] wrote: Is there an easy way to determine the yesterday's date(year-month-day) from the python datetime library if I know today's date? from datetime import datetime, timedelta today = datetime.today() yesterday = today - timedelta(1) # See the .month, .day, and .year fields of yesterday Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Enumerating Regular Expressions
In article [EMAIL PROTECTED], James Stroud [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: James Stroud wrote: You mean like re.compile(r'.*') ? No. I mean like: regex = re.compile(r'a|b') regex.enumerate() a b You see the difficulty don't you? How will the computer know in advance that the regex matches only a finite set of possible strings? You don't. Hence, you want something that behaves like a generator, and will produce the strings one at a time. Preferably, for the purposes of useful computation, in some canonical order. I'm sorry to say I don't know of an existing Python module to do this, although you could write one for at least the basic regular expression operators if you wanted. The basic problem isn't all that hard to solve, though the full generality of the re module's input syntax is far more expressive than truly regular expressions from language theory. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Splice two lists
In article [EMAIL PROTECTED], [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: Is there a good way to splice two lists together without resorting to a manual loop? Say I had 2 lists: l1 = [a,b,c] l2 = [1,2,3] And I want a list: [a,1,b,2,c,3] as the result. I've been searching around but I can't seem to find a good example. Here's one possibility: list(reduce(lambda s, t: s + t, zip(L1, L2), ())) -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: How to do an 'inner join' with dictionaries
In article [EMAIL PROTECTED], [EMAIL PROTECTED] wrote: Let's say I have two dictionaries: dict1 is 1:23, 2:76, 4:56 dict2 is 23:A, 76:B, 56:C How do I get a dictionary that is 1:A, 2:B, 4:C The following should work: j = dict((k, dict2[dict1[k]]) for k in dict1 if dict1[k] in dict2) Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: can't solve an exercise-help me with it
In article [EMAIL PROTECTED],
hossam [EMAIL PROTECTED] wrote:
I'm studying python newly and have an exercise that is difficult for me as a
beginner.Here it is :
Write a program that approximates the value of pi by summing the terms of
this series:
4/1-4/3+4/5-4/7+4/9-4/11+ The program should prompt the user for n, the
number of terms to sum and then output the sum of the first n terms of this
series.
any help would be appreciated.
benni
The series you're describing is basically expanding the Taylor series
for 4 * atan(1), where
i
oo -1
atan(1) = SUM --
i = 0 2i + 1
(or in LaTeX: \sum_{i=0}^{\infty}\frac{-1^i}{2i+1})
To enumerate the terms of this series, you can treat i as a counter, and
compute the numerator and denominator either directly or from their
previous values. The sum can be accumulated into a separate variable.
So, for instance,
. sum = 0. ; num = 1. ; den = 1.
. for i in xrange(n):
. sum += num / den
. num = -num
. den += 2
.
. pi = 4 * sum # approximately...
Keep in mind that this approximation for atan(1) converges very slowly,
so you will need to do quite a lot of terms before it will converge (you
need about a thousand terms to get 2 significant figures after the
decimal point). You might have better results using Machin's formula,
atan(1) = 4 * atan(1/5) - atan(1/239)
Or, pi = 16 * atan(1/5) - 4 * atan(1/239). The Taylor series will
converge more quickly for atan(1/5) and atan(1/239).
(LaTeX: \mathrm{atan}(x) = \sum_{i=0}^\infty\frac{(-1^i)x^i}{2i+1})
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: Burrows-Wheeler (BWT) Algorithm in Python
In article [EMAIL PROTECTED], [EMAIL PROTECTED] wrote: Michael J. Fromberger: I can send you a Python implementation I wrote, if you like; but if you're interested in better understanding how the transform works, I would recommend you try writing your own implementation. I'd like to see it, if you want you can put it somewhere or send it directly. You can grab a copy here: http://www.cs.dartmouth.edu/~sting/sw/bwt.py Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Burrows-Wheeler (BWT) Algorithm in Python
In article [EMAIL PROTECTED], DENG [EMAIL PROTECTED] wrote: Hi, all I've used Python Bz2 module for times and want to kown something about Burrows-Wheeler (BWT) algorithm, the Bz2 module is wrriten in C, is there a version in Python too? BWT http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-124.ht ml Python Bz2 module http://labix.org/python-bz2 It is perfectly possible to implement the BWT in Python. I can send you a Python implementation I wrote, if you like; but if you're interested in better understanding how the transform works, I would recommend you try writing your own implementation. It's not very difficult to do, though for large inputs you may find performance to be an issue. Mark Nelson wrote a nice user-friendly article on the BWT for the Sep. 1996 issue of Dr. Dobbs, you might have a look: http://www.dogma.net/markn/articles/bwt/bwt.htm I hope this helps you get started. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Where to find python c-sources
In article [EMAIL PROTECTED], Tor Erik Sønvisen [EMAIL PROTECTED] wrote: Hi I need to browse the socket-module source-code. I believe it's contained in the file socketmodule.c, but I can't locate this file... Where should I look? I recommend you look in the Modules subdirectory of the Python source tree. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Parser suggestion
In article [EMAIL PROTECTED], Jorge Godoy [EMAIL PROTECTED] wrote: I'm needing a parser to retrieve some information from source code -- including parts of code -- from Fortran, to use in a project with a documentation system. Any recommendations on a Python app or parser that I could use for that? There seems to be a great diversity of parsing tools available for Python programmers. Here are a few suggestions to get you started: PLY (Python Lex/Yacc) http://www.dabeaz.com/ply/ PyParsing http://pyparsing.sourceforge.net/ SPARK (Scanning Parsing And Rewriting Kit) http://pages.cpsc.ucalgary.ca/~aycock/spark/ You might also find the following an interesting read, if this sort of thing interests you: http://www.python.org/sigs/parser-sig/towards-standard.html Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Crypto.Cipher.ARC4, bust or me doing something wrong?
In article [EMAIL PROTECTED], Michael Sparks [EMAIL PROTECTED] wrote: Hi, I suspect this is a bug with AMK's Crypto package from http://www.amk.ca/python/code/crypto , but want to check to see if I'm being dumb before posting a bug report. I'm looking at using this library and to familiarise myself writing small tests with each of the ciphers. When I hit Crypto.Cipher.ARC4 I've found that I can't get it to decode what it encodes. This might be a case of PEBKAC, but I'm trying the following: from Crypto.Cipher import ARC4 as cipher key = obj = cipher.new(key) obj.encrypt(This is some random text) ')f\xd4\xf6\xa6Lm\x9a%}\x8a\x95\x8ef\x00\xd6:\x12\x00!\xf3k\xafX' X=_ X ')f\xd4\xf6\xa6Lm\x9a%}\x8a\x95\x8ef\x00\xd6:\x12\x00!\xf3k\xafX' obj.decrypt(X) '\x87\xe1\x83\xc1\x93\xdb\xed\x93U\xe4_\x92}\x9f\xdb\x84Y\xa3\xd4b\x9eHu~' Clearly this decode doesn't match the encode. Me being dumb or bug? Any comments welcome :) Michael, Since ARC4 is a stream cipher, the keystream changes over time -- with ARC4, after each character enciphered. To decrypt successfully, you need to make sure the decrypting keystream exactly matches the encrypting one. In your example, you used a different keystream to decrypt than you used to encrypt -- in this case, a little further downstream of the original encryption key. Contrast your experience above with the following transcript: from Crypto.Cipher import ARC4 as cipher enc = cipher.new(abcdefgh) dec = cipher.new(abcdefgh) x = enc.encrypt(This is some random text) x \x05o\xd5XH|\xa4\xfc\xf7z\xecd\xe92\xfb\x05rR'\xbf\xc0F\xfc\xde y = dec.decrypt(x) y 'This is some random text' enc.decrypt(x) 'M|[bI\x1ciG6A]\x13Hz\xb0\x19\xca\xf1-\x9a\x1a2\x9e%' I hope this helps clear up your confusion. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: encryption with python
In article [EMAIL PROTECTED], [EMAIL PROTECTED] wrote: Hi! I was wondering if someone can recommend a good encryption algorithm written in python. My goal is to combine two different numbers and encrypt them to create a new number that cann't be traced back to the originals. It would be great if there exists a library already written to do this, and if there is, can somebody please point me to it?? I recommend you investigate PyCrypto: http://www.amk.ca/python/code/crypto http://sourceforge.net/projects/pycrypto Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Proposal: add sys to __builtins__
In article [EMAIL PROTECTED], Rick Wotnaz [EMAIL PROTECTED] wrote: You're right that there is no necessity for such a change. I was not actually talking about importing *any* module in every case, but rather about importing, say, 'sys' when, for example, sys.argv appeared in the code and no import had been specified. I think I must have missed that post; I will go back and look at it. However, while I'm here, how would your proposal deal with code like this: import foobar # ... some while later ... def f( ... ): ... global foobar, sys sys = foobar ... # ... some while even later ... f( ... ) sys.wallaby(Fear and loathing!) In particular, we have no import of sys, but the name sys is meaningful as a local alias for a different module. I'm not saying you couldn't deal with this, but it rules out some of the more obvious ways of detecting and automatically handling this kind of substitution. Naturally, you might well ask, why would you do such a fool thing? To this I can only respond: Never underestimate the ingenuity of fools. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Proposal: add sys to __builtins__
In article [EMAIL PROTECTED], Rick Wotnaz [EMAIL PROTECTED] wrote: Michael Hoffman [EMAIL PROTECTED] wrote in news:[EMAIL PROTECTED]: What would people think about adding sys to __builtins__ so that import sys is no longer necessary? This is something I must add to every script I write that's not a one-liner since they have this idiom at the bottom: if __name__ == __main__: sys.exit(main(sys.argv[1:])) [...] In short, given the wide use of sys, its unambiguous nature, and the fact that it really is built-in already, although not exposed as such, I think we would be better off if sys were always allowed even without an import statement. +1 here. As far as I'm concerned, both os and sys could be special- cased that way. That said, I would guess the likelihood of that happening is 0. While I'm mildly uncomfortable with the precedent that would be set by including the contents of sys as built-ins, I must confess my objections are primarily aesthetic: I don't want to see the built-in namespace any more cluttered than is necessary -- or at least, any more than it already is. But os is another matter -- the os module contains things which might well not be present in an embedded Python environment (e.g., file and directory structure navigation, stat). Do you really want to force everyone to deal with that? Is it so much more work to add import os to those Python programs that require it? Of course, you might counter why should we force everybody else to type `import os' just in case somebody wants to imbed Python? But then, why don't we just include the whole standard library in __builtins__? Or, since that would be too much, maybe we survey the user community and include the top fifteen most included modules! Where do you draw the line? Do you really want to hard-code user opinions into the language? Right now, we have a nice, simple yet effective mechanism for controlling the contents of our namespaces. I don't think this would be a worthwhile change. -1. -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Replacing large number of substrings
In article [EMAIL PROTECTED],
Will McGugan [EMAIL PROTECTED] wrote:
Hi,
Is there a simple way of replacing a large number of substrings in a
string? I was hoping that str.replace could take a dictionary and use it
to replace the occurrences of the keys with the dict values, but that
doesnt seem to be the case.
To clarify, something along these lines..
dict_replace( a b c, dict(a=x, b=y) )
x y c
Hi, Will,
Perhaps the following solution might appeal to you:
. import re
.
. def replace_many(s, r):
. Replace substrings of s. The parameter r is a dictionary in
. which each key is a substring of s to be replaced and the
. corresponding value is the string to replace it with.
.
. exp = re.compile('|'.join(re.escape(x) for x in r.keys()))
. return exp.sub(lambda m: r.get(m.group()), s)
Cheers,
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: Permutation Generator
In article [EMAIL PROTECTED], Talin [EMAIL PROTECTED] wrote: I'm sure I am not the first person to do this, but I wanted to share this: a generator which returns all permutations of a list: def permute( lst ): if len( lst ) == 1: yield lst else: head = lst[:1] for x in permute( lst[1:] ): yield head + x yield x + head return You're right that you're not the first person to do this: Many others have also posted incorrect permutation generators. Have you tried your code on some simple test cases? list(permute([1, 2, 3])) == [[1, 2, 3], [2, 3, 1], [1, 3, 2], [3, 2, 1]] Notably absent from this list are [2, 1, 3] and [2, 3, 1]. The problem gets worse with longer lists. The basic problem is that x needs to be able to occur in ALL positions, not just the beginning and the end. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
In article [EMAIL PROTECTED],
Andrea Griffini [EMAIL PROTECTED] wrote:
On Sat, 18 Jun 2005 13:35:16 -, Grant Edwards [EMAIL PROTECTED]
wrote:
AFAICT, the main use for do/while in C is when you want to
define a block of code with local variables as a macro:
When my job was squeezing most out of the CPU (videogame
industry) I remember that the asm code generated by
while (sz-- 0)
{
/* do some stuff */
}
was indeed worse than
do
{
/* do some stuff */
} while (--sz);
because of the initial empty-loop test (conditional jumps
were bad, and forward conditional jumps were worse).
So where at least one iteration was guaranteed the do-while
loop was a better choice.
Hmm. I don't know what compiler you were using, but in my experience
it's fairly typical to compile while(test ...) { body ... } as
j test
body: body
...
test: test
...
je body ;; or whatever your condition is
To turn this into a do { body ... } while(test ...), you need only
remove the initial j test instruction. Even if forward conditional
jumps are bad for your particular architecture, this method avoids the
problem neatly.
Personally, I'd be pretty suspicious of the quality of a compiler that
produced radically different code for these two constructs without some
damned good reason.
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: Unique Elements in a List
In article [EMAIL PROTECTED],
[EMAIL PROTECTED] [EMAIL PROTECTED] wrote:
Is there an easy way to grab the Unique elements from a list?
For Example:
data = [0.1,0.5,0.6,0.4,0.1,0.5,0.6,0.9]
what I am looking for is the unique elements 0.4 and 0.9 with their
index from the list.
Probably something like a Hash Table approach!!
I would like to get this done without unnecessary overhead.And the list
could be essentially anything strings,floats,int etc...
Or is it already avaliable as an attr to a list or an array?
I dont remember seeing anything like that.
From your comments downthread, it seems you want to find those elements
of the input sequence which occur exactly once, and return not only
these elements, but also their positions.
One reasonable solution might be as follows:
def unique_elts(seq):
elts = {}
for pos, elt in enumerate(seq):
elts.setdefault(elt, []).append(pos)
return [ (x, p[0]) for (x, p) in elts.iteritems()
if len(p) == 1 ]
This returns a list of tuples of the form (x, pos), where x is an
element of seq that occurs exactly once, and pos is its index in the
original sequence. This implementation traverses the input sequence
exactly once, and requires storage proportional to the length of the
input.
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
Re: variables exist
In article [EMAIL PROTECTED],
[EMAIL PROTECTED] (fabian) wrote:
how testing if a variable exists in python as isset in php??
Would the following be a satisfactory implementation?
def isset(varname, lloc = locals()):
return varname in lloc or varname in globals()
I believe this works as desired:
x = 5
def f(y):
... z = 10
... print isset('z') ## == True
... print isset('y') ## == True
... print isset('x') ## == True
...
Tests:
f(1) ## As shown above
print isset('z') ## == False
print isset('y') ## == False
print isset('x') ## == True
print isset('varname') ## == False
print isset('lloc')## == False
lloc = foo!
print isset('lloc')## == True
Perhaps this is not the most elegant solution, but I believe it handles
scoping correctly.
-M
--
Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
--
http://mail.python.org/mailman/listinfo/python-list
