Re: [Tutor] dictionary keys
Alex Kleider wrote: > On 2014-04-08 14:34, Peter Otten wrote: > >> That's a change in Python 3 where dict.keys() no longer creates a list, >> but >> instead creates a view on the underlying dict data thus saving time and >> space. In the rare case where you actually need a list you can >> explicitly >> create one with >> >> ips = list(ipDic) > Thanks, Peter, for this clarification. I want to present the list > sorted so probably this is the rare case of which you spoke where I > would need to use l = list(myDict) rather than the view. You can create and sort the list in a single step: l = sorted(myDict) ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] question about strip() and list comprehension
> > > if line.strip() > > Is that stripping the line of white space at the same time that it is > testing it? > > Two features about Python: 1. Strings are immutable, so the above is computing what a whitespace-stripped line would look like. So that means that 'line.strip()' is doing just a computation: it's not mutating the original line, but computing a new string that has its leading and trailing whitespace stripped away. 2. Empty strings are treated as false values. I'm not happy with how loose Python treats truth, and would rather prefer: if line.strip() != "": ... so that the thing being tested is explicitly either True or False. I like my truth to be black and white, but I suppose I'll have to grimace and bear the fuzziness. :P Together, we see those two features allow us to look at the test in the Python code: if line.strip(): ... and rephrase it in English as: "If the line consists of at least one non-whitespace character: ..." ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] dictionary keys
On 2014-04-08 14:34, Peter Otten wrote: That's a change in Python 3 where dict.keys() no longer creates a list, but instead creates a view on the underlying dict data thus saving time and space. In the rare case where you actually need a list you can explicitly create one with ips = list(ipDic) That's because the above is a session using Python 2. Compare: $ python3 Python 3.3.2+ (default, Feb 28 2014, 00:52:16) [GCC 4.8.1] on linux Type "help", "copyright", "credits" or "license" for more information. dict(a=1, b=2).keys() dict_keys(['b', 'a']) $ python2 Python 2.7.5+ (default, Feb 27 2014, 19:37:08) [GCC 4.8.1] on linux2 Type "help", "copyright", "credits" or "license" for more information. dict(a=1, b=2).keys() ['a', 'b'] PS: You can get a view in Python 2, too, with dict.viewkeys() Thanks, Peter, for this clarification. I want to present the list sorted so probably this is the rare case of which you spoke where I would need to use l = list(myDict) rather than the view. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] question about strip() and list comprehension
On Tue, Apr 08, 2014 at 02:38:13PM -0600, Jared Nielsen wrote:
> Hello,
> Could someone explain why and how this list comprehension with strip()
> works?
>
> f = open('file.txt')
> t = [t for t in f.readlines() if t.strip()]
> f.close()
> print "".join(t)
>
> I had a very long file of strings filled with blank lines I wanted to
> remove. I did some Googling and found the above code snippet, but no clear
> explanation as to why it works. I'm particularly confused by how "if
> t.strip()" is removing the blank lines.
It isn't. Rather, what it is doing is *preserving* the non-blank lines.
The call to strip() removes any leading and trailing whitespace, so if
the line is blank of contains nothing but whitespace, it reduces down to
the empty string:
py> ''.strip()
''
Like other empty sequences and containers, the empty string is
considered to be "like False", falsey:
py> bool('')
False
So your list cmprehension (re-written to use a more meaningful name)
which looks like this:
[line for line in f.readlines() if line.strip()
iterates over each line in the file, tests if there is anything left
over after stripping the leading/trailing whitespace, and only
accumulates the lines that are non-blank. It is equivalent to this
for-loop:
accumulator = []
for line in f.readlines():
if line.strip(): # like "if bool(line.strip())"
accumulator.append(line)
> I also don't fully understand the 'print "".join(t)'.
I presume you understand what print does :-) so it's only the "".join(t)
that has you confused. This is where the interactive interpreter is
brilliant, you can try things out for yourself and see what they do. Do
you know how to start the interactive interpreter?
(If not, ask and we'll tell you.)
py> t = ['Is', 'this', 'the', 'right', 'place', 'for', 'an', 'argument?']
py> ''.join(t)
'Isthistherightplaceforanargument?'
py> ' '.join(t)
'Is this the right place for an argument?'
py> '--+--'.join(t)
'Is--+--this--+--the--+--right--+--place--+--for--+--an--+--argument?'
In your case, you have a series of lines, so each line will end with a
newline:
py> t = ['line 1\n', 'line 2\n', 'line 3\n']
py> ''.join(t)
'line 1\nline 2\nline 3\n'
py> print ''.join(t)
line 1
line 2
line 3
> The above didn't remove the leading white space on several lines, so I made
> the following addition:
>
> f = open('file.txt')
> t = [t for t in f.readlines() if t.strip()]
> f.close()
> s = [x.lstrip() for x in t]
> print "".join(s)
You can combine those two list comps into a single one:
f = open('file.txt')
lines = [line.lstrip() for line in f.readlines() if line.strip()]
f.close()
--
Steven
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Re: [Tutor] question about strip() and list comprehension
Jared Nielsen writes: > I had a very long file of strings filled with blank lines I wanted to > remove. I did some Googling and found the above code snippet The code you found is one of several syntactic shortcuts in Python, which allow creating a sequence directly from an expression in your code. For explanations, look at the documentation for “generator expression” and “display” (the latter have syntax for list comprehension, set comprehension, and dict comprehension). https://docs.python.org/3/reference/expressions.html#generator-expressions> https://docs.python.org/3/reference/expressions.html#displays-for-lists-sets-and-dictionaries> > I'm particularly confused by how "if t.strip()" is removing the blank > lines. The ‘str.strip’ method returns a new string, constructed from the original by removing all leading and trailing whitespace https://docs.python.org/3/library/stdtypes.html#str.strip>. A string, like any object, can be used in a boolean context; that's why “if some_expression” works for any expression https://docs.python.org/3/reference/expressions.html#boolean-operations>. For an expression that returns a string, the value in a boolean context will be false if the string is empty, and true for any other string. So, in the list comprehension you found: the “if t.strip()” is getting a new string, testing it in a boolean context which will be false when the string is empty, and that condition is what determines which values will end up in the sequence. > I also don't fully understand the 'print "".join(t)'. Each text string object has a ‘join’ method, which uses that string and the specified sequence to construct a new string, joining all the items together https://docs.python.org/3/library/stdtypes.html#str.join>. > The above didn't remove the leading white space on several lines Right. The new “stripped” string is not used except in the “if” clause, to determine which values will end up in the new list. The original values themselves are unchanged by the process, and end up in the new list as they began. > List comprehensions are still magic to me. How would I go about > incorporating lstrip() in the first list comprehension? In general, if something is a mystery to you, look up the formal description of the method or syntax in the documentation and carefully follow what it's telling you. Have a careful read of the documentation for those concepts, experiment based on your reading, and see what questions you have after that. -- \ “Smoking cures weight problems. Eventually.” —Steven Wright | `\ | _o__) | Ben Finney ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] question about strip() and list comprehension
Hi,
On 8 April 2014 22:38, Jared Nielsen wrote:
> Hello,
> Could someone explain why and how this list comprehension with strip()
> works?
>
> f = open('file.txt')
> t = [t for t in f.readlines() if t.strip()]
> f.close()
> print "".join(t)
>
> I had a very long file of strings filled with blank lines I wanted to
> remove. I did some Googling and found the above code snippet, but no clear
> explanation as to why it works. I'm particularly confused by how "if
> t.strip()" is removing the blank lines. I also don't fully understand the
> 'print "".join(t)'.
The list comprehension loops through each item in f.readlines(), and
outputs each item (adds it to the output list being constructed), if
and only if the "if" filter condition is true. Now, to understand "if
t.strip()" you need to understand that Python allows objects and items
which are not explicitly bool types in contexts where a boolean is
required, such as in if statements.
In the case of strings, a blank/empty string is considered False,
while a non-blank/empty string is considered True. As a consequence,
if t.strip() is equivalent to writing if t.strip() != '', and so its
presence effectively suppresses adding empty lines from the file into
the output list t. An empty string is said to be "falsy" while a
non-empty string is said to be "truthy". For more see Section 5.1
(Truth value testing), here:
https://docs.python.org/2/library/stdtypes.html
As for the question about the print statement, firstly read the
following documentation page that describes the string str.join()
method: https://docs.python.org/2/library/stdtypes.html#str.join
>From this, you should be able to infer that what "".join(t) does is to
effectively construct a new output string by concatenating all the
items in t, using a blank string as the delimiter, e.g. the result is
effectively to just concatenate all the strings directly with no
additional delimiter.
> The above didn't remove the leading white space on several lines, so I made
> the following addition:
>
> f = open('file.txt')
> t = [t for t in f.readlines() if t.strip()]
> f.close()
> s = [x.lstrip() for x in t]
> print "".join(s)
>
> List comprehensions are still magic to me. How would I go about
> incorporating lstrip() in the first list comprehension?
The output expression for each item output in the list comprehension,
that's the bit n front of the "for", is something you specify/control.
Now, in your original code, you just output the line that was read
from the file verbatim (e.g. "t"), but nothing forces this on you --
instead you can write any expression you like, including calling
"lstrip" on t, as in your question, e.g simply.
t = [t.lstrip() for t in f.readlines() if t.strip()]
HTH,
Walter
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Re: [Tutor] question about strip() and list comprehension
> Could someone explain why and how this list comprehension with strip()
> works?
>
> f = open('file.txt')
> t = [t for t in f.readlines() if t.strip()]
> f.close()
> print "".join(t)
Hi Jared,
Let me rewrite this without the list comprehension, while preserving behavior.
##
inputFile = open('file.txt')
lines = []
for line in inputFile.readlines():
if line.strip():
lines.append(line)
inputFile.close()
print "".join(lines)
##
I am changing the names of the variables from the original code
because I find it very difficult to distinguish 't' from 'f'
sometimes, and because those names are very tied in my mind to
something else entirely ("true" and "false").
Does the above code make more sense to you than the version using the
list comprehension syntax, or is there something there that is still
confusing?
Good luck to you.
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Re: [Tutor] dictionary keys
Alex Kleider wrote:
> I've got a fairly large script that uses a dictionary (called 'ipDic')
> each
> value of which is a dictionary which in turn also has values which are
> not
> simple types.
> Instead of producing a simple list,
> """
> ips = ipDic.keys()
> print(ips)
> """
> yields
> """
> dict_keys(['61.147.107.120', '76.191.204.54', '187.44.1.153'])
> """
>
> Searching my code for 'dict_keys' yields nothing. I've no idea where it
> comes from.
>
>
> Can anyone shed light on why instead of getting the
> I
> get "dict_keys( )"?
>
> (Using Python3, on Ubuntu 12.4)
That's a change in Python 3 where dict.keys() no longer creates a list, but
instead creates a view on the underlying dict data thus saving time and
space. In the rare case where you actually need a list you can explicitly
create one with
ips = list(ipDic)
> I've been unable to reproduce this behaviour using simpler dictionaries
> which seem to work as I expect:
d = dict(a=1, b=2, c=3)
d
> {'a': 1, 'c': 3, 'b': 2}
d.keys()
> ['a', 'c', 'b']
print(d.keys())
> ['a', 'c', 'b']
That's because the above is a session using Python 2. Compare:
$ python3
Python 3.3.2+ (default, Feb 28 2014, 00:52:16)
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> dict(a=1, b=2).keys()
dict_keys(['b', 'a'])
$ python2
Python 2.7.5+ (default, Feb 27 2014, 19:37:08)
[GCC 4.8.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> dict(a=1, b=2).keys()
['a', 'b']
PS: You can get a view in Python 2, too, with dict.viewkeys()
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[Tutor] dictionary keys
I've got a fairly large script that uses a dictionary (called 'ipDic')
each
value of which is a dictionary which in turn also has values which are
not
simple types.
Instead of producing a simple list,
"""
ips = ipDic.keys()
print(ips)
"""
yields
"""
dict_keys(['61.147.107.120', '76.191.204.54', '187.44.1.153'])
"""
Searching my code for 'dict_keys' yields nothing. I've no idea where it
comes from.
I've been unable to reproduce this behaviour using simpler dictionaries
which seem to work as I expect:
d = dict(a=1, b=2, c=3)
d
{'a': 1, 'c': 3, 'b': 2}
d.keys()
['a', 'c', 'b']
print(d.keys())
['a', 'c', 'b']
Can anyone shed light on why instead of getting the
I
get "dict_keys( )"?
(Using Python3, on Ubuntu 12.4)
Thanks,
Alex
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[Tutor] question about strip() and list comprehension
Hello,
Could someone explain why and how this list comprehension with strip()
works?
f = open('file.txt')
t = [t for t in f.readlines() if t.strip()]
f.close()
print "".join(t)
I had a very long file of strings filled with blank lines I wanted to
remove. I did some Googling and found the above code snippet, but no clear
explanation as to why it works. I'm particularly confused by how "if
t.strip()" is removing the blank lines. I also don't fully understand the
'print "".join(t)'.
The above didn't remove the leading white space on several lines, so I made
the following addition:
f = open('file.txt')
t = [t for t in f.readlines() if t.strip()]
f.close()
s = [x.lstrip() for x in t]
print "".join(s)
List comprehensions are still magic to me. How would I go about
incorporating lstrip() in the first list comprehension?
Many thanks,
J.
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Re: [Tutor] Block highlighters in python
On Tue, Apr 08, 2014 at 11:18:40AM +0530, Santosh Kumar wrote:
> Is there a way by which we can highlight a block in the python ?
>
> i have a huge code and i want to see a block of ``if`` code. How do i
> achieve this in VIM or any other editor.
>
> Note: In perl if i put my cursor on one "{" it will hightlight the other
> closed "}".
>
> Do we have any such facility in python ?
This has nothing to do with Python, it is completely to do with the
editing capabilities of your editor. If you are using Windows Notepad,
it is very weak and has nearly no functionality. If you are using Emacs,
it can do nearly anything, perhaps with a little bit of scripting.
I don't use Vim, so I cannot answer your question. But in my case, I
always use four spaces for indents so I can easily see changes in indent
level:
# not this
if condition:
do_this()
while condition:
do_something()
do_that()
# make it obvious
if condition:
do_this()
while condition:
do_something()
do_that()
--
Steven
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Re: [Tutor] Inheritance in classes
On Tue, Apr 08, 2014 at 11:14:36AM +0530, Santosh Kumar wrote:
> Can i mask the parent attibutes in the child. let me give a quick example.
>
> In [1]: class a:
>...: value1 = 1
>...: value2 = 2
>...:
>
> In [2]: class b(a):
>...: value3 = 3
>...:
All of value1, value2, value3 here are *class attributes*, bound to the
class, not the instance. In Java terms, that is similar to static
variables.
For the purpose of your example, that is not very important, but it can
make a difference.
> In [3]: obj1 = b()
>
> In [4]: obj1.value1
> Out[4]: 1
>
> In [5]: obj1.value2
> Out[5]: 2
>
> In [6]: obj1.value3
> Out[6]: 3
>
> If you notice in the below example you will see that the child class object
> ``obj1`` has inherited all the attibutes of the parent class.
That is how object oriented programming is supposed to work. If you
don't want to inherit the attributes of class "a", you should not
inherit from class "a".
> Is there a
> way by which i can make the child class not inherit some of the properites
> of parent class.
There is, but *you should not do this*. This is poor design, and
violates the Liskov Substitution Principle:
https://en.wikipedia.org/wiki/Liskov_substitution_principle
http://www.oodesign.com/liskov-s-substitution-principle.html
But what we can do is inherit from a, but over-ride access to one of the
attributes and fake an attribute error. But really, you should not do
this -- it is poor design.
py> class A:
... value1 = 23
... value2 = 42
...
py> class B(A):
... value3 = 17
... @property
... def value1(self):
... raise AttributeError("Fake!")
...
py> obj = B()
py> obj.value1
Traceback (most recent call last):
File "", line 1, in
File "", line 5, in value1
AttributeError: Fake!
py> obj.value2
42
py> obj.value3
17
--
Steven
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Re: [Tutor] Constructs
On Tue, Apr 08, 2014 at 11:10:57AM +0530, Santosh Kumar wrote: > 1 #!/usr/bin/python > 2 > 3 class shape: > 4 def __init__(self,x,y): > 5 self.x = x > 6 self.y = y > 7 description = "This shape has not been described yet" > 8 author = "Nobody has claimed to make this shape yet" Notice that the body of the __init__ method is indented (by two spaces -- four is recommended) from the method header. The body ends once the indentation returns to the previous level. So this piece of code has three levels of indentation: Level 0: "class shape" is not indented; Level 1: "def __init__" is 1 indent in; Level 2: the body of the method is 2 indents in; Level 1: the "description" and "author" lines are outdented from 2 back to 1. Because "description" and "author" are indented level with the __init__ definition (NOT the body of the method, the def header) that makes them *class attributes*. They are bound to the class itself, not the instance, and are shared by all instances. In Java terms they would be called "static variables". > 9 > 10 def __init__(self,x,y,z): > 11 self.x = x > 12 self.y = y > 13 self.z = z This now overwrites the existing __init__ method with a new method, also called __init__, that takes three arguments instead of two. Because methods are values exactly the same as strings, floats, bools and so forth, you can only have one method with the same name. If you wrote: x = 1 x = 2 of course you would expect that the second assignment to x overwrites the first assignment -- x cannot have two different values at the same time. The same applies to methods: you cannot have __init__ set to a method taking arguments x, y, z and a method taking arguments x, y at the same time. The newest assignment wins. Java methods are fixed at compile-time, so it is easy for the compiler to decide which constructor method to call at compile-time: if there are two arguments, call the first method, if there are three, call the second. But Python methods are values, and can be replaced on the fly at run-time. Python can do run-time polymorphism, but not in the same way that you do it with Java. Instead, you should define the method with default arguments: def __init__(self, x, y, z=None): self.x = x self.y = y if z is not None: self.z = z > 14 print "The values are %d,%d,%d" %(self.x,self.y,self.z) This line is outdented relative to the body of the __init__ method, so it too is at the class scope. That means that the print statement will be executed at class definition time. The problem is, at class definition time, there is no instance yet, even the class doesn't exist yet, so the reference to "self" will fail with NameError. I think what you want is for the print to be indented one more level, so it is inside the method: def __init__(self,x,y,z): self.x = x self.y = y self.z = z print "The values are %d,%d,%d" % (self.x, self.y, self.z) -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Inheritance in classes
2014-04-08 7:44 GMT+02:00 Santosh Kumar : > Can i mask the parent attibutes in the child. let me give a quick example. > > In [1]: class a: >...: value1 = 1 >...: value2 = 2 >...: > > In [2]: class b(a): >...: value3 = 3 >...: > > In [3]: obj1 = b() > > In [4]: obj1.value1 > Out[4]: 1 > > In [5]: obj1.value2 > Out[5]: 2 > > In [6]: obj1.value3 > Out[6]: 3 > > If you notice in the below example you will see that the child class object > ``obj1`` has inherited all the attibutes of the parent class. Is there a way > by which i can make the child class not inherit some of the properites of > parent class. > > > -- > D. Santosh Kumar > > > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor > Hi, This is the default behaviour. You can ovrcome that, but it will require extra work. For instance you could make use of pseudoprivate attributes (any attribute starting with double underscore, but not ending with dtwo underscores); or some managing attributes tool: *) __getattr__, __getattribute__ are generic ways to manage attribute fetching *) properties and descriptors allow a more specific way to control attributes (one by one) But, be careful. These tools can be very tricky at first. Hope it helps. Best ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Block highlighters in python
On 08/04/14 06:48, Santosh Kumar wrote:
Is there a way by which we can highlight a block in the python ?
i have a huge code and i want to see a block of ``if`` code. How do i
achieve this in VIM or any other editor.
Note: In perl if i put my cursor on one "{" it will hightlight the other
closed "}".
Do we have any such facility in python ?
This is not a feature of Python but of the IDE or editor.
I don't know of any editor that does that although several
have folding that can collapse a block (one example is Scite)
so it should be possible.
Some of the Python specific IDEs or the Python plugins for
Netbeans or Eclipse etc might support it.
In vim, if you put blank lines between your blocks the {}
keys will move yo to beginning/end of block (ie paragraph).
That's how I tend to do it... But that doesn't match where
you have nested blocks, inside a function say and you want
the outer block...
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.flickr.com/photos/alangauldphotos
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Re: [Tutor] Inheritance in classes
On 08/04/14 06:44, Santosh Kumar wrote: Can i mask the parent attibutes in the child. let me give a quick example. In [1]: class a: ...: value1 = 1 ...: value2 = 2 ...: In [2]: class b(a): ...: value3 = 3 ...: Note that these are class variables and not instance variables. In [3]: obj1 = b() In [4]: obj1.value1 Out[4]: 1 In [6]: obj1.value3 Out[6]: 3 If you notice in the below example you will see that the child class object ``obj1`` has inherited all the attibutes of the parent class. Yes that's what inheritance means. there a way by which i can make the child class not inherit some of the properites of parent class. No. But you can change the inherited values by masking them with your local versions, which could be None. class c(a): value1 = None obj2 = c() print(obj2.value1) -> None HTH -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Constructs
On 08/04/14 06:40, Santosh Kumar wrote: 1 #!/usr/bin/python 2 3 class shape: 4 def __init__(self,x,y): 5 self.x = x 6 self.y = y 7 description = "This shape has not been described yet" 8 author = "Nobody has claimed to make this shape yet" 9 10 def __init__(self,x,y,z): 11 self.x = x 12 self.y = y 13 self.z = z 14 print "The values are %d,%d,%d" %(self.x,self.y,self.z) 15 16 triangle = shape(100,20,30) 17 rectange = shape(20,30) I am getting NameError exceptions when i am trying to achieve these. python third.py Traceback (most recent call last): File "third.py", line 3, in class shape: File "third.py", line 14, in shape print "The values are %d,%d,%d" %(self.x,self.y,self.z) NameError: name 'self' is not defined can we have two constructs within the same class. My requiment is very simple, when i want to pass two values the shape class should take two arguments and when i pass on three it should take 3 arguments . Is this possible ? No, its not possible in Python, there is no overloading of methods. The normal way to accept variable numbers of parameters is to use default values def __init__(self, a, b, c=None): if c is None: # do one thing else: # do another But in your case you should probably be defining subclasses since triangle and rectangle are probably going to need different method implementations for most things. The name error however is nothing to do with that. The problem there is that you have put the print statement outside of the method so it gets executed during class definition. At that stage there is no instance and therefore no self value. I suspect you wanted to have it indented to the same level as the self assignment lines? -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
