Re: [Tutor] Some questions about my yen-USD.py
Dnia czwartek, 7 września 2006 14:37, Andrei napisał: get me into trouble with the flakiness of float(n)? In testing I didn't find any problems, but .. Nah. Float accuracy is only a problem if you need around lots of significant digits (16 or so). I wouldn't bet. Such a simple thing as 0.1 can't be represented correctly on Float... That's what 'decimal' is for. See that: 0.1 + 0.1 + 0.1 - 0.3 5.5511151231257827e-17 from decimal import Decimal Decimal(0.1)+ Decimal(0.1)+ Decimal(0.1)-Decimal(0.3) Decimal(0.0) For more see: http://docs.python.org/lib/module-decimal.html -- Pawel Kraszewski http://www.kraszewscy.net ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] [tutor] how to cast to stucture
Dnia wtorek, 5 września 2006 08:32, [EMAIL PROTECTED] napisał: i have a complex data in binary file and i want to read its fields... the C way is to read file in buffer and then cast it to proper structure. Is there a way to do the same in Python or i have to read the data byte by byte ? cite: - struct -- Interpret strings as packed binary data This module performs conversions between Python values and C structs represented as Python strings. It uses format strings (explained below) as compact descriptions of the lay-out of the C structs and the intended conversion to/from Python values. This can be used in handling binary data stored in files or from network connections, among other sources. - see http://docs.python.org/lib/module-struct.html -- Pawel Kraszewski http://www.kraszewscy.net ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] beginner: using optional agument in __init__ breaks my code
Dnia niedziela, 25 czerwca 2006 22:45, Bob Gailer napisał: To get the behavior I think you want try: def __init__(self, q = None): if not q: q = [] self.queue = q I would disagree... This sure works for empty arguments, but: u = [1,2,3] a = Queue(u) b = Queue(u) a.insert(12) print b gives [1, 2, 3, 12] which is still wrong. However assigning self.queue a COPY of parameter (empty or not) gives the desired result: def __init__(self, q = []): self.queue = q[:] -- Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Test If File System is mounted in Linux
Dnia piątek, 17 marca 2006 19:22, Bill Campbell napisał: If you're sure it's a Linux system, fine. Like /etc/mtab, this isn't portable. Looking at some of the systems we have here: Fortezza mentioned it the way I assumed he has Linux: If there a semi-standard way to test if a file system has been mounted or not using Python? In the Linux command line, I can type mount and see all mounted file system, and then see if the one I am looking for is But, generally - you are right. Each system has its own rules for accessing mount table. Anyway - this would be a great exercise to make a portable pure-python library to get this information. -- Pawel www.kraszewscy.net ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Test If File System is mounted in Linux
Dnia piątek, 17 marca 2006 18:41, Adam napisał: The more general problem is to get a list of mounted file systems. How about just reading the mtab? That's usually /etc/mtab it should be readable as a user and it means you don't have to rely on any other programs. Or always-up-to-date user readable '/proc/mounts' ? -- Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Cannot Understand
Dnia piątek, 10 marca 2006 19:31, Edgar Antonio Rodriguez Velazco napisał: f = lambda n: n-1 + abs(n-1) and f(n-1)*n or 1 Oh God! This smells lispish! Haven't seen a jevel like this before, but I LOVE it! - First, let's cut that into pieces: R1 n-1 + abs(n-1) This is a funny way to test if n1. Indeed, for n1 it works as 2*n-2 (especially, it is non-zero), for n=1 it gives a constant 0 R2 f(n-1)*n It is a formula for you-know-what R3 1 Well... This is a ONE, you know. - Then, few notes on 'and' and 'or' operators. For correctness of many algorithms, programmers introduced 'partial evaluation'. What does it mean? L1 A and B When A is FALSE, no matter what the B is, the whole result is FALSE. Therefore, we DON'T NEED to evaluate B. When A is TRUE, the result is yet unknown and we must evaluate B L2 A or B When A is TRUE, no matter what the B is, the whole result is TRUE. Therefore, we DON'T NEED to evaluate B. When A is FALSE, the result is yet unknown and we must evaluate B L3 'and' has higher priority than 'or' - Putting things together: f = lambda n: n-1 + abs(n-1) and f(n-1)*n or 1 which may be rewritten due to law L3 as (R1 and R2) or R3 First check is R1 If R1==false (it means n=1), then whole brace is automaticaly false due to law L1. Moreover, due to the same law, R2 is NOT calculated. We now have the value of brace, which is FALSE. We now have FALSE or R3, so due to law L2 we now must evaluate R3 to get the final (lambda) result. The total result is value of R3, this means 1. If R1==true (it means n1), due to law L1 we must evaluate R2 to get the brace result. This is done by recursively calling lambda function with argument n-1. Let's call the returned value a RES. When RES is non-zero (it actualy always is, due function it implements) we have non-zero result of the whole brace. due to law L2, we don't need to evaluate R3 and calculated result RES is the return value of lambda function. -- Pawel Kraszewski P.S. This might be pythonized and de-recursived this way: ff = lambda n: reduce(lambda x, y: x*y, xrange(1,n+1)) - Spoiler below... Scroll down Of course, as a last resort this may be rewritten in plan english as: def f(n): if n1: return n*f(n-1) else: return 1 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Repeating a routine
Dnia środa, 22 lutego 2006 12:43, John Connors napisał: I know goto and gosub are evil, bad habits but I'm starting to miss them. You SHOULD miss them. GOSUB is called a function call in Python (you call a function by its name, rather than by its starting line number) Someone misinformed you: GOSUB is perfectly good. It's GOTO that's evil. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Exception repeated in a loop
Dnia wtorek, 6 grudnia 2005 16:29, Jan Eden napisał: Hi, I use the following loop to parse some HTML code: for record in data: try: parser.feed(record['content']) except HTMLParseError, (msg): print !!!Parsing error in, record['page_id'], : , msg Now after HTMLParser encounters a parse error in one record, it repeats to execute the except statement for all following records - why is that? Short answer: because you told Python to do so... Long answer: My hint for students having such problems is to execute their code with a pencil on a hardcopy. They read aloud what the program currently does - usually they spot the error during the first reading. Your code being read loud 1. begin loop 2. attempt to execute parser.feed 3. abort attempt if it fails, showing the error 4. take next loop So - you take next loop regardless of the failure or not. There are two ways out of here. I wrote them aloud, to transcribe into python as an excersize: (Notice the difference between this and your original) I) 1. attempt to 2. begin loop 3. abort attempt if it fails, showing the error 4. take next loop II) 1. begin loop 2. attempt to execute parser.feed 3. abort attempt if it fails, showing the error AND breaking the loop 4. take next loop Hope this helps, -- Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Python DB
Dnia czwartek, 22 września 2005 10:43, Matt Williams napisał: I've been looking for a simple python based DB for a fairly simple app. Only a single user, single access needed, but with a dynamic structure (which needs to be extensible very easily). For pure Python implementation you might try SnakeSQL (http://www.pythonweb.org/projects/snakesql/): ---[CITE]--- SnakeSQL is a pure Python SQL database written to remove the dependence of the Python Web Modules on 3rd party drivers for non-Python databases like MySQL but designed to be a useful database in its own right. The database supports the simplest possible subset of ANSI SQL 92 including NULLs (something other pure Python databases such as Gadfly do not currently support) as well as more advanced features such as foreign key constraints and simple joins. The database is fully DB-API 2.0 compliant and is written in layers so that it can easily be extended to support different storage mechanisms. Currently implemented are a fast binary DBM driver (default) and a slower CSV file driver (handy for viewing table contents when designing and developing an application or database structure). ---[/CITE]--- If you don't need pure-Python,take a look at gadfly (http://gadfly.sourceforge.net) If the data isn't too big, you may even try XML storage with DOM access (PyXML http://pyxml.sourceforge.net/) HTH, -- Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterating over sorted dictionary keys in one line
Dnia niedziela, 18 września 2005 19:20, Marcin Komorowski napisał: I know that one of the ways to iterate over sorted dictionary keys is: keylist = dictionary.keys() keylist.sort() for key in keylist: ... Indeed, 'sort' sorts its argument in-place and returns 'none'. Bur Python 2.4+ has a new thing: 'sorted'. This leaves its argument intact and returns a sorted copy. for mykey in sorted(dictionary.keys()): # things to do See this: Python 2.4.1 (#1, Jun 19 2005, 01:02:56) [GCC 3.4.4 (Gentoo 3.4.4, ssp-3.4.4-1.0, pie-8.7.8)] on linux2 Type help, copyright, credits or license for more information. a = [4,3,2,1] print sorted(a) # sorted FUNCTION returns sorted list [1, 2, 3, 4] a # and original list remains unchanged [4, 3, 2, 1] print a.sort() # sort METHOD returns nothing None a # instead it changes (sorts) its argument [1, 2, 3, 4] Best regards -- Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] fourier transform
Hello, I have a signal that I want to do a fourier transform on. I tried using FFT.fft(data).real but I don't understand the output. what is output from this function, and why doesn't it require amplitude and time as inputs? Please write the package you use for FFT. Standard Python doesn't have one. Perhaps you use some custom Python? Like Enthought Python? If so, the exhaustive guide is delivered with it in CHM (windows help) format. 1. In general FFT algorithms require you to supply just N complex values (complex contains both amplitude and phase of signal). If you supply real values, the system assumes phase=0 for each sample and takes given value for amplitude. 2. You DO supply a time information, you just didn't think of it this way: each consecutive sample is taken one fixed clock tick later, so position of sample in data gives you the time it was taken. 3. Results of FFT are in the form of complex vector of the same length as data. Starting from position 0 it gives you an constant (DC factor), position 1 an amplitude and phase (remember - complex number gives you both amplitude and phase) of wave of the length table/2 and so on. If you take real value of this, you discard part of the information. AFAIR - taking real part gives you sine component, taking imaginary part gives you cosine component, taking absolute value gives you total amplitude and taking angle gives you phase. 4. The answer is symmetrical - usually you take only half of it. I don't remember the exact difference between the halves, but you may find it in any article on FFT. Best regards, Pawel Kraszewski ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Polish translation of my tutor
Dnia pitek, 17 grudnia 2004 20:59, Alan Gauld napisa: |Are there any Polish speakers on the tutor list who would like |to check a new version of my tutorial? There are no formal links |to it yet as there are only a few pages but it can be found at: That would be me. First of all content=text/html; charset=ISO-6682 Ehm... Never heard of ISO-6682... Already contacted the author. The page looks OK, when you force Windows CP 1250. -- Pawel KraszewskiFreeBSD/Linux E-Mail/Jabber Phone ICQ GG [EMAIL PROTECTED]+48 604 777447 45615564 69381 ___ Tutor maillist - [EMAIL PROTECTED] http://mail.python.org/mailman/listinfo/tutor
