🌐 Mailman 3 September 2010 - Python-list - python.org

Re: "Strong typing vs. strong testing"
by namekuseijin Oct. 17, 2010

Oct. 17, 2010

On 27 set, 05:46, TheFlyingDutchman <zzbba...(a)aol.com> wrote: > On Sep 27, 12:58 am, p...(a)informatimago.com (Pascal J. Bourguignon) > wrote: > > RG <rNOSPA...(a)flownet.com> writes: > > > In article > > > <7df0eb06-9be1-4c9c-8057-e9fdb7f0b...(a)q16g2000prf.googlegroups.com>, > > > TheFlyingDutchman <zzbba...(a)aol.com> wrote: > > > >> On Sep 22, 10:26 pm, "Scott L. Burson" <Sc...(a)ergy.com> wrote: > > >> > This might have been mentioned here before, but I just came across it: a > > >> > 2003 essay by Bruce Eckel on how reliable systems can get built in > > >> > dynamically-typed languages. It echoes things we've all said here, but > > >> > I think it's interesting because it describes a conversion experience: > > >> > Eckel started out in the strong-typing camp and was won over. > > > >> > https://docs.google.com/View?id=dcsvntt2_25wpjvbbhk > > > >> > -- Scott > > > >> If you are writing a function to determine the maximum of two numbers > > >> passed as arguents in a dynamic typed language, what is the normal > > >> procedure used by Eckel and others to handle someone passing in > > >> invalid values - such as a file handle for one varible and an array > > >> for the other? > > > > The normal procedure is to hit such a person over the head with a stick > > > and shout "FOO". > > > Moreover, the functions returning the maximum may be able to work on > > non-numbers, as long as they're comparable. What's more, there are > > numbers that are NOT comparable by the operator you're thinking about!. > > > So to implement your specifications, that function would have to be > > implemented for example as: > > > (defmethod lessp ((x real) (y real)) (< x y)) > > (defmethod lessp ((x complex) (y complex)) > > (or (< (real-part x) (real-part y)) > > (and (= (real-part x) (real-part y)) > > (< (imag-part x) (imag-part y))))) > > > (defun maximum (a b) > > (if (lessp a b) b a)) > > > And then the client of that function could very well add methods: > > > (defmethod lessp ((x symbol) (y t)) (lessp (string x) y)) > > (defmethod lessp ((x t) (y symbol)) (lessp x (string y))) > > (defmethod lessp ((x string) (y string)) (string< x y)) > > > and call: > > > (maximum 'hello "WORLD") --> "WORLD" > > > and who are you to forbid it!? > > > -- > > __Pascal Bourguignon__ http://www.informatimago.com/-Hide quoted text - > > > - Show quoted text - > > in C I can have a function maximum(int a, int b) that will always > work. Never blow up, and never give an invalid answer. If someone > tries to call it incorrectly it is a compile error. > In a dynamic typed language maximum(a, b) can be called with incorrect > datatypes. Even if I make it so it can handle many types as you did > above, it could still be inadvertantly called with a file handle for a > parameter or some other type not provided for. So does Eckel and > others, when they are writing their dynamically typed code advocate > just letting the function blow up or give a bogus answer, or do they > check for valid types passed? If they are checking for valid types it > would seem that any benefits gained by not specifying type are lost by > checking for type. And if they don't check for type it would seem that > their code's error handling is poor. that is a lie. Compilation only makes sure that values provided at compilation-time are of the right datatype. What happens though is that in the real world, pretty much all computation depends on user provided values at runtime. See where are we heading? this works at compilation time without warnings: int m=numbermax( 2, 6 ); this too: int a, b, m; scanf( "%d", &a ); scanf( "%d", &b ); m=numbermax( a, b ); no compiler issues, but will not work just as much as in python if user provides "foo" and "bar" for a and b... fail. What you do if you're feeling insecure and paranoid? Just what dynamically typed languages do: add runtime checks. Unit tests are great to assert those. Fact is: almost all user data from the external words comes into programs as strings. No typesystem or compiler handles this fact all that graceful...

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if the else short form
by Tracubik Oct. 13, 2010

Oct. 13, 2010

Hi all, I'm studying PyGTK tutorial and i've found this strange form: button = gtk.Button(("False,", "True,")[fill==True]) the label of button is True if fill==True, is False otherwise. i have googled for this form but i haven't found nothing, so can any of you pass me any reference/link to this particular if/then/else form? thanks Nico

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Design: Module interface vs. library interface
by Stefan Schwarzer Oct. 10, 2010

Oct. 10, 2010

Hello, I maintain ftputil [1], an FTP client library. Users of the library may use it like this: | import ftputil | | with ftputil.FTPHost(server, userid, password) as ftp_host: | # for example, make a remote file and read its content | with ftp_host.open("remote_name", "rb") as remote_fobj: | data = remote_fobj.read() Even though "ftp files" are defined in module `ftp_file` [2] via class `FTPFile`, users of the ftputil library should never use `FTPFile` directly; it's up to `FTPHost.open` to generate these file objects. Now, from the "point of view" of the _module_ `ftp_file`, I'd use | # ftp_file.py | | # used outside this module | __all__ = ['FTPFile'] | | class FTPFile(object): | ... | # ftputil.py | | import ftp_file | | class FTPHost(object): | ... | def open(name, mode): | ... | return ftp_file.FTPFile(...) On the other hand, from the point of view of the _library_ ftputil, `FTPFile` isn't part of the official API, so I may as well write | # ftp_file.py | | # be explicit, no user-public interface | __all__ = [] | | class FTPFile(object): | ... Which approach would you prefer and why? Or some other approach? Would you use a different approach if the library-internal class was named `_FTPFile` instead of `FTPFile`? [1] http://ftputil.sschwarzer.net [2] http://ftputil.sschwarzer.net/trac/browser/ftp_file.py Stefan

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Please help: pylint does not work with Emacs23 on Windows
by Dsrt Egle Oct. 5, 2010

Oct. 5, 2010

Hi, I am trying to use Pylint with Emacs on Windows XP. My Emacs version is EmacsW32 23.1, pylint is 0.21.3 with Python 2.5. After easy_install pylint, I added the code block below to Emacs init file, copied form Emacs Wiki. (when (load "flymake" t) (defun flymake-pylint-init () (let* ((temp-file (flymake-init-create-temp-buffer-copy 'flymake-create-temp-inplace)) (local-file (file-relative-name temp-file (file-name-directory buffer-file-name)))) (list "epylint" (list local-file)))) (add-to-list 'flymake-allowed-file-name-masks '("\\.py\\'" flymake-pylint-init))) When I invoke the flymake-mode on a .py file, I can see the flymake starting the syntax check, the mode status changed to flymake*, and then goes back to flymake after several seconds. But no error is reported, and no syntax error highlighting. I tried to use pylint in command line, and it works with command "pylint test.py", reporting syntax errors on the same file. I even tried to clear out my .emacs file, but it doesn't help. Could someone help me on this? Thanks a lot.

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Re: how to get partition information of a hard disk with python
by Nobody Oct. 5, 2010

Oct. 5, 2010

On Wed, 22 Sep 2010 00:31:04 +0200, Hellmut Weber wrote: > I'm looking for a possibility to access the partiton inforamtion of a > hard disk of my computer from within a python program. Have you considered parsing /proc/partitions?

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namespace hacking question
by kj Oct. 5, 2010

Oct. 5, 2010

This is a recurrent situation: I want to initialize a whole bunch of local variables in a uniform way, but after initialization, I need to do different things with the various variables. What I end up doing is using a dict: d = dict() for v in ('spam', 'ham', 'eggs'): d[v] = init(v) foo(d['spam']) bar(d['ham']) baz(d['eggs']) This is fine, but I'd like to get rid of the tedium of typing all those extra d['...']s. I.e., what I would *like* to do is something closer to this: d = locals() for v in ('spam', 'ham', 'eggs'): d[v] = init(v) foo(spam) bar(ham) baz(eggs) ...but this results in errors like "NameError: global name 'spam' is not defined". But the problem is deeper than the fact that the error above would suggest, because even this fails: spam = ham = eggs = None d = locals() for v in ('spam', 'ham', 'eggs'): d[v] = init(v) foo(spam) # calls foo(None) bar(ham) # calls bar(None) baz(eggs) # calls baz(None) In other words, setting the value of locals()['x'] does not set the value of the local variable x. I also tried a hack using eval: for v in ('spam', 'ham', 'eggs'): eval "%s = init('%s')" % (v, v) but the "=" sign in the eval string resulted in a "SyntaxError: invalid syntax". Is there any way to use a loop to set a whole bunch of local variables (and later refer to these variables by their individual names)? TIA! kj

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Re: sequence multiplied by -1
by John Nagle Oct. 4, 2010

Oct. 4, 2010

On 9/25/2010 4:45 AM, Thomas Jollans wrote: > On Saturday 25 September 2010, it occurred to Yingjie Lan to exclaim: >> Hi, >> >> I noticed that in python3k, multiplying a sequence by a negative integer is >> the same as multiplying it by 0, and the result is an empty sequence. It >> seems to me that there is a more meaningful symantics. The concept that the multiply operator should be overloaded to do something l33t on sequences was a Python design mistake. It leads to semantics like this: x = 10 y = "10" x*2 20 y*2 '1010' int(y*2) 1010 int(y)*2 20 That's awful. It's the kind of thing a novice C++ programmer would write shortly after they discovered operator overloading. Now, ask yourself, when you get a numeric value back from a database, is it an integer or a string? How about the CSV module? Are you sure? John Nagle

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Re: sequence multiplied by -1
by Stefan Schwarzer Oct. 3, 2010

Oct. 3, 2010

Hi, On 2010-09-25 14:11, Yingjie Lan wrote: > Having more than one way of doing things sometimes is good. In my opinion this _isn't_ a situation where it's good. :) L[::-1] is only marginally longer than -1 * L I think this small gain doesn't justify "violating" this "Python Zen" rule (from `import this`): There should be one-- and preferably only one --obvious way to do it. One could argue that using L[::-1] isn't "obvious" but I believe the rule refers to being obvious to people who have used Python for a while. Besides that, multiplying a list with a negative value to give a reverse list isn't so intuitive either. :-) On 2010-09-25 13:45, Thomas Jollans wrote: > Multiplying a list by a negative integer should produce a list of negative > length, which does not exist. IMHO, the only correct behaviour would be to > raise an exception, though one could argue that there are practical benefits > for the operation to succeed for any integer operand. I agree with raising an exception, probably a ValueError. Stefan

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Re: Determine sockets in use by python
by Jean-Paul Calderone Oct. 1, 2010

Oct. 1, 2010

On Sep 29, 4:08 pm, Jim Mellander <jmellan...(a)lbl.gov> wrote: > > On Wed, Sep 29, 2010 at 11:05 AM, Gary Herron <gher...(a)digipen.edu> wrote: > > On 09/29/2010 09:50 AM, Jim Mellander wrote: > > >> Hi: > > >> I'm a newbie to python, although not to programming. Briefly, I am > >> using a binding to an external library used for communication in a > >> client-server context, with the server in python. Typically, I would > >> set this up with event callbacks, and then enter a select loop, which, > >> most the time idles and processes input events when the socket shows > >> activity, kinda like: > > >> while True: > >> socket.select((my_socket),(),()) > >> process_event() > > >> Unfortunately, the API does not expose the socket to the script level, > >> and the developer recommends a busy loop: > > >> while True: > >> sleep(1) > >> process_event() > > >> which I hope to avoid, for many reasons. If the socket can be exposed > >> to the script level, then the problem would be solved. > > >> Failing that, it would be nice to be able to pythonically determine > >> the sockets in use and select on those. Does anyone have any > >> suggestions on how to proceed? > > >> Thanks in advance > > > It's certain that any answer to this will depend on which operating system > > you are using. So do tell: What OS? > > Hi Gary: > > Certainly not windows.... I'm developing on OS/X but for production > probably Linux and FreeBSD > > (I'm hoping for something a bit more portable than running 'lsof' and > parsing the output, but appreciate any/all advice) > Linux has /proc/self/fd and OS X has /dev/fd. Those both suppose you have some way of determining which file descriptor corresponds to the socket or sockets that the library is using, of course. Vastly better would be to convince the author to expose that information via a real API. Jean-Paul

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Re: Python becoming orphaned over ssh
by Jean-Paul Calderone Oct. 1, 2010

Oct. 1, 2010

On Sep 30, 9:08 am, David <wizza...(a)gmail.com> wrote: > On Wed, Sep 29, 2010 at 6:49 PM, John Nagle <na...(a)animats.com> wrote: > > Python's signal handling for multithread and multiprocess programs > > leaves something to be desired. > > Thanks for the confirmation (that I'm not missing something obvious). > > I've reported a bug for this behavior in the Python issue tracker. > > In the meanwhile, I've made a workaround function called > "check_call_and_monitor_ppids", that behaves like > subprocess.check_call, except that it regularly checks if the parent > pid "chain" (up to init process) changes during execution, and then > terminates the subprocess and raises an exception. > > Actually I tried this before, and it didn't work. But strangely, it > seems to work fine so long as I don't try to print any warning > messages to stderr or stdout from the Python script (though, the > called tool itself may print to stdout or stderr without problems). > Quite peculiar... > > Anyway, I hope that one of the Python developers will fix this sometime. > > David. Python ignores SIGPIPE by default. The default SIGPIPE behavior is to exit. This is sort of what people on POSIX expect. If you're talking to another process over a pipe and that process goes away, and then you write to the pipe, you get a SIGPIPE and you exit (of course, if it takes you 20 minutes before you do another write, then it's 20 minutes before you exit). But with SIGPIPE ignored, a Python process won't do exactly this. Instead, you'll get an exception from the write. If you don't handle the exception, then it'll propagate to the top-level and you'll exit. Just like with a "normal" process. Except you also get the option to doing something other than exiting. Pretty nice. But signal dispositions are inherited by child processes. So you run ping from your short Python program, and it inherits SIGPIPE being ignored. And it's written in C, not Python, so when it writes to the pipe, there's no exception. So ping never gets any indication that it should exit. No Python writes ever happen in this scenario. The SSH supplied stdout is shared with the ping process, which writes to it directly. You can fix this by resetting the signal disposition of SIGPIPE for the ping process: #!/usr/bin/python import signal def reset(): signal.signal(signal.SIGPIPE, signal.SIG_DFL) from subprocess import check_call check_call(['ping', 'www.google.com'], preexec_fn=reset) Very likely the subprocess module should be resetting the disposition of signals that Python itself has fiddled with (and resetting any other unusual state that the child is going to inherit, but nothing else comes immediately to mind).

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