mirror of https://github.com/python/cpython
intermediate version -- why not...
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Guido van Rossum
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Misc/FAQ
170
Misc/FAQ
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@ -6,7 +6,7 @@ Reply-to: guido@cwi.nl (Guido van Rossum)
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Approved: news-answers-request@MIT.Edu
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Archive-name: python-faq/part1
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Version: 1.18
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Version: 1.19--
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Last-modified: 2 January 1995
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This article contains answers to Frequently Asked Questions about
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@ -24,7 +24,7 @@ Author's address:
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P.O. Box 94079
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1090 GB Amsterdam
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The Netherlands
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Email: guido@cwi.nl
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Email: <guido@cwi.nl>
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The latest version of this FAQ is available by anonymous ftp from
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<URL:ftp://ftp.cwi.nl/pub/python/python-FAQ>. It will also be posted
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@ -42,9 +42,6 @@ the archive by e-mail if you have no ftp access. You send a e-mail
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message to <mail-server@rtfm.mit.edu> containing the single word help
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in the message body to receive instructions.
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Skip Montanaro <skip@automatrix.com> maintains an HTML version of this
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FAQ, <URL:http://www.automatrix.com/~skip/python-faq.html>.
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This FAQ is divided in the following chapters:
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1. General information and availability
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@ -153,6 +150,8 @@ Here's an overview of the questions per chapter:
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4.28. Q. How can I create a stand-alone binary from a Python script?
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4.29. Q. Is there a special lib for writing CGI scripts in Python?
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4.30. Q. What other WWW tools are there for Python?
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4.31. Q. How do I run a subprocess with pipes connected to both input
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and output?
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5. Extending Python
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5.1. Q. Can I create my own functions in C?
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@ -161,7 +160,7 @@ Here's an overview of the questions per chapter:
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5.4. Q. How can I evaluate an arbitrary Python expression from C?
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5.5. Q. How do I extract C values from a Python object?
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5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length?
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5.7. Q. What happened to mktuple(), featuring in an example in the
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5.7. Q. What happened to mktuple(), featured in an example in the
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Extensions manual?
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5.8. Q. How do I call an object's method from C?
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5.9. Q. How do I catch the output from print_error()?
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@ -235,7 +234,7 @@ down).
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1.2. Q. Why is it called Python?
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A. Apart from being a computer wizard, I'm also a fan of "Monty
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A. Apart from being a computer scientist, I'm also a fan of "Monty
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Python's Flying Circus" (a BBC comedy series from the seventies, in
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the -- unlikely -- case you didn't know). It occurred to me one day
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that I needed a name that was short, unique, and slightly mysterious.
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@ -297,8 +296,7 @@ A. There is a newsgroup, comp.lang.python <URL:news:comp.lang.python>,
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and a mailing list. The newsgroup and mailing list are gatewayed into
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each other -- if you can read news it's unnecessary to subscribe to
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the mailing list. Send e-mail to <python-list-request@cwi.nl> to
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(un)subscribe to the mailing list
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<URL:mailto:python-list-request@cwi.nl>. Hypermail archives of
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(un)subscribe to the mailing list. Hypermail archives of
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(nearly) everything posted to the mailing list (and thus the
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newsgroup) are available on our WWW server,
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<URL:http://www.cwi.nl/~guido/hypermail/index.html>. The raw archives
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@ -419,7 +417,7 @@ included in the list above, send me email!
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2.3. Q. Are there any commercial projects going on using Python?
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A. Several companies have revealed to me that they are planning or
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considering to use Python in a future product.
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considering use of Python in a future product.
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The furthest is Sunrise Software, who already have a product out using
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Python -- they use Python for a GUI management application and an SNMP
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@ -540,10 +538,10 @@ A. Get STDWIN 0.9.9 <URL:ftp://ftp.cwi.nl/pub/stdwin/stdwin0.9.9.tar.gz>.
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3.4. Q. Link errors building Python with STDWIN 0.9.9.
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A. Probably routines like 'tereate', 'tenew' etc. The STDWIN 0.9.9
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distribution requires that you add TWO libraries from stdwin to the
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line for stdwin in the Setupfile. Use something like this (all on one
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line!):
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A. The linker probably complains that it can't find routines like
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'tecreate', 'tenew' etc. The STDWIN 0.9.9 distribution requires that
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you add TWO libraries from stdwin to the line for stdwin in the
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Setupfile. Use something like this (all on one line!):
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stdwin stdwinmodule.c -I$(STDWIN)/H $(STDWIN)/Packs/textedit/libtextedit.a $(STDWIN)/Ports/x11/libstdwin.a -lX11
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@ -601,7 +599,7 @@ A. Consider using readline 2.0. From the Python 1.1 README:
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- You can use the GNU readline library to improve the interactive user
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interface: this gives you line editing and command history when
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calling python interactively. You need to configure build the GNU
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calling python interactively. You need to configure and build the GNU
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readline library before running the configure script. Its sources are
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no longer distributed with Python; you can ftp them from any GNU
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mirror site, or from its home site
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@ -664,31 +662,25 @@ HAVE_PROTOTYPES.
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3.13. Q. Trouble with posix.listdir on NeXTSTEP 3.2.
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A. The problem seems to be that that the NeXT posix library and the
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NeXT dynamic loading library are incompatible. Mike Carlton reports
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that the following worked for him (from a clean 1.1 distribution):
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A. If you built 1.2, you probably forgot to pass -posix as mentioned
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in the README file -- this links with the right version of the
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libraries. If you built an earlier version, that's also your problem
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(edit Makefile to add it to OPT), but you also need to remove this
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NeXT-specific section from import.c:
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1) ./configure
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2) edited config.status and changed
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OPT='-O'
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to
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OPT='-posix -O'
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3) edited Python/import.c and commented out the section
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#if defined(NeXT) || defined(WITH_RLD)
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#define DYNAMIC_LINK
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#define USE_RLD
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#endif
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this disables dynamic loading
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4) make
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3.14. Q. Other trouble building Python on platform X.
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A. Please email me the details <URL:mailto:guido@cwi.nl> and I'll look
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into it. Please provide as many details as possible. In particular,
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if you don't tell me what type of computer and what operating system
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(and version) you are using it will be difficult for me to figure out
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what is the matter. If you get a specific error message, please email
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it to me too.
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A. Please email the details to <guido@cwi.nl> and I'll look into it.
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Please provide as many details as possible. In particular, if you
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don't tell me what type of computer and what operating system (and
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version) you are using it will be difficult for me to figure out what
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is the matter. If you get a specific error message, please email it
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to me too.
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4. Programming in Python
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@ -850,10 +842,10 @@ again (into the same Python process), the changes don't seem to take
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place. What is going on?
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A. For reasons of efficiency as well as consistency, Python only reads
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the module file on the first time a module is imported (otherwise a
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the module file on the first time a module is imported. (Otherwise a
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program consisting of many modules, each of which imports the same
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basic module, would read the basic module over and over again). To
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force a changed module being read again, do this:
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basic module, would read the basic module over and over again.) To
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force rereading of a changed module, do this:
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import modname
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reload(modname)
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@ -896,7 +888,7 @@ machines).
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NOTE: if the complaint is about "Tkinter" (upper case T) and you have
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already configured module "tkinter" (lower case t), the solution is
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*not* to rename tkinter to Tkinter or vice versa. There is probably
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something wring with your module search path. Check out the value of
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something wrong with your module search path. Check out the value of
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sys.path.
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For X-related modules (Xt and Xm) you will have to do more work: they
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@ -931,7 +923,7 @@ is also very likely that in the future the standard Python GUI API
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will be based on or at least look very much like the Tkinter
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interface. For more info about Tk, including pointers to the source,
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see John Ousterhout's home page
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<URL:http://playground.Sun.COM:80/~ouster/>.
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<URL:http://playground.sun.com/~ouster/>.
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- The standard Python distribution comes with an interface to STDWIN,
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a platform-independent low-level windowing interface. You have to ftp
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@ -977,13 +969,16 @@ Athena widget sets. Last I heard about it it was included in the WAFE
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A. There's an interface to SYBASE by John Redford
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<jredford@lehman.com>.
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There's also an interface to metalbase by Lance Ellinghaus
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There's an interface to metalbase by Lance Ellinghaus
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<lance@markv.com>; it is part of the separate Extensions distribution
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<URL:ftp://ftp.cwi.nl/pub/python/extensions.tar.gz>.
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Anthony Baxter <anthony.baxter@aaii.oz.au> has written an interface to
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mSQL (mini-SQL). <URL:ftp://ftp.cwi.nl/pub/python/PymSQL.tar.gz>.
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Tom Culliton <culliton@clark.net> has written an Oracle module. Write
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him to get a copy of a late BETA version.
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4.15. Q. Is it possible to write obfuscated one-liners in Python?
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A. Yes. See the following three examples, due to Ulf Bartelt:
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@ -1074,31 +1069,34 @@ setenv(), and some have none at all).
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However if all you want is to pass environment variables to the
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commands run by os.system() or os.popen(), there's a simple solution:
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prefix the command string with a couple of variable assignments and
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export statements. I guess the following would be universal for popen
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(untested):
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export statements. The following would be universal for popen:
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import os
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from commands import mkarg # nifty routine to add shell quoting
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def epopen(cmd, mode, env = {}):
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# env is a dictionary of environment variables
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prefix = ''
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for key, value in env.values():
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prefix = prefix + '%s=%s\n' % (key, mkarg(value))
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for key, value in env.items():
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prefix = prefix + '%s=%s\n' % (key, mkarg(value)[1:])
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prefix = prefix + 'export %s\n' % key
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return os.popen(prefix + cmd, mode)
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4.19. Q. What is a class?
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A. A class is the particular object type that is created by executing
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a class statement.
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a class statement. Class objects are used as templates, to create
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class instance objects, which embody both the data structure and
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program routines specific to a datatype.
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4.20. Q. What is a method?
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A. A method is a function that you normally call as
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x.name(arguments...) for some object x. The word is used for methods
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x.name(arguments...) for some object x. The term is used for methods
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of classes and class instances as well as for methods of built-in
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objects. The latter have a completely different implementation and
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only share the way their calls look in Python code.
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objects. (The latter have a completely different implementation and
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only share the way their calls look in Python code.) Methods of
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classes (and class instances) are defined as functions inside the
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class definition.
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4.21. Q. What is self?
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@ -1196,13 +1194,6 @@ modules which are actually used in the program. It then compiles the
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generated C code and links it with the rest of the Python interpreter
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to form a self-contained binary which acts exactly like your script.
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Unfortunately, the current version is very platform-specific, because
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each platform has its own compilation flags and libraries to link
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with. You will probably have to edit the freeze.py file to point it
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to the right directories and tell it about the compilation and link
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flags for your platform. A new version will be released with Python
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1.1 -- if you want a peek write to my colleague <Jack.Jansen@cwi.nl>.
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4.29. Q. Is there a special lib for writing CGI scripts in Python?
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A. There's documentation and code for a cgi.py module by Michael McLay
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@ -1229,6 +1220,73 @@ called Dancer. An alpha version can be FTP'ed from
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few articles about Dancer in the (hyper)mail archive
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<URL:http://www.cwi.nl/~guido/hypermail/python-1994q3/index.html>.)
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4.31. Q. How do I run a subprocess with pipes connected to both input
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and output?
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A. This is really a UNIX question. Also, in general, it is unwise to
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do so, because you can easily cause a deadlock where the parent
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process is blocked waiting for output from the child, while the child
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is blocked waiting for input from the child. This can be caused
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because the parent expects the child to output more text than it does,
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or it can be caused by data being stuck in stdio buffers due to lack
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of flushing. The Python parent can of course explicitly flush the data
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it sends to the child before it reads any output, but if the child is
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a naive C program it can easily have been written to never explicitly
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flush its output, even if it is interactive, since flushing is
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normally automatic.
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In many cases, all you really need is to run some data through a
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command and get the result back. Unless the data is infinite in size,
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the easiest (and often the most efficient!) way to do this is to write
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it to a temporary file and run the command with that temporary file as
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input. The standard module tempfile exports a function mktemp() which
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generates unique temporary file names.
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If after reading all of the above you still want to connect two pipes
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to a subprocess's standard input and output, here's a simple solution,
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due to Jack Jansen:
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import os
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import sys
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import string
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MAXFD = 100 # Max number of file descriptors in this system
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def popen2(cmd):
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cmd = string.split(cmd)
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p2cread, p2cwrite = os.pipe()
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c2pread, c2pwrite = os.pipe()
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pid = os.fork()
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if pid == 0:
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# Child
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os.close(0)
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os.close(1)
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if os.dup(p2cread) <> 0:
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sys.stderr.write('popen2: bad read dup\n')
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if os.dup(c2pwrite) <> 1:
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sys.stderr.write('popen2: bad write dup\n')
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for i in range(3, MAXFD):
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try:
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os.close(i)
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except:
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pass
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try:
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os.execv(cmd[0], cmd)
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finally:
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os._exit(1)
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os.close(p2cread)
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tochild = os.fdopen(p2cwrite, 'w')
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os.close(c2pwrite)
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fromchild = os.fdopen(c2pread, 'r')
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return fromchild, tochild
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Note that many interactive programs (e.g. vi) don't work well with
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pipes substituted for standard input and output. You will have to use
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pseudo ttys ("ptys") instead of pipes. There is some undocumented
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code to use these in the library module pty.py -- I'm afraid you're on
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your own here. What's *really* needed is a Python interface to Don
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Libes' expect library -- any takers?
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5. Extending Python
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===================
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@ -1283,7 +1341,7 @@ some value before you pass the tuple to Python code --
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newtupleobject(n) initializes them to NULL, which isn't a valid Python
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value.
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5.7. Q. What happened to mktuple(), featuring in an example in the
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5.7. Q. What happened to mktuple(), featured in an example in the
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Extensions manual?
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A. It's a typo, I meant newtupleobject() (see previous question).
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@ -1537,7 +1595,7 @@ implementation requires thread support for C.
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6.11. Q. Why can't lambda forms contain statements?
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A. Python lambda forms cannot contain statements because Python's
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syntactic framework can't handle statements nested inside functions.
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syntactic framework can't handle statements nested inside expressions.
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However, in Python, this is not a serious problem. Unlike lambda
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forms in other languages, where they add functionality, Python lambdas
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@ -1690,7 +1748,7 @@ A. You still need to copy the files from the distribution directory
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"python/Lib" to your system. If you don't have the full distribution,
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you can get the file pythonlib<version>.tar.gz from most ftp sites
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carrying Python; this is a subset of the distribution containing just
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those file, e.g.
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those files, e.g.
|
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<URL:ftp://ftp.cwi.nl/pub/python/pythonlib1.1.tar.gz>.
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Once you have installed the library, you need to point sys.path to it.
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Reference in New Issue