Python on Windows FAQ
a...@nyman.amk.ca
the more specific questions deal with one Windows bug or another; it's
not clear how many of these bugs are still relevant for users of
Windows XP or 2000.
Please *don't* send me comments about the questions, because I can't
tell which comments are correct and which aren't, so I'm not planning
to make any changes to the file. A Windows-using maintainer is
desperately needed. If you want to be the maintainer, just grab a
copy of the reStructured Text source from
http://www.python.org/doc/faq/windows.ht, edit away, and send your
updated draft to the python.org webmaster address.
--amk
Title: Python Windows FAQ
Content-type: text/x-rst
====================================
Python Windows FAQ
====================================
:Date: $Date: 2003/08/14 20:41:20 $
:Version: $Revision: 1.2 $
:Web site: http://www.python.org/
.. contents::
.. sectnum::
How do I run a Python program under Windows?
----------------------------------------------------
This is not necessarily a straightforward question. If you are already
familiar with running programs from the Windows command line then
everything will seem obvious; otherwise, you might need a little more
guidance. There are also differences between Windows 95, 98, NT, ME,
2000 and XP which can add to the confusion.
Unless you use some sort of integrated development environment,
you will end up *typing* Windows commands into what is variously referred
to as a "DOS window" or "Command prompt window". Usually you can
create such a window from your Start menu; under Windows 2000
the menu selection is
"Start | Programs | Accessories | Command Prompt". You should be
able to recognize when you have started such a window because you will
see a Windows "command prompt", which usually looks like this::
C:\>
The letter may be different, and there might be other things after it,
so you might just as easily see something like::
D:\Steve\Projects\Python>
depending on how your computer has been set up and what else you have
recently done with it. Once you have started such a window, you are
well on the way to running Python programs.
You need to realize that your Python scripts have to be processed by
another program called the Python interpreter. The
interpreter reads your script, compiles it into bytecodes, and then executes the bytecodes to run your
program. So, how do you arrange for the interpreter to handle your
Python?
First, you need to make sure that your command window recognises the
word "python" as an instruction to start the interpreter. If you have
opened a command window, you should try entering the command ``python``
and hitting return. You should then see something like::
Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
You have started the interpreter in "interactive mode". That means you
can enter Python statements or expressions interactively and have them
executed or evaluated while you wait. This is one of Python's
strongest features. Check it by entering a few expressions of your
choice and seeing the results::
>>> print "Hello"
Hello
>>> "Hello" * 3
HelloHelloHello
Many people use the interactive mode as a convenient yet highly
programmable calculator.
When you want to end your interactive Python session, hold the Ctrl
key down while you enter a Z, then hit the "Enter" key to get back to
your Windows command prompt.
You may also find that you have a
Start-menu entry such as "Start | Programs | Python 2.2 | Python
(command line)" that results in you seeing the ">>>" prompt in a new
window. If so, the window will disappear after you enter the
Ctrl-Z character; Windows is running a single "python" command in the window,
and closes it when you terminate the interpreter.
If the ``python`` command, instead of displaying the interpreter
prompt ">>>", gives you a message like::
'python' is not recognized as an internal or external command,
operable program or batch file.
or::
Bad command or filename
then you need to make sure that your computer knows where to find the
Python interpreter. To do this you will have to modify a setting
called PATH, which is a list of directories where Windows
will look for programs. You should arrange for Python's
installation directory to be added to the PATH of every command window
as it starts. If you installed Python fairly recently then the command ::
dir C:\py*
will probably tell you where it is installed; the usual location is
something like C:\Python23. Otherwise you will be reduced to a search of your
whole disk ... use "Tools | Find"
or hit the "Search" button and look for "python.exe". Supposing you
discover that Python is installed in the C:\Python23 directory (the
default at the time of writing), you should make sure that
entering the command ::
c:\Python23\python
starts up the interpreter as above (and don't forget you'll need a
"CTRL-Z" and an "Enter" to get out of it). Once you have verified the
directory, you need to add it to the start-up routines your computer
goes through. For older versions of Windows the easiest way to do
this is to edit the C:\AUTOEXEC.BAT file. You would want to add a line
like the following to AUTOEXEC.BAT::
PATH C:\Python23;%PATH%
For Windows NT, 2000 and (I assume) XP, you will need to add a string
such as ::
;C:\Python23
to the current setting for the PATH environment variable, which you
will find in the properties window of "My Computer" under the
"Advanced" tab. Note that if you have sufficient privilege you might
get a choice of installing the settings either for the Current User or
for System. The latter is preferred if you want everybody to be able
to run Python on the machine.
If you aren't confident doing any of these manipulations yourself, ask
for help! At this stage you may want to reboot your system
to make absolutely sure the new setting has taken effect. You probably
won't need to reboot for Windows NT, XP or 2000. You can also avoid it in
earlier versions by editing the file C:\WINDOWS\COMMAND\CMDINIT.BAT
instead of AUTOEXEC.BAT.
You should now be able to start a new command window, enter ``python``
at the "C:>" (or whatever) prompt, and see the ">>>" prompt that
indicates the Python interpreter is reading interactive commands.
Let's suppose you have a program called "pytest.py" in directory
"C:\Steve\Projects\Python". A session to run that program might look
like this::
C:\> cd \Steve\Projects\Python
C:\Steve\Projects\Python> python pytest.py
Because you added a file name to the command to start the interpreter,
when it starts up it reads the Python script in the named file,
compiles it, executes it, and terminates, so you see another "C:\>"
prompt. You might also have entered ::
C:\> python \Steve\Projects\Python\pytest.py
if you hadn't wanted to change your current directory.
Under NT, 2000 and XP you may well find that the installation process
has also arranged that the command ``pytest.py``
(or, if the file isn't in the current directory,
``C:\Steve\Projects\Python\pytest.py``)
will automatically recognize the ".py" extension and run the Python
interpreter on the named file. Using this feature is fine, but *some*
versions of Windows have bugs which mean that this form isn't exactly
equivalent to using the interpreter explicitly, so be careful.
The important things to remember are:
1. Start Python from the Start Menu, or make sure the PATH is set
correctly so Windows can find the Python interpreter. ::
python
should give you a '>>>" prompt from the Python interpreter. Don't
forget the CTRL-Z and ENTER to terminate the interpreter (and, if you
started the window from the Start Menu, make the window disappear).
2. Once this works, you run programs with commands::
python {program-file}
3. When you know the commands to use you can build Windows shortcuts
to run the Python interpreter on any of your scripts, naming
particular working directories, and adding them to your menus.
Take a look at ::
python --help
if your needs are complex.
4. Interactive mode (where you see the ">>>" prompt) is best used
for checking that individual statements and expressions do
what you think they will, and for developing code by experiment.
How do I make python scripts executable?
----------------------------------------------
On Windows 2000, the standard Python installer already associates the
.py extension with a file type (Python.File) and gives that file type
an open command that runs the interpreter (D:\Program
Files\Python\python.exe "%1" %*). This is enough to make scripts
executable from the command prompt as 'foo.py'. If you'd rather be
able to execute the script by simple typing 'foo' with no extension
you need to add .py to the PATHEXT environment variable.
On Windows NT, the steps taken by the installer as described above
allow you to run a script with 'foo.py', but a longtime bug in the NT
command processor prevents you from redirecting the input or output of
any script executed in this way. This is often important.
The incantation for making a Python script executable under WinNT
is to give the file an extension of .cmd and add the following as the first
line::
@setlocal enableextensions & python -x %~f0 %* & goto :EOF
Where is Freeze for Windows?
------------------------------------
"Freeze" is a program that allows you to ship a Python program
as a single stand-alone executable file. It is *not* a compiler;
your programs don't run any faster, but they are more easily
distributable, at least to platforms with the same OS and CPU. Read the
README file of the freeze program for more disclaimers.
You can use freeze on Windows, but you must download the source
tree (see http://www.python.org/download/download_source.html).
The freeze program is in the ``Tools\freeze`` subdirectory of the source
tree.
You need the Microsoft VC++ compiler, and you probably need to build
Python. The required project files are in the PCbuild directory.
Is a ``*.pyd`` file the same as a DLL?
------------------------------------------
Yes, .pyd files are dll's, but there are a few differences. If you
have a DLL named foo.pyd, then it must have a function initfoo(). You
can then write Python "import foo", and Python will search for foo.pyd
(as well as foo.py, foo.pyc) and if it finds it, will attempt to call
initfoo() to initialize it. You do not link your .exe with foo.lib,
as that would cause Windows to require the DLL to be present.
Note that the search path for foo.pyd is PYTHONPATH, not the same as
the path that Windows uses to search for foo.dll. Also, foo.pyd need
not be present to run your program, whereas if you linked your program
with a dll, the dll is required. Of course, foo.pyd is required if
you want to say "import foo". In a DLL, linkage is declared in the
source code with __declspec(dllexport). In a .pyd, linkage is defined
in a list of available functions.
How can I embed Python into a Windows application?
----------------------------------------------------------
Embedding the Python interpreter in a Windows app can be summarized as
follows:
1. Do _not_ build Python into your .exe file directly. On Windows,
Python must be a DLL to handle importing modules that are themselves
DLL's. (This is the first key undocumented fact.) Instead, link to
pythonNN.dll; it is typically installed in c:\Windows\System.
NN is the Python version, a number such as "23" for Python 2.3.
You can link to Python statically or dynamically. Linking
statically means linking against pythonNN.lib, while dynamically
linking means linking against pythonNN.dll. The drawback to
dynamic linking is that your app won't run if pythonNN.dll does not
exist on your system.
(General note: pythonNN.lib is the so-called "import lib" corresponding
to python.dll. It merely defines symbols for the linker.)
Linking dynamically greatly simplifies link options; everything happens
at run time. Your code must load pythonNN.dll using the Windows
LoadLibraryEx() routine. The code must also use access routines and
data in pythonNN.dll (that is, Python's C API's) using pointers
obtained by the Windows GetProcAddress() routine. Macros can make
using these pointers transparent to any C code that calls routines in
Python's C API.
Borland note: convert pythonNN.lib to OMF format using Coff2Omf.exe
first.
2. If you use SWIG, it is easy to create a Python "extension module"
that will make the app's data and methods available to Python. SWIG
will handle just about all the grungy details for you. The result is C
code that you link *into* your .exe file (!) You do _not_ have to
create a DLL file, and this also simplifies linking.
3. SWIG will create an init function (a C function) whose name depends
on the name of the extension module. For example, if the name of the
module is leo, the init function will be called initleo(). If you use
SWIG shadow classes, as you should, the init function will be called
initleoc(). This initializes a mostly hidden helper class used by the
shadow class.
The reason you can link the C code in step 2 into your .exe file is that
calling the initialization function is equivalent to importing the
module into Python! (This is the second key undocumented fact.)
4. In short, you can use the following code to initialize the Python
interpreter with your extension module. ::
#include "python.h"
...
Py_Initialize(); // Initialize Python.
initmyAppc(); // Initialize (import) the helper class.
PyRun_SimpleString("import myApp") ; // Import the shadow class.
5. There are two problems with Python's C API which will become apparent
if you use a compiler other than MSVC, the compiler used to build
pythonNN.dll.
Problem 1: The so-called "Very High Level" functions that take FILE *
arguments will not work in a multi-compiler environment because each compiler's
notion of a struct FILE will be different. From an implementation
standpoint these are very _low_ level functions.
Problem 2: SWIG generates the following code when generating wrappers to
void functions::
Py_INCREF(Py_None);
_resultobj = Py_None;
return _resultobj;
Alas, Py_None is a macro that expands to a reference to a complex data
structure called _Py_NoneStruct inside pythonNN.dll. Again, this code
will fail in a mult-compiler environment. Replace such code by::
return Py_BuildValue("");
It may be possible to use SWIG's %typemap command to make the change
automatically, though I have not been able to get this to work (I'm a
complete SWIG newbie).
6. Using a Python shell script to put up a Python interpreter window
from inside your Windows app is not a good idea; the resulting window
will be independent of your app's windowing system. Rather, you (or the
wxPythonWindow class) should create a "native" interpreter window. It
is easy to connect that window to the Python interpreter. You can
redirect Python's i/o to _any_ object that supports read and write, so
all you need is a Python object (defined in your extension module) that
contains read() and write() methods.
How do I use Python for CGI?
-------------------------------------------------------
On the Microsoft IIS server or on the Win95 MS Personal Web Server you
set up Python in the same way that you would set up any other
scripting engine.
Run regedt32 and go to::
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W3SVC\Parameters\ScriptMap
and enter the following line (making any specific changes that your system may need)::
.py :REG_SZ: c:\<path to python>\python.exe -u %s %s
This line will allow you to call your script with a simple reference like:
http://yourserver/scripts/yourscript.py
provided "scripts" is an "executable" directory for your server (which
it usually is by default).
The "-u" flag specifies unbuffered and binary mode for stdin - needed when working with binary data.
In addition, it is recommended that using
".py" may not be a good idea for the file extensions when used in this
context (you might want to reserve ``*.py`` for support modules and
use ``*.cgi`` or ``*.cgp`` for "main program" scripts).
In order to set up Internet Information Services 5 to use Python for CGI processing, please see the following links:
http://www.e-coli.net/pyiis_server.html (for Win2k Server)
http://www.e-coli.net/pyiis.html (for Win2k pro)
Configuring Apache is much simpler.
In the Apache configuration file ``httpd.conf``, add the following line at
the end of the file::
ScriptInterpreterSource Registry
Then, give your Python CGI-scripts the extension .py and put them in the cgi-bin directory.
How do I keep editors from inserting tabs into my Python source?
------------------------------------------------------------------
The FAQ does not recommend using tabs, and `the Python style guide <http://www.python.org/peps/pep-0008.html>`_
recommends 4 spaces for distributed Python code; this is also the
Emacs python-mode default.
Under any editor, mixing tabs and spaces is a bad idea. MSVC is no
different in this respect, and is easily configured to use spaces:
Take Tools -> Options -> Tabs, and for file type "Default" set "Tab
size" and "Indent size" to 4, and select the "Insert spaces" radio
button.
If you suspect mixed tabs and spaces are causing problems in leading
whitespace, run Python with the -t switch or run
``Tools/Scripts/tabnanny.py`` to check a directory tree in batch mode.
How do I check for a keypress without blocking?
----------------------------------------------------
Use the msvcrt module. This is a standard Windows-specific extension
module. It defines a function kbhit() which checks whether a keyboard
hit is present, and getch() which gets one character without echoing
it.
How do I emulate os.kill() in Windows?
---------------------------------------------
Use win32api::
def kill(pid):
"""kill function for Win32"""
import win32api
handle = win32api.OpenProcess(1, 0, pid)
return (0 != win32api.TerminateProcess(handle, 0))
Why does os.path.isdir() fail on NT shared directories?
--------------------------------------------------------------
The solution appears to be always append the "\\" on
the end of shared drives. ::
>>> import os
>>> os.path.isdir( '\\\\rorschach\\public')
0
>>> os.path.isdir( '\\\\rorschach\\public\\')
1
It helps to think of share points as being like drive letters.
Example::
k: is not a directory
k:\ is a directory
k:\media is a directory
k:\media\ is not a directory
The same rules apply if you substitute "k:" with "\\conky\foo"::
\\conky\foo is not a directory
\\conky\foo\ is a directory
\\conky\foo\media is a directory
\\conky\foo\media\ is not a directory
cgi.py (or other CGI programming) doesn't work sometimes on NT or win95!
-----------------------------------------------------------------------------
Be sure you have the latest python.exe, that you are using
python.exe rather than a GUI version of Python and that you
have configured the server to execute ::
"...\python.exe -u ..."
for the CGI execution. The -u (unbuffered) option on NT and
Win95 prevents the interpreter from altering newlines in the
standard input and output. Without it post/multipart requests
will seem to have the wrong length and binary (e.g. GIF)
responses may get garbled (resulting in broken images, PDF files,
and other binary downloads failing).
Why doesn't os.popen() work in PythonWin on NT?
-------------------------------------------------------
The reason that os.popen() doesn't work from within PythonWin is due
to a bug in Microsoft's C Runtime Library (CRT). The CRT assumes you
have a Win32 console attached to the process.
You should use the win32pipe module's popen() instead which doesn't
depend on having an attached Win32 console.
Example::
import win32pipe
f = win32pipe.popen('dir /c c:\\')
print f.readlines()
f.close()
Why doesn't os.popen()/win32pipe.popen() work on Win9x?
---------------------------------------------------------------
There is a bug in Win9x that prevents os.popen/win32pipe.popen* from
working. The good news is there is a way to work around this problem.
The Microsoft Knowledge Base article that you need to lookup is:
Q150956. You will find links to the knowledge base at:
http://www.microsoft.com/kb.
PyRun_SimpleFile() crashes on Windows but not on Unix; why?
------------------------------------------------------------
This is very sensitive to the compiler vendor, version and (perhaps)
even options. If the FILE* structure in your embedding program isn't
the same as is assumed by the Python interpreter it won't work.
The Python 1.5.* DLLs (python15.dll) are all compiled
with MS VC++ 5.0 and with multithreading-DLL options (``/MD``).
If you can't change compilers or flags, try using Py_RunSimpleString().
A trick to get it to run an arbitrary file is to construct a call to
execfile() with the name of your file as argument.
Also note that you can not mix-and-match Debug and Release versions. If you wish to use the Debug Multithreaded DLL, then your module _must_ have an "_d" appended to the base name.
Importing _tkinter fails on Windows 95/98: why?
------------------------------------------------
Sometimes, the import of _tkinter fails on Windows 95 or 98,
complaining with a message like the following::
ImportError: DLL load failed: One of the library files needed
to run this application cannot be found.
It could be that you haven't installed Tcl/Tk, but if you did
install Tcl/Tk, and the Wish application works correctly,
the problem may be that its installer didn't
manage to edit the autoexec.bat file correctly. It tries to add a
statement that changes the PATH environment variable to include
the Tcl/Tk 'bin' subdirectory, but sometimes this edit doesn't
quite work. Opening it with notepad usually reveals what the
problem is.
(One additional hint, noted by David Szafranski: you can't use
long filenames here; e.g. use C:\PROGRA~1\Tcl\bin instead of
C:\Program Files\Tcl\bin.)
How do I extract the downloaded documentation on Windows?
------------------------------------------------------------
Sometimes, when you download the documentation package to a Windows
machine using a web browser, the file extension of the saved file
ends up being .EXE. This is a mistake; the extension should be .TGZ.
Simply rename the downloaded file to have the .TGZ extension, and
WinZip will be able to handle it. (If your copy of WinZip doesn't,
get a newer one from http://www.winzip.com.)
Missing cw3215mt.dll (or missing cw3215.dll)
----------------------------------------------------
Sometimes, when using Tkinter on Windows, you get an error that
cw3215mt.dll or cw3215.dll is missing.
Cause: you have an old Tcl/Tk DLL built with cygwin in your path
(probably C:\Windows). You must use the Tcl/Tk DLLs from the
standard Tcl/Tk installation (Python 1.5.2 comes with one).
Warning about CTL3D32 version from installer
----------------------------------------------------
The Python installer issues a warning like this::
This version uses CTL3D32.DLL whitch is not the correct version.
This version is used for windows NT applications only.
[Tim Peters]
This is a Microsoft DLL, and a notorious
source of problems. The message means what it says: you have the wrong version
of this DLL for your operating system. The Python installation did not
cause this -- something else you installed previous to this overwrote the
DLL that came with your OS (probably older shareware of some sort, but
there's no way to tell now). If you search for "CTL3D32" using any search
engine (AltaVista, for example), you'll find hundreds and hundreds of web
pages complaining about the same problem with all sorts of installation
programs. They'll point you to ways to get the correct version reinstalled
on your system (since Python doesn't cause this, we can't fix it).
David A Burton has written a little program to fix this. Go to
http://www.burtonsys.com/download.html and click on "ctl3dfix.zip"