
Python on Windows FAQ
*********************


Contents
^^^^^^^^

* Python on Windows FAQ

  * How do I run a Python program under Windows?

  * How do I make Python scripts executable?

  * Why does Python sometimes take so long to start?

  * Where is Freeze for Windows?

  * Is a ``*.pyd`` file the same as a DLL?

  * How can I embed Python into a Windows application?

  * How do I use Python for CGI?

  * How do I keep editors from inserting tabs into my Python source?

  * How do I check for a keypress without blocking?

  * How do I emulate os.kill() in Windows?

  * Why does os.path.isdir() fail on NT shared directories?

  * cgi.py (or other CGI programming) doesn't work sometimes on NT or
    win95!

  * Why doesn't os.popen() work in PythonWin on NT?

  * Why doesn't os.popen()/win32pipe.popen() work on Win9x?

  * PyRun_SimpleFile() crashes on Windows but not on Unix; why?

  * Importing _tkinter fails on Windows 95/98: why?

  * How do I extract the downloaded documentation on Windows?

  * Missing cw3215mt.dll (or missing cw3215.dll)

  * Warning about CTL3D32 version from installer


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.


[image]
^^^^^^^

Python Development on XP

This series of screencasts aims to get you up and running with Python
on Windows XP.  The knowledge is distilled into 1.5 hours and will get
you up and running with the right Python distribution, coding in your
choice of IDE, and debugging and writing solid code with unit-tests.

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.


[image]
^^^^^^^

Adding Python to DOS Path

Python is not added to the DOS path by default.  This screencast will
walk you through the steps to add the correct entry to the *System
Path*, allowing Python to be executed from the command-line by all
users.

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


Why does Python sometimes take so long to start?
================================================

Usually Python starts very quickly on Windows, but occasionally there
are bug reports that Python suddenly begins to take a long time to
start up.  This is made even more puzzling because Python will work
fine on other Windows systems which appear to be configured
identically.

The problem may be caused by a misconfiguration of virus checking
software on the problem machine.  Some virus scanners have been known
to introduce startup overhead of two orders of magnitude when the
scanner is configured to monitor all reads from the filesystem.  Try
checking the configuration of virus scanning software on your systems
to ensure that they are indeed configured identically. McAfee, when
configured to scan all file system read activity, is a particular
offender.


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/source).  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 ``python*NN*.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 in two different ways.  Load-time linking
   means linking against ``python*NN*.lib``, while run-time linking
   means linking against ``python*NN*.dll``.  (General note:
   ``python*NN*.lib`` is the so-called "import lib" corresponding to
   ``python*NN*.dll``.  It merely defines symbols for the linker.)

   Run-time linking greatly simplifies link options; everything
   happens at run time.  Your code must load ``python*NN*.dll`` using
   the Windows ``LoadLibraryEx()`` routine.  The code must also use
   access routines and data in ``python*NN*.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 ``python*NN*.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,
**PEP 8**, 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?
======================================

Prior to Python 2.7 and 3.2, to terminate a process, you can use
``ctypes``:

   import ctypes

   def kill(pid):
       """kill function for Win32"""
       kernel32 = ctypes.windll.kernel32
       handle = kernel32.OpenProcess(1, 0, pid)
       return (0 != kernel32.TerminateProcess(handle, 0))

In 2.7 and 3.2, ``os.kill()`` is implemented similar to the above
function, with the additional feature of being able to send CTRL+C and
CTRL+BREAK to console subprocesses which are designed to handle those
signals. See ``os.kill()`` for further details.


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 "\conkyfoo":

   \\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://support.microsoft.com/.


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 which 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/downloads.html and click on "ctl3dfix.zip".
