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


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.


[image: Python Development on XP][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 7 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:\YourName\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.:

   C:\Users\YourName> python

You should then see something like:

   Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:55:48) [MSC v.1600 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 3.3 ‣ 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][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:\Python33".  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:\Python33" directory (the default at the time of
writing), you should make sure that entering the command

   c:\Python33\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 can add it to the system path to make it easier to
start Python by just running the "python" command. This is currently
an option in the installer as of CPython 3.3.

More information about environment variables can be found on the Using
Python on Windows page.


How do I make Python scripts executable?
========================================

On Windows, 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.


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.


How do I make an executable from a Python script?
=================================================

See http://cx-freeze.sourceforge.net/ for a distutils extension that
allows you to create console and GUI executables from Python code.
py2exe, the most popular extension for building Python 2.x-based
executables, does not yet support Python 3 but a version that does is
in development.


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
"PyInit_foo()".  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 "PyInit_foo()" 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
   “33” for Python 3.3.

   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 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.

Python raises "IndentationError" or "TabError" if mixed tabs and
spaces are causing problems in leading whitespace. You may also run
the "tabnanny" module 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.


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 https://www.winzip.com.)
