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.

Unless you use some sort of integrated development environment, you
will end up *typing* Windows commands into what is referred to as a
“Command prompt window”.  Usually you can create such a window from
your search bar by searching for "cmd".  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 “py” as an instruction to start the interpreter.  If you have
opened a command window, you should try entering the command "py" and
hitting return:

   C:\Users\YourName> py

You should then see something like:

   Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 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, call the "exit()" function or 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.x ‣ Python (command line) that results in you
seeing the ">>>" prompt in a new window.  If so, the window will
disappear after you call the "exit()" function or enter the "Ctrl-Z"
character; Windows is running a single “python” command in the window,
and closes it when you terminate the interpreter.

Now that we know the "py" command is recognized, you can give your
Python script to it. You’ll have to give either an absolute or a
relative path to the Python script. Let’s say your Python script is
located in your desktop and is named "hello.py", and your command
prompt is nicely opened in your home directory so you’re seeing
something similar to:

   C:\Users\YourName>

So now you’ll ask the "py" command to give your script to Python by
typing "py" followed by your script path:

   C:\Users\YourName> py Desktop\hello.py
   hello


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 How can I create a stand-alone binary from a Python script? for a
list of tools that can be used to make executables.


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.

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.
