Python on Windows FAQ (2024)

How do I run a Python program under Windows?¶

This is not necessarily a straightforward question. If you are already familiarwith running programs from the Windows command line then everything will seemobvious; otherwise, you might need a little more guidance.

Unless you use some sort of integrated development environment, you will end uptyping Windows commands into what is referred to as a“Command prompt window”. Usually you can create such a window from yoursearch bar by searching for cmd. You should be able to recognizewhen you have started such a window because you will see a Windows “commandprompt”, which usually looks like this:

C:\>

The letter may be different, and there might be other things after it, so youmight just as easily see something like:

D:\YourName\Projects\Python>

depending on how your computer has been set up and what else you have recentlydone with it. Once you have started such a window, you are well on the way torunning Python programs.

You need to realize that your Python scripts have to be processed by anotherprogram called the Python interpreter. The interpreter reads your script,compiles it into bytecodes, and then executes the bytecodes to run yourprogram. 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 acommand window, you should try entering the command py and hittingreturn:

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 win32Type "help", "copyright", "credits" or "license" for more information.>>>

You have started the interpreter in “interactive mode”. That means you can enterPython statements or expressions interactively and have them executed orevaluated while you wait. This is one of Python’s strongest features. Check itby 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 programmablecalculator. When you want to end your interactive Python session,call the exit() function or hold the Ctrl key downwhile you enter a Z, then hit the “Enter” key to getback 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 youseeing the >>> prompt in a new window. If so, the window will disappearafter you call the exit() function or enter the Ctrl-Zcharacter; 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 yourPython script to it. You’ll have to give either an absolute or arelative path to the Python script. Let’s say your Python script islocated in your desktop and is named hello.py, and your commandprompt is nicely opened in your home directory so you’re seeing somethingsimilar to:

C:\Users\YourName>

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

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

How do I make Python scripts executable?¶

On Windows, the standard Python installer already associates the .pyextension with a file type (Python.File) and gives that file type an opencommand 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 bugreports that Python suddenly begins to take a long time to start up. This ismade even more puzzling because Python will work fine on other Windows systemswhich appear to be configured identically.

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

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 tomake 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 DLLnamed foo.pyd, then it must have a function PyInit_foo(). You can thenwrite Python “import foo”, and Python will search for foo.pyd (as well asfoo.py, foo.pyc) and if it finds it, will attempt to call PyInit_foo() toinitialize it. You do not link your .exe with foo.lib, as that would causeWindows to require the DLL to be present.

Note that the search path for foo.pyd is PYTHONPATH, not the same as the paththat Windows uses to search for foo.dll. Also, foo.pyd need not be present torun your program, whereas if you linked your program with a dll, the dll isrequired. Of course, foo.pyd is required if you want to say import foo. Ina 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 mustbe a DLL to handle importing modules that are themselves DLL’s. (This is thefirst key undocumented fact.) Instead, link to pythonNN.dll; it istypically installed in C:\Windows\System. NN is the Python version, anumber such as “33” for Python 3.3.

   You can link to Python in two different ways. Load-time linking meanslinking against pythonNN.lib, while run-time linking means linkingagainst pythonNN.dll. (General note: pythonNN.lib is theso-called “import lib” corresponding to pythonNN.dll. It merelydefines symbols for the linker.)

   Run-time linking greatly simplifies link options; everything happens at runtime. Your code must load pythonNN.dll using the WindowsLoadLibraryEx() routine. The code must also use access routines and datain pythonNN.dll (that is, Python’s C API’s) using pointers obtainedby the Windows GetProcAddress() routine. Macros can make using thesepointers 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 willmake the app’s data and methods available to Python. SWIG will handle justabout all the grungy details for you. The result is C code that you linkinto your .exe file (!) You do not have to create a DLL file, and thisalso simplifies linking.

3. SWIG will create an init function (a C function) whose name depends on thename 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 initializesa 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 thatcalling the initialization function is equivalent to importing the moduleinto Python! (This is the second key undocumented fact.)

4. In short, you can use the following code to initialize the Python interpreterwith 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 youuse 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 eachcompiler’s notion of a struct FILE will be different. From an implementationstandpoint these are very low level functions.

   Problem 2: SWIG generates the following code when generating wrappers to voidfunctions:

   Py_INCREF(Py_None);_resultobj = Py_None;return _resultobj;

   Alas, Py_None is a macro that expands to a reference to a complex datastructure called _Py_NoneStruct inside pythonNN.dll. Again, this code willfail 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 changeautomatically, though I have not been able to get this to work (I’m acomplete SWIG newbie).

6. Using a Python shell script to put up a Python interpreter window from insideyour Windows app is not a good idea; the resulting window will be independentof your app’s windowing system. Rather, you (or the wxPythonWindow class)should create a “native” interpreter window. It is easy to connect thatwindow 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 Emacspython-mode default.

Under any editor, mixing tabs and spaces is a bad idea. MSVC is no different inthis respect, and is easily configured to use spaces: Take Tools‣ Options ‣ Tabs, and for file type “Default” set “Tab size” and “Indentsize” to 4, and select the “Insert spaces” radio button.

Python raises IndentationError or TabError if mixed tabsand spaces are causing problems in leading whitespace.You may also run the tabnanny module to check a directory treein 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 ispresent, and getch() which gets one character without echoing it.

How do I solve the missing api-ms-win-crt-runtime-l1-1-0.dll error?¶

This can occur on Python 3.5 and later when using Windows 8.1 or earlier without all updates having been installed.First ensure your operating system is supported and is up to date, and if that does not resolve the issue,visit the Microsoft support pagefor guidance on manually installing the C Runtime update.