Python on Windows FAQ¶

How do I run a Python program under Windows?В¶

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 variously referred to as a “DOS window” or “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:

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

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:

You should then see something like:

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:

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:

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

How do I make Python scripts executable?В¶

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?В¶

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?В¶

See cx_Freeze 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?В¶

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?В¶

Embedding the Python interpreter in a Windows app can be summarized as follows:

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.

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.

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.)

In short, you can use the following code to initialize the Python interpreter with your extension module.

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:

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:

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).

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?В¶

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?В¶

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.

Frequently Asked Questions about using Python on Windows

Trouble installing a package with pip install

There are a number of reasons why an installation will fail—in many cases the right solution is to contact the package developer.

A common cause for trouble is trying to install into a location that you do not have permission to modify. For example, the default install location might require Administrative privileges, but by default Python will not have them. The best solution is to create a virtual environment and install there.

Some packages include native code that requires a C or C++ compiler to install. In general, package developers should publish pre-compiled versions, but often do not. Some of these packages might work if you install Build Tools for Visual Studio and select the C++ option, however in most cases you will need to contact the package developer.

Trouble installing pip with WSL

When installing a package (like Flask) with pip on Windows Subsystem for Linux (WSL or WSL2), for example python3 -m pip install flask , you may specifically encounter an error like this:

When researching this problem you may be led down several rabbit holes, none of which are particularly productive with a WSL linux distribution. (Warning: on WSL do not try editing resolv.conf , that file is a symbolic link and modifying it is a can of worms). Unless you are running an aftermarket firewall, the likely solution is to simply re-install pip:

*Further discussion in the WSL product repo on GitHub. Thanks to our user community for contributing this issue to the docs.

What is py.exe?

You may end up with multiple versions of Python installed on your machine because you are working on different types of Python projects. Because these all use the python command, it may not be obvious which version of Python you are using. As a standard, it is recommended to use the python3 command (or python3.7 to select a specific version).

The py.exe launcher will automatically select the most recent version of Python you’ve installed. You can also use commands like py -3.7 to select a particular version, or py —list to see which versions can be used. HOWEVER, the py.exe launcher will only work if you are using a version of Python installed from python.org. When you install Python from the Microsoft Store, the py command is not included. For Linux, macOS, WSL and the Microsoft Store version of Python, you should use the python3 (or python3.7 ) command.

Why does running python.exe open the Microsoft Store?

To help new users find a good installation of Python, we added a shortcut to Windows that will take you directly to the latest version of the community’s package published in the Microsoft Store. This package can be installed easily, without administrator permissions, and will replace the default python and python3 commands with the real ones.

Running the shortcut executable with any command-line arguments will return an error code to indicate that Python was not installed. This is to prevent batch files and scripts from opening the Store app when it was probably not intended.

If you install Python using the installers from python.org and select the «add to PATH» option, the new python command will take priority over the shortcut. Note that other installers may add python at a lower priority than the built-in shortcut.

You can disable the shortcuts without installing Python by opening «Manage app execution aliases» from Start, finding the «App Installer» Python entries and switching them to «Off».

Why don’t file paths work in Python when I copy-paste them?

Python strings use “escapes” for special characters. For example, to insert a new line character into a string, you would type \n . Because file paths on Windows use backslashes, some parts might be being converted into special characters.

To paste a path as a string in Python, add the r prefix. This indicates that it is a raw string, and no escape characters will be used except for \” (you might need to remove the last backslash in your path). So your path might look like: r»C:\Users\MyName\Documents\Document.txt»

When working with paths in Python, we recommend using the standard pathlib module. This will let you convert the string to a rich Path object that can do path manipulations consistently whether it uses forward slashes or backslashes, making your code work better across different operating systems.

What is PYTHONPATH?

The PYTHONPATH environment variable is used by Python to specify a list of directories that modules can be imported from. When running, you can inspect the sys.path variable to see which directories will be searched when you import something.

To set this variable from the Command Prompt, use: set PYTHONPATH=list;of;paths .

To set this variable from PowerShell, use: $env:PYTHONPATH=’list;of;paths’ just before you launch Python.

Setting this variable globally through the Environment Variables settings is not recommended, as it may be used by any version of Python instead of the one that you intend to use.

Where can I find help with packaging and deployment?

Docker: VSCode extension helps you quickly package and deploy with Dockerfile and docker-compose.yml templates (generate the proper Docker files for your project).

Azure Kubernetes Service (AKS) enables you to deploy and manage containerized applications while scaling resources on demand.

What if I need to work across different machines?

Settings Sync allows you to synchronize your VS Code settings across different installations using GitHub. If you work on different machines, this helps keep your environment consistent across them.

What if I’m used to using PyCharm, Atom, Sublime Text, Emacs, or Vim?

The VSCode extension Keymaps can help your environment feel right at home.

How do Mac shortcut keys map to Windows shortcut keys?

Some of the keyboard buttons and system shortcuts are slightly different between a Windows machine and a Macintosh. This Mac to Windows transition guide covers the basics.