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This document aims to give an overview of Windows-specific behaviour you should know about when using Python on Microsoft Windows.
Unlike most Unix systems and services, Windows does not include a system supported installation of Python. To make Python available, the CPython team has compiled Windows installers (MSI packages) with every release for many years. These installers are primarily intended to add a per-user installation of Python, with the core interpreter and library being used by a single user. The installer is also able to install for all users of a single machine, and a separate ZIP file is available for application-local distributions.
Four Python 3.5 installers are available for download - two each for the 32-bit and 64-bit versions of the interpreter. The web installer is a small initial download, and it will automatically download the required components as necessary. The offline installer includes the components necessary for a default installation and only requires an internet connection for optional features. See Installing Without Downloading for other ways to avoid downloading during installation.
After starting the installer, one of two options may be selected:
If you select “Install Now”:
Selecting “Customize installation” will allow you to select the features to install, the installation location and other options or post-install actions. To install debugging symbols or binaries, you will need to use this option.
To perform an all-users installation, you should select “Customize installation”. In this case:
All of the options available in the installer UI can also be specified from the command line, allowing scripted installers to replicate an installation on many machines without user interaction. These options may also be set without suppressing the UI in order to change some of the defaults.
To completely hide the installer UI and install Python silently, pass the /quiet option. To skip past the user interaction but still display progress and errors, pass the /passive option. The /uninstall option may be passed to immediately begin removing Python - no prompt will be displayed.
All other options are passed as name=value, where the value is usually 0 to disable a feature, 1 to enable a feature, or a path. The full list of available options is shown below.
| Name | Description | Default |
|---|---|---|
| InstallAllUsers | Perform a system-wide installation. | 0 |
| TargetDir | The installation directory | Selected based on InstallAllUsers |
| DefaultAllUsersTargetDir | The default installation directory for all-user installs | %ProgramFiles%\Python X.Y or %ProgramFiles(x86)%\Python X.Y |
| DefaultJustForMeTargetDir | The default install directory for just-for-me installs | %LocalAppData%\Programs\PythonXY or %LocalAppData%\Programs\PythonXY-32 |
| DefaultCustomTargetDir | The default custom install directory displayed in the UI | (empty) |
| AssociateFiles | Create file associations if the launcher is also installed. | 1 |
| CompileAll | Compile all .py files to .pyc. | 0 |
| PrependPath | Add install and Scripts directories tho PATH and .PY to PATHEXT | 0 |
| Shortcuts | Create shortcuts for the interpreter, documentation and IDLE if installed. | 1 |
| Include_doc | Install Python manual | 1 |
| Include_debug | Install debug binaries | 0 |
| Include_dev | Install developer headers and libraries | 1 |
| Include_exe | Install python.exe and related files | 1 |
| Include_launcher | Install Python Launcher for Windows. | 1 |
| InstallLauncherAllUsers | Installs Python Launcher for Windows for all users. | 1 |
| Include_lib | Install standard library and extension modules | 1 |
| Include_pip | Install bundled pip and setuptools | 1 |
| Include_symbols | Install debugging symbols (*.pdb) | 0 |
| Include_tcltk | Install Tcl/Tk support and IDLE | 1 |
| Include_test | Install standard library test suite | 1 |
| Include_tools | Install utility scripts | 1 |
| LauncherOnly | Only installs the launcher. This will override most other options. | 0 |
| SimpleInstall | Disable most install UI | 0 |
| SimpleInstallDescription | A custom message to display when the simplified install UI is used. | (empty) |
For example, to silently install a default, system-wide Python installation, you could use the following command (from an elevated command prompt):
python-3.5.0.exe /quiet InstallAllUsers=1 PrependPath=1 Include_test=0
To allow users to easily install a personal copy of Python without the test suite, you could provide a shortcut with the following command. This will display a simplified initial page and disallow customization:
python-3.5.0.exe InstallAllUsers=0 Include_launcher=0 Include_test=0
SimpleInstall=1 SimpleInstallDescription="Just for me, no test suite."
(Note that omitting the launcher also omits file associations, and is only recommended for per-user installs when there is also a system-wide installation that included the launcher.)
The options listed above can also be provided in a file named unattend.xml alongside the executable. This file specifies a list of options and values. When a value is provided as an attribute, it will be converted to a number if possible. Values provided as element text are always left as strings. This example file sets the same options and the previous example:
<Options>
<Option Name="InstallAllUsers" Value="no" />
<Option Name="Include_launcher" Value="0" />
<Option Name="Include_test" Value="no" />
<Option Name="SimpleInstall" Value="yes" />
<Option Name="SimpleInstallDescription">Just for me, no test suite</Option>
</Options>
As some features of Python are not included in the initial installer download, selecting those features may require an internet connection. To avoid this need, all possible components may be downloaded on-demand to create a complete layout that will no longer require an internet connection regardless of the selected features. Note that this download may be bigger than required, but where a large number of installations are going to be performed it is very useful to have a locally cached copy.
Execute the following command from Command Prompt to download all possible required files. Remember to substitute python-3.5.0.exe for the actual name of your installer, and to create layouts in their own directories to avoid collisions between files with the same name.
python-3.5.0.exe /layout [optional target directory]
You may also specify the /quiet option to hide the progress display.
With ongoing development of Python, some platforms that used to be supported earlier are no longer supported (due to the lack of users or developers). Check PEP 11 for details on all unsupported platforms.
See Python for Windows for detailed information about platforms with pre-compiled installers.
See also
Besides the standard CPython distribution, there are modified packages including additional functionality. The following is a list of popular versions and their key features:
Note that these packages may not include the latest versions of Python or other libraries, and are not maintained or supported by the core Python team.
To run Python conveniently from a command prompt, you might consider changing some default environment variables in Windows. While the installer provides an option to configure the PATH and PATHEXT variables for you, this is only reliable for a single, system-wide installation. If you regularly use multiple versions of Python, consider using the Python Launcher for Windows.
Windows allows environment variables to be configured permanently at both the User level and the System level, or temporarily in a command prompt.
To temporarily set environment variables, open Command Prompt and use the set command:
C:\>set PATH=C:\Program Files\Python 3.5;%PATH%
C:\>set PYTHONPATH=%PYTHONPATH%;C:\My_python_lib
C:\>python
These changes will apply to any further commands executed in that console, and will be inherited by any applications started from the console.
Including the variable name within percent signs will expand to the existing value, allowing you to add your new value at either the start or the end. Modifying PATH by adding the directory containing python.exe to the start is a common way to ensure the correct version of Python is launched.
To permanently modify the default environment variables, click Start and search for ‘edit environment variables’, or open System properties, Advanced system settings and click the Environment Variables button. In this dialog, you can add or modify User and System variables. To change System variables, you need non-restricted access to your machine (i.e. Administrator rights).
Note
Windows will concatenate User variables after System variables, which may cause unexpected results when modifying PATH.
The PYTHONPATH variable is used by all versions of Python 2 and Python 3, so you should not permanently configure this variable unless it only includes code that is compatible with all of your installed Python versions.
See also
Changed in version 3.5.
Besides using the automatically created start menu entry for the Python interpreter, you might want to start Python in the command prompt. The installer for Python 3.5 and later has an option to set that up for you.
On the first page of the installer, an option labelled “Add Python 3.5 to PATH” can be selected to have the installer add the install location into the PATH. The location of the Scripts\ folder is also added. This allows you to type python to run the interpreter, and pip or . Thus, you can also execute your scripts with command line options, see Command line documentation.
If you don’t enable this option at install time, you can always re-run the installer, select Modify, and enable it. Alternatively, you can manually modify the PATH using the directions in Excursus: Setting environment variables. You need to set your PATH environment variable to include the directory of your Python installation, delimited by a semicolon from other entries. An example variable could look like this (assuming the first two entries already existed):
C:\WINDOWS\system32;C:\WINDOWS;C:\Program Files\Python 3.5
New in version 3.3.
The Python launcher for Windows is a utility which aids in locating and executing of different Python versions. It allows scripts (or the command-line) to indicate a preference for a specific Python version, and will locate and execute that version.
Unlike the PATH variable, the launcher will correctly select the most appropriate version of Python. It will prefer per-user installations over system-wide ones, and orders by language version rather than using the most recently installed version.
System-wide installations of Python 3.3 and later will put the launcher on your PATH. The launcher is compatible with all available versions of Python, so it does not matter which version is installed. To check that the launcher is available, execute the following command in Command Prompt:
py
You should find that the latest version of Python 2.x you have installed is started - it can be exited as normal, and any additional command-line arguments specified will be sent directly to Python.
If you have multiple versions of Python 2.x installed (e.g., 2.6 and 2.7) you will have noticed that Python 2.7 was started - to launch Python 2.6, try the command:
py -2.6
If you have a Python 3.x installed, try the command:
py -3
You should find the latest version of Python 3.x starts.
If you see the following error, you do not have the launcher installed:
'py' is not recognized as an internal or external command,
operable program or batch file.
Per-user installations of Python do not add the launcher to PATH unless the option was selected on installation.
New in version 3.5.
If the launcher is run with no explicit Python version specification, and a virtual environment (created with the standard library venv module or the external virtualenv tool) active, the launcher will run the virtual environment’s interpreter rather than the global one. To run the global interpreter, either deactivate the virtual environment, or explicitly specify the global Python version.
Let’s create a test Python script - create a file called hello.py with the following contents
#! python
import sys
sys.stdout.write("hello from Python %s\n" % (sys.version,))
From the directory in which hello.py lives, execute the command:
py hello.py
You should notice the version number of your latest Python 2.x installation is printed. Now try changing the first line to be:
#! python3
Re-executing the command should now print the latest Python 3.x information. As with the above command-line examples, you can specify a more explicit version qualifier. Assuming you have Python 2.6 installed, try changing the first line to #! python2.6 and you should find the 2.6 version information printed.
The launcher should have been associated with Python files (i.e. .py, .pyw, .pyc files) when it was installed. This means that when you double-click on one of these files from Windows explorer the launcher will be used, and therefore you can use the same facilities described above to have the script specify the version which should be used.
The key benefit of this is that a single launcher can support multiple Python versions at the same time depending on the contents of the first line.
If the first line of a script file starts with #!, it is known as a “shebang” line. Linux and other Unix like operating systems have native support for such lines and are commonly used on such systems to indicate how a script should be executed. This launcher allows the same facilities to be using with Python scripts on Windows and the examples above demonstrate their use.
To allow shebang lines in Python scripts to be portable between Unix and Windows, this launcher supports a number of ‘virtual’ commands to specify which interpreter to use. The supported virtual commands are:
For example, if the first line of your script starts with
#! /usr/bin/python
The default Python will be located and used. As many Python scripts written to work on Unix will already have this line, you should find these scripts can be used by the launcher without modification. If you are writing a new script on Windows which you hope will be useful on Unix, you should use one of the shebang lines starting with /usr.
Any of the above virtual commands can be suffixed with an explicit version (either just the major version, or the major and minor version) - for example /usr/bin/python2.7 - which will cause that specific version to be located and used.
The /usr/bin/env form of shebang line has one further special property. Before looking for installed Python interpreters, this form will search the executable PATH for a Python executable. This corresponds to the behaviour of the Unix env program, which performs a PATH search.
The shebang lines can also specify additional options to be passed to the Python interpreter. For example, if you have a shebang line:
#! /usr/bin/python -v
Then Python will be started with the -v option
Two .ini files will be searched by the launcher - py.ini in the current user’s “application data” directory (i.e. the directory returned by calling the Windows function SHGetFolderPath with CSIDL_LOCAL_APPDATA) and py.ini in the same directory as the launcher. The same .ini files are used for both the ‘console’ version of the launcher (i.e. py.exe) and for the ‘windows’ version (i.e. pyw.exe)
Customization specified in the “application directory” will have precedence over the one next to the executable, so a user, who may not have write access to the .ini file next to the launcher, can override commands in that global .ini file)
In some cases, a version qualifier can be included in a command to dictate which version of Python will be used by the command. A version qualifier starts with a major version number and can optionally be followed by a period (‘.’) and a minor version specifier. If the minor qualifier is specified, it may optionally be followed by “-32” to indicate the 32-bit implementation of that version be used.
For example, a shebang line of #!python has no version qualifier, while #!python3 has a version qualifier which specifies only a major version.
If no version qualifiers are found in a command, the environment variable PY_PYTHON can be set to specify the default version qualifier - the default value is “2”. Note this value could specify just a major version (e.g. “2”) or a major.minor qualifier (e.g. “2.6”), or even major.minor-32.
If no minor version qualifiers are found, the environment variable PY_PYTHON{major} (where {major} is the current major version qualifier as determined above) can be set to specify the full version. If no such option is found, the launcher will enumerate the installed Python versions and use the latest minor release found for the major version, which is likely, although not guaranteed, to be the most recently installed version in that family.
On 64-bit Windows with both 32-bit and 64-bit implementations of the same (major.minor) Python version installed, the 64-bit version will always be preferred. This will be true for both 32-bit and 64-bit implementations of the launcher - a 32-bit launcher will prefer to execute a 64-bit Python installation of the specified version if available. This is so the behavior of the launcher can be predicted knowing only what versions are installed on the PC and without regard to the order in which they were installed (i.e., without knowing whether a 32 or 64-bit version of Python and corresponding launcher was installed last). As noted above, an optional “-32” suffix can be used on a version specifier to change this behaviour.
Examples:
In addition to environment variables, the same settings can be configured in the .INI file used by the launcher. The section in the INI file is called [defaults] and the key name will be the same as the environment variables without the leading PY_ prefix (and note that the key names in the INI file are case insensitive.) The contents of an environment variable will override things specified in the INI file.
For example:
[defaults]
python=3.1
[defaults]
python=3
python3=3.1
If an environment variable PYLAUNCH_DEBUG is set (to any value), the launcher will print diagnostic information to stderr (i.e. to the console). While this information manages to be simultaneously verbose and terse, it should allow you to see what versions of Python were located, why a particular version was chosen and the exact command-line used to execute the target Python.
Python usually stores its library (and thereby your site-packages folder) in the installation directory. So, if you had installed Python to C:\Python\, the default library would reside in C:\Python\Lib\ and third-party modules should be stored in C:\Python\Lib\site-packages\.
This is how sys.path is populated on Windows:
If a pyvenv.cfg file is found alongside the main executable or in the directory one level above the executable, the following variations apply:
The end result of all this is:
For those who want to bundle Python into their application or distribution, the following advice will prevent conflicts with other installations:
These will ensure that the files in a system-wide installation will not take precedence over the copy of the standard library bundled with your application. Otherwise, your users may experience problems using your application. Note that the first suggestion is the best, as the other may still be susceptible to non-standard paths in the registry and user site-packages.
Even though Python aims to be portable among all platforms, there are features that are unique to Windows. A couple of modules, both in the standard library and external, and snippets exist to use these features.
The Windows-specific standard modules are documented in MS Windows Specific Services.
The PyWin32 module by Mark Hammond is a collection of modules for advanced Windows-specific support. This includes utilities for:
PythonWin is a sample MFC application shipped with PyWin32. It is an embeddable IDE with a built-in debugger.
See also
cx_Freeze is a distutils extension (see Extending Distutils) which wraps Python scripts into executable Windows programs (*.exe files). When you have done this, you can distribute your application without requiring your users to install Python.
If you want to compile CPython yourself, first thing you should do is get the source. You can download either the latest release’s source or just grab a fresh checkout.
The source tree contains a build solution and project files for Microsoft Visual Studio 2015, which is the compiler used to build the official Python releases. These files are in the PCbuild directory.
Check PCbuild/readme.txt for general information on the build process.
For extension modules, consult Building C and C++ Extensions on Windows.
See also
New in version 3.5.
The embedded distribution is a ZIP file containing a minimal Python environment. It is intended for acting as part of another application, rather than being directly accessed by end-users.
When extracted, the embedded distribution is (almost) fully isolated from the user’s system, including environment variables, system registry settings, and installed packages. The standard library is included as pre-compiled and optimized .pyc files in a ZIP, and python3.dll, python35.dll, python.exe and pythonw.exe are all provided. Tcl/tk (including all dependants, such as Idle), pip and the Python documentation are not included.
Note
The embedded distribution does not include the Microsoft C Runtime and it is the responsibility of the application installer to provide this. The runtime may have already been installed on a user’s system previously or automatically via Windows Update, and can be detected by finding ucrtbase.dll in the system directory.
Third-party packages should be installed by the application installer alongside the embedded distribution. Using pip to manage dependencies as for a regular Python installation is not supported with this distribution, though with some care it may be possible to include and use pip for automatic updates. In general, third-party packages should be treated as part of the application (“vendoring”) so that the developer can ensure compatibility with newer versions before providing updates to users.
The two recommended use cases for this distribution are described below.
An application written in Python does not necessarily require users to be aware of that fact. The embedded distribution may be used in this case to include a private version of Python in an install package. Depending on how transparent it should be (or conversely, how professional it should appear), there are two options.
Using a specialized executable as a launcher requires some coding, but provides the most transparent experience for users. With a customized launcher, there are no obvious indications that the program is running on Python: icons can be customized, company and version information can be specified, and file associations behave properly. In most cases, a custom launcher should simply be able to call Py_Main with a hard-coded command line.
The simpler approach is to provide a batch file or generated shortcut that directly calls the python.exe or pythonw.exe with the required command-line arguments. In this case, the application will appear to be Python and not its actual name, and users may have trouble distinguishing it from other running Python processes or file associations.
With the latter approach, packages should be installed as directories alongside the Python executable to ensure they are available on the path. With the specialized launcher, packages can be located in other locations as there is an opportunity to specify the search path before launching the application.
Applications written in native code often require some form of scripting language, and the embedded Python distribution can be used for this purpose. In general, the majority of the application is in native code, and some part will either invoke python.exe or directly use python3.dll. For either case, extracting the embedded distribution to a subdirectory of the application installation is sufficient to provide a loadable Python interpreter.
As with the application use, packages can be installed to any location as there is an opportunity to specify search paths before initializing the interpreter. Otherwise, there is no fundamental differences between using the embedded distribution and a regular installation.
See also