Py File Runner

In this tutorial, learn how to execute Python program or code on Windows. Execute Python program on Command prompt or use Python IDLE GUI mode to run Python code. Create your file in.py extension and execute using the step-step process given here. Python is a well known high-level programming language. The Python script is basically a file containing code written in Python. The file containing python script has the extension ‘.py’ or can also have the extension ‘.pyw’ if it is being run on a windows machine. Run Python on Windows. To run Python programs on windows, there are a few steps you need to do. Save your code with the.py extension. Lets call it hello.py. Open a command prompt, by pressing start, run and type “cmd” (in the text field) and press OK. Go to the directory of your Python file with the cd command. At this point, you're ready to run your first Python application in PyCharm. Run your application. Use either of the following ways to run your code: Right-click the editor and select Run 'Car' from the context menu. Press Ctrl+Shift+F10. Since this Python script contains a main function, you can click an icon in the gutter. If you hover your.

Released:

Invoke py.test as distutils command with dependency resolution

Project description

Setup scripts can use pytest-runner to add setup.py test support for pytestrunner.

Deprecation Notice

pytest-runner depends on deprecated features of setuptools and relies on features that break securitymechanisms in pip. For example ‘setup_requires’ and ‘tests_require’ bypass pip --require-hashes.See also pypa/setuptools#1684.

It is recommended that you:

• Remove 'pytest-runner' from your setup_requires, preferably removing the setup_requires option.
• Remove 'pytest' and any other testing requirements from tests_require, preferably removing the tests_requires option.
• Select a tool to bootstrap and then run tests such as tox.

Usage

• Add ‘pytest-runner’ to your ‘setup_requires’. Pin to ‘>=2.0,<3dev’ (orsimilar) to avoid pulling in incompatible versions.
• Include ‘pytest’ and any other testing requirements to ‘tests_require’.
• Invoke tests with setup.py pytest.
• Pass --index-url to have test requirements downloaded from an alternateindex URL (unnecessary if specified for easy_install in setup.cfg).
• Pass additional py.test command-line options using --addopts.
• Set permanent options for the python setup.py pytest command (like index-url)in the [pytest] section of setup.cfg.
• Set permanent options for the py.test run (like addopts or pep8ignore) in the [pytest]section of pytest.ini or tox.ini or put them in the [tool:pytest]section of setup.cfg. See pytest issue 567.
• Optionally, set test=pytest in the [aliases] section of setup.cfgto cause python setup.py test to invoke pytest.

Example

The most simple usage looks like this in setup.py:

Additional dependencies require to run the tests (e.g. mock or pytestplugins) may be added to tests_require and will be downloaded andrequired by the session before invoking pytest.

Follow this search on githubfor examples of real-world usage.

Standalone Example

This technique is deprecated - if you have standalone scriptsyou wish to invoke with dependencies, use pip-run.

Although pytest-runner is typically used to add pytest testrunner support to maintained packages, pytest-runner mayalso be used to create standalone tests. Consider this examplefailure,reported in jsonpickle #117or this MongoDB testdemonstrating a technique that works even when dependenciesare required in the test.

Either example file may be cloned or downloaded and simply run onany system with Python and Setuptools. It will download thespecified dependencies and run the tests. Afterward, the thecloned directory can be removed and with it all trace ofinvoking the test. No other dependencies are needed and nosystem configuration is altered.

Then, anyone trying to replicate the failure can do so easilyand with all the power of pytest (rewritten assertions,rich comparisons, interactive debugging, extensibility throughplugins, etc).

As a result, the communication barrier for describing andreplicating failures is made almost trivially low.

Considerations

Conditional Requirement

Because it uses Setuptools setup_requires, pytest-runner will install itselfon every invocation of setup.py. In some cases, this causes delays forinvocations of setup.py that will never invoke pytest-runner. To help avoidthis contingency, consider requiring pytest-runner only when pytestis invoked:

Security Contact

To report a security vulnerability, please use theTidelift security contact.Tidelift will coordinate the fix and disclosure.

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This document aims to give an overview of Windows-specific behaviour you shouldknow about when using Python on Microsoft Windows.

Unlike most Unix systems and services, Windows does not include a systemsupported installation of Python. To make Python available, the CPython teamhas compiled Windows installers (MSI packages) with every release for many years. These installersare primarily intended to add a per-user installation of Python, with thecore interpreter and library being used by a single user. The installer is alsoable to install for all users of a single machine, and a separate ZIP file isavailable for application-local distributions.

As specified in PEP 11, a Python release only supports a Windows platformwhile Microsoft considers the platform under extended support. This means thatPython 3.9 supports Windows 8.1 and newer. If you require Windows 7support, please install Python 3.8.

There are a number of different installers available for Windows, each withcertain benefits and downsides.

The full installer contains all components and is the best option fordevelopers using Python for any kind of project.

The Microsoft Store package is a simple installation of Python that is suitable forrunning scripts and packages, and using IDLE or other development environments.It requires Windows 10, but can be safely installed without corrupting otherprograms. It also provides many convenient commands for launching Python andits tools.

The nuget.org packages are lightweight installations intended for continuousintegration systems. It can be used to build Python packages or run scripts,but is not updateable and has no user interface tools.

The embeddable package is a minimal package of Python suitable forembedding into a larger application.

3.1. The full installer¶

3.1.1. Installation steps¶

Four Python 3.9 installers are available for download - two each for the32-bit and 64-bit versions of the interpreter. The web installer is a smallinitial download, and it will automatically download the required components asnecessary. The offline installer includes the components necessary for adefault installation and only requires an internet connection for optionalfeatures. See Installing Without Downloading for other ways to avoid downloadingduring installation.

After starting the installer, one of two options may be selected:

If you select “Install Now”:

• You will not need to be an administrator (unless a system update for theC Runtime Library is required or you install the Python Launcher for Windows for allusers)

• Python will be installed into your user directory

• The Python Launcher for Windows will be installed according to the option at the bottomof the first page

• The standard library, test suite, launcher and pip will be installed

• If selected, the install directory will be added to your PATH

• Shortcuts will only be visible for the current user

Selecting “Customize installation” will allow you to select the features toinstall, 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 “Customizeinstallation”. In this case:

• You may be required to provide administrative credentials or approval

• Python will be installed into the Program Files directory

• The Python Launcher for Windows will be installed into the Windows directory

• Optional features may be selected during installation

• The standard library can be pre-compiled to bytecode

• If selected, the install directory will be added to the system PATH

• Shortcuts are available for all users

3.1.2. Removing the MAX_PATH Limitation¶

Windows historically has limited path lengths to 260 characters. This meant thatpaths longer than this would not resolve and errors would result.

In the latest versions of Windows, this limitation can be expanded toapproximately 32,000 characters. Your administrator will need to activate the“Enable Win32 long paths” group policy, or set LongPathsEnabled to 1in the registry keyHKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlFileSystem.

This allows the open() function, the os module and most otherpath functionality to accept and return paths longer than 260 characters.

After changing the above option, no further configuration is required.

Changed in version 3.6: Support for long paths was enabled in Python.

3.1.3. Installing Without UI¶

All of the options available in the installer UI can also be specified from thecommand line, allowing scripted installers to replicate an installation on manymachines without user interaction. These options may also be set withoutsuppressing 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 displayprogress and errors, pass the /passive option. The /uninstalloption may be passed to immediately begin removing Python - no prompt will bedisplayed.

All other options are passed as name=value, where the value is usually0 to disable a feature, 1 to enable a feature, or a path. The full listof available options is shown below.

For example, to silently install a default, system-wide Python installation,you could use the following command (from an elevated command prompt):

To allow users to easily install a personal copy of Python without the testsuite, you could provide a shortcut with the following command. This willdisplay a simplified initial page and disallow customization:

(Note that omitting the launcher also omits file associations, and is onlyrecommended for per-user installs when there is also a system-wide installationthat included the launcher.)

The options listed above can also be provided in a file named unattend.xmlalongside 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 ifpossible. Values provided as element text are always left as strings. Thisexample file sets the same options as the previous example:

3.1.4. Installing Without Downloading¶

As some features of Python are not included in the initial installer download,selecting those features may require an internet connection. To avoid thisneed, all possible components may be downloaded on-demand to create a completelayout that will no longer require an internet connection regardless of theselected features. Note that this download may be bigger than required, butwhere a large number of installations are going to be performed it is veryuseful to have a locally cached copy.

Execute the following command from Command Prompt to download all possiblerequired files. Remember to substitute python-3.9.0.exe for the actualname of your installer, and to create layouts in their own directories toavoid collisions between files with the same name.

You may also specify the /quiet option to hide the progress display.

3.1.5. Modifying an install¶

Once Python has been installed, you can add or remove features through thePrograms and Features tool that is part of Windows. Select the Python entry andchoose “Uninstall/Change” to open the installer in maintenance mode.

“Modify” allows you to add or remove features by modifying the checkboxes -unchanged checkboxes will not install or remove anything. Some options cannot bechanged in this mode, such as the install directory; to modify these, you willneed to remove and then reinstall Python completely.

“Repair” will verify all the files that should be installed using the currentsettings and replace any that have been removed or modified.

“Uninstall” will remove Python entirely, with the exception of thePython Launcher for Windows, which has its own entry in Programs and Features.

3.2. The Microsoft Store package¶

The Microsoft Store package is an easily installable Python interpreter thatis intended mainly for interactive use, for example, by students.

To install the package, ensure you have the latest Windows 10 updates andsearch the Microsoft Store app for “Python 3.9”. Ensure that the appyou select is published by the Python Software Foundation, and install it.

Warning

Python will always be available for free on the Microsoft Store. If youare asked to pay for it, you have not selected the correct package.

After installation, Python may be launched by finding it in Start.Alternatively, it will be available from any Command Prompt or PowerShellsession by typing python. Further, pip and IDLE may be used by typingpip or idle. IDLE can also be found in Start.

All three commands are also available with version number suffixes, forexample, as python3.exe and python3.x.exe as well aspython.exe (where 3.x is the specific version you want to launch,such as 3.9). Open “Manage App Execution Aliases” through Start toselect which version of Python is associated with each command. It isrecommended to make sure that pip and idle are consistent withwhichever version of python is selected.

Virtual environments can be created with python-mvenv and activatedand used as normal.

If you have installed another version of Python and added it to yourPATH variable, it will be available as python.exe rather than theone from the Microsoft Store. To access the new installation, usepython3.exe or python3.x.exe.

The py.exe launcher will detect this Python installation, but will preferinstallations from the traditional installer.

To remove Python, open Settings and use Apps and Features, or else findPython in Start and right-click to select Uninstall. Uninstalling willremove all packages you installed directly into this Python installation, butwill not remove any virtual environments

3.2.1. Known Issues¶

Because of restrictions on Microsoft Store apps, Python scripts may not havefull write access to shared locations such as TEMP and the registry.Instead, it will write to a private copy. If your scripts must modify theshared locations, you will need to install the full installer.

For more detail on the technical basis for these limitations, please consultMicrosoft’s documentation on packaged full-trust apps, currently available atdocs.microsoft.com/en-us/windows/msix/desktop/desktop-to-uwp-behind-the-scenes

3.3. The nuget.org packages¶

The nuget.org package is a reduced size Python environment intended for use oncontinuous integration and build systems that do not have a system-wideinstall of Python. While nuget is “the package manager for .NET”, it also worksperfectly fine for packages containing build-time tools.

Visit nuget.org for the most up-to-date informationon using nuget. What follows is a summary that is sufficient for Pythondevelopers.

The nuget.exe command line tool may be downloaded directly fromhttps://aka.ms/nugetclidl, for example, using curl or PowerShell. With thetool, the latest version of Python for 64-bit or 32-bit machines is installedusing:

To select a particular version, add a -Version3.x.y. The output directorymay be changed from ., and the package will be installed into asubdirectory. By default, the subdirectory is named the same as the package,and without the -ExcludeVersion option this name will include the specificversion installed. Inside the subdirectory is a tools directory thatcontains the Python installation:

In general, nuget packages are not upgradeable, and newer versions should beinstalled side-by-side and referenced using the full path. Alternatively,delete the package directory manually and install it again. Many CI systemswill do this automatically if they do not preserve files between builds.

Alongside the tools directory is a buildnative directory. Thiscontains a MSBuild properties file python.props that can be used in aC++ project to reference the Python install. Including the settings willautomatically use the headers and import libraries in your build.

The package information pages on nuget.org arewww.nuget.org/packages/pythonfor the 64-bit version and www.nuget.org/packages/pythonx86 for the 32-bit version.

3.4. The embeddable package¶

The embedded distribution is a ZIP file containing a minimal Python environment.It is intended for acting as part of another application, rather than beingdirectly accessed by end-users.

When extracted, the embedded distribution is (almost) fully isolated from theuser’s system, including environment variables, system registry settings, andinstalled packages. The standard library is included as pre-compiled andoptimized .pyc files in a ZIP, and python3.dll, python37.dll,python.exe and pythonw.exe are all provided. Tcl/tk (including alldependants, such as Idle), pip and the Python documentation are not included.

Note

The embedded distribution does not include the Microsoft C Runtime and it isthe responsibility of the application installer to provide this. Theruntime may have already been installed on a user’s system previously orautomatically via Windows Update, and can be detected by findingucrtbase.dll in the system directory.

Third-party packages should be installed by the application installer alongsidethe embedded distribution. Using pip to manage dependencies as for a regularPython installation is not supported with this distribution, though with somecare it may be possible to include and use pip for automatic updates. Ingeneral, third-party packages should be treated as part of the application(“vendoring”) so that the developer can ensure compatibility with newerversions before providing updates to users.

The two recommended use cases for this distribution are described below.

3.4.1. Python Application¶

An application written in Python does not necessarily require users to be awareof that fact. The embedded distribution may be used in this case to include aprivate version of Python in an install package. Depending on how transparent itshould be (or conversely, how professional it should appear), there are twooptions.

Using a specialized executable as a launcher requires some coding, but providesthe most transparent experience for users. With a customized launcher, there areno obvious indications that the program is running on Python: icons can becustomized, company and version information can be specified, and fileassociations behave properly. In most cases, a custom launcher should simply beable to call Py_Main with a hard-coded command line.

The simpler approach is to provide a batch file or generated shortcut thatdirectly calls the python.exe or pythonw.exe with the requiredcommand-line arguments. In this case, the application will appear to be Pythonand not its actual name, and users may have trouble distinguishing it from otherrunning Python processes or file associations.

With the latter approach, packages should be installed as directories alongsidethe Python executable to ensure they are available on the path. With thespecialized launcher, packages can be located in other locations as there is anopportunity to specify the search path before launching the application.

3.4.2. Embedding Python¶

Applications written in native code often require some form of scriptinglanguage, and the embedded Python distribution can be used for this purpose. Ingeneral, the majority of the application is in native code, and some part willeither invoke python.exe or directly use python3.dll. For either case,extracting the embedded distribution to a subdirectory of the applicationinstallation is sufficient to provide a loadable Python interpreter.

As with the application use, packages can be installed to any location as thereis an opportunity to specify search paths before initializing the interpreter.Otherwise, there is no fundamental differences between using the embeddeddistribution and a regular installation.

3.5. Alternative bundles¶

Besides the standard CPython distribution, there are modified packages includingadditional functionality. The following is a list of popular versions and theirkey features:

Installer with multi-platform compatibility, documentation, PyWin32

Popular scientific modules (such as numpy, scipy and pandas) and theconda package manager.

A “comprehensive Python analysis environment” with editors and otherdevelopment tools.

Windows-specific distribution with prebuilt scientific packages andtools for building packages.

Note that these packages may not include the latest versions of Python orother libraries, and are not maintained or supported by the core Python team.

3.6. Configuring Python¶

To run Python conveniently from a command prompt, you might consider changingsome default environment variables in Windows. While the installer provides anoption to configure the PATH and PATHEXT variables for you, this is onlyreliable for a single, system-wide installation. If you regularly use multipleversions of Python, consider using the Python Launcher for Windows.

3.6.1. Excursus: Setting environment variables¶

Windows allows environment variables to be configured permanently at both theUser level and the System level, or temporarily in a command prompt.

To temporarily set environment variables, open Command Prompt and use theset command:

These changes will apply to any further commands executed in that console, andwill be inherited by any applications started from the console.

Including the variable name within percent signs will expand to the existingvalue, allowing you to add your new value at either the start or the end.Modifying PATH by adding the directory containingpython.exe to the start is a common way to ensure the correct versionof Python is launched.

To permanently modify the default environment variables, click Start and searchfor ‘edit environment variables’, or open System properties, Advancedsystem settings and click the Environment Variables button.In this dialog, you can add or modify User and System variables. To changeSystem variables, you need non-restricted access to your machine(i.e. Administrator rights).

Note

Windows will concatenate User variables after System variables, which maycause unexpected results when modifying PATH.

The PYTHONPATH variable is used by all versions of Python 2 andPython 3, so you should not permanently configure this variable unless itonly includes code that is compatible with all of your installed Pythonversions.

See also

Environment variables in Windows NT

The SET command, for temporarily modifying environment variables

The SETX command, for permanently modifying environment variables

How To Manage Environment Variables in Windows XP

Setting Environment variables, Louis J. Farrugia

3.6.2. Finding the Python executable¶

Besides using the automatically created start menu entry for the Pythoninterpreter, you might want to start Python in the command prompt. Theinstaller has an option to set that up for you.

On the first page of the installer, an option labelled “Add Python to PATH”may be selected to have the installer add the install location into thePATH. The location of the Scripts folder is also added.This allows you to type python to run the interpreter, andpip for the package installer. Thus, you can also execute yourscripts with command line options, see Command line documentation.

If you don’t enable this option at install time, you can always re-run theinstaller, select Modify, and enable it. Alternatively, you can manuallymodify the PATH using the directions in Excursus: Setting environment variables. Youneed to set your PATH environment variable to include the directoryof your Python installation, delimited by a semicolon from other entries. Anexample variable could look like this (assuming the first two entries alreadyexisted):

3.7. UTF-8 mode¶

Windows still uses legacy encodings for the system encoding (the ANSI CodePage). Python uses it for the default encoding of text files (e.g.locale.getpreferredencoding()).

This may cause issues because UTF-8 is widely used on the internetand most Unix systems, including WSL (Windows Subsystem for Linux).

You can use UTF-8 mode to change the default text encoding to UTF-8.You can enable UTF-8 mode via the -Xutf8 command line option, orthe PYTHONUTF8=1 environment variable. See PYTHONUTF8 forenabling UTF-8 mode, and Excursus: Setting environment variables for how to modifyenvironment variables.

When UTF-8 mode is enabled:

• locale.getpreferredencoding() returns 'UTF-8' instead ofthe system encoding. This function is used for the default textencoding in many places, including open(), Popen,Path.read_text(), etc.

• sys.stdin, sys.stdout, and sys.stderrall use UTF-8 as their text encoding.

• You can still use the system encoding via the “mbcs” codec.

Note that adding PYTHONUTF8=1 to the default environment variableswill affect all Python 3.7+ applications on your system.If you have any Python 3.7+ applications which rely on the legacysystem encoding, it is recommended to set the environment variabletemporarily or use the -Xutf8 command line option.

Note

Even when UTF-8 mode is disabled, Python uses UTF-8 by defaulton Windows for:

• Console I/O including standard I/O (see PEP 528 for details).

• The filesystem encoding (see PEP 529 for details).

3.8. Python Launcher for Windows¶

The Python launcher for Windows is a utility which aids in locating andexecuting of different Python versions. It allows scripts (or thecommand-line) to indicate a preference for a specific Python version, andwill locate and execute that version.

Unlike the PATH variable, the launcher will correctly select the mostappropriate version of Python. It will prefer per-user installations oversystem-wide ones, and orders by language version rather than using the mostrecently installed version.

The launcher was originally specified in PEP 397.

3.8.1. Getting started¶

3.8.1.1. From the command-line¶

System-wide installations of Python 3.3 and later will put the launcher on yourPATH. The launcher is compatible with all available versions ofPython, so it does not matter which version is installed. To check that thelauncher is available, execute the following command in Command Prompt:

You should find that the latest version of Python you have installed isstarted - it can be exited as normal, and any additional command-linearguments specified will be sent directly to Python.

If you have multiple versions of Python installed (e.g., 2.7 and 3.9) youwill have noticed that Python 3.9 was started - to launch Python 2.7, trythe command:

If you want the latest version of Python 2.x you have installed, try thecommand:

You should find the latest version of Python 2.x starts.

If you see the following error, you do not have the launcher installed:

Per-user installations of Python do not add the launcher to PATHunless the option was selected on installation.

3.8.1.2. Virtual environments¶

If the launcher is run with no explicit Python version specification, and avirtual environment (created with the standard library venv module orthe external virtualenv tool) active, the launcher will run the virtualenvironment’s interpreter rather than the global one. To run the globalinterpreter, either deactivate the virtual environment, or explicitly specifythe global Python version.

Py File Runner Online

3.8.1.3. From a script¶

Let’s create a test Python script - create a file called hello.py with thefollowing contents

From the directory in which hello.py lives, execute the command:

You should notice the version number of your latest Python 2.x installationis printed. Now try changing the first line to be:

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 explicitversion qualifier. Assuming you have Python 2.6 installed, try changing thefirst line to #!python2.6 and you should find the 2.6 versioninformation printed.

Note that unlike interactive use, a bare “python” will use the latestversion of Python 2.x that you have installed. This is for backwardcompatibility and for compatibility with Unix, where the command pythontypically refers to Python 2.

3.8.1.4. From file associations¶

The launcher should have been associated with Python files (i.e. .py,.pyw, .pyc files) when it was installed. This means thatwhen you double-click on one of these files from Windows explorer the launcherwill be used, and therefore you can use the same facilities described above tohave the script specify the version which should be used.

The key benefit of this is that a single launcher can support multiple Pythonversions at the same time depending on the contents of the first line.

3.8.2. Shebang Lines¶

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 nativesupport for such lines and they are commonly used on such systems to indicatehow a script should be executed. This launcher allows the same facilities tobe used with Python scripts on Windows and the examples above demonstrate theiruse.

To allow shebang lines in Python scripts to be portable between Unix andWindows, this launcher supports a number of ‘virtual’ commands to specifywhich interpreter to use. The supported virtual commands are:

• /usr/bin/envpython

• /usr/bin/python

• /usr/local/bin/python

• python

For example, if the first line of your script starts with

The default Python will be located and used. As many Python scripts writtento work on Unix will already have this line, you should find these scripts canbe used by the launcher without modification. If you are writing a new scripton Windows which you hope will be useful on Unix, you should use one of theshebang 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).Furthermore the 32-bit version can be requested by adding “-32” after theminor version. I.e. /usr/bin/python2.7-32 will request usage of the32-bit python 2.7.

New in version 3.7: Beginning with python launcher 3.7 it is possible to request 64-bit versionby the “-64” suffix. Furthermore it is possible to specify a major andarchitecture without minor (i.e. /usr/bin/python3-64).

The /usr/bin/env form of shebang line has one further special property.Before looking for installed Python interpreters, this form will search theexecutable PATH for a Python executable. This corresponds to thebehaviour of the Unix env program, which performs a PATH search.

3.8.3. Arguments in shebang lines¶

The shebang lines can also specify additional options to be passed to thePython interpreter. For example, if you have a shebang line:

Then Python will be started with the -v option

3.8.4. Customization¶

3.8.4.1. Customization via INI files¶

Two .ini files will be searched by the launcher - py.ini in the currentuser’s “application data” directory (i.e. the directory returned by calling theWindows function SHGetFolderPath with CSIDL_LOCAL_APPDATA) and py.ini in thesame 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 overthe 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.

3.8.4.2. Customizing default Python versions¶

In some cases, a version qualifier can be included in a command to dictatewhich version of Python will be used by the command. A version qualifierstarts with a major version number and can optionally be followed by a period(‘.’) and a minor version specifier. Furthermore it is possible to specifyif a 32 or 64 bit implementation shall be requested by adding “-32” or “-64”.

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 environmentvariable PY_PYTHON can be set to specify the default versionqualifier. If it is not set, the default is “3”. The variable canspecify any value that may be passed on the command line, such as “3”,“3.7”, “3.7-32” or “3.7-64”. (Note that the “-64” option is onlyavailable with the launcher included with Python 3.7 or newer.)

If no minor version qualifiers are found, the environment variablePY_PYTHON{major} (where {major} is the current major version qualifieras determined above) can be set to specify the full version. If no such optionis found, the launcher will enumerate the installed Python versions and usethe latest minor release found for the major version, which is likely,although not guaranteed, to be the most recently installed version in thatfamily.

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 bepreferred. This will be true for both 32-bit and 64-bit implementations of thelauncher - a 32-bit launcher will prefer to execute a 64-bit Python installationof the specified version if available. This is so the behavior of the launchercan be predicted knowing only what versions are installed on the PC andwithout regard to the order in which they were installed (i.e., without knowingwhether a 32 or 64-bit version of Python and corresponding launcher wasinstalled last). As noted above, an optional “-32” or “-64” suffix can beused on a version specifier to change this behaviour.

Examples:

• If no relevant options are set, the commands python andpython2 will use the latest Python 2.x version installed andthe command python3 will use the latest Python 3.x installed.

• The commands python3.1 and python2.7 will not consult anyoptions at all as the versions are fully specified.

• If PY_PYTHON=3, the commands python and python3 will both usethe latest installed Python 3 version.

• If PY_PYTHON=3.1-32, the command python will use the 32-bitimplementation of 3.1 whereas the command python3 will use the latestinstalled Python (PY_PYTHON was not considered at all as a majorversion was specified.)

• If PY_PYTHON=3 and PY_PYTHON3=3.1, the commandspython and python3 will both use specifically 3.1

In addition to environment variables, the same settings can be configuredin the .INI file used by the launcher. The section in the INI file iscalled [defaults] and the key name will be the same as theenvironment variables without the leading PY_ prefix (and note thatthe key names in the INI file are case insensitive.) The contents ofan environment variable will override things specified in the INI file.

For example:

• Setting PY_PYTHON=3.1 is equivalent to the INI file containing:

• Setting PY_PYTHON=3 and PY_PYTHON3=3.1 is equivalent to the INI filecontaining:

3.8.5. Diagnostics¶

If an environment variable PYLAUNCH_DEBUG is set (to any value), thelauncher will print diagnostic information to stderr (i.e. to the console).While this information manages to be simultaneously verbose and terse, itshould allow you to see what versions of Python were located, why aparticular version was chosen and the exact command-line used to execute thetarget Python.

3.9. Finding modules¶

Python usually stores its library (and thereby your site-packages folder) in theinstallation directory. So, if you had installed Python toC:Python, the default library would reside inC:PythonLib and third-party modules should be stored inC:PythonLibsite-packages.

To completely override sys.path, create a ._pth file with the samename as the DLL (python37._pth) or the executable (python._pth) andspecify one line for each path to add to sys.path. The file based on theDLL name overrides the one based on the executable, which allows paths to berestricted for any program loading the runtime if desired.

When the file exists, all registry and environment variables are ignored,isolated mode is enabled, and site is not imported unless one line in thefile specifies importsite. Blank paths and lines starting with # areignored. Each path may be absolute or relative to the location of the file.Import statements other than to site are not permitted, and arbitrary codecannot be specified.

Note that .pth files (without leading underscore) will be processed normallyby the site module when importsite has been specified.

Py File Runner

When no ._pth file is found, this is how sys.path is populated onWindows:

• An empty entry is added at the start, which corresponds to the currentdirectory.

• If the environment variable PYTHONPATH exists, as described inEnvironment variables, its entries are added next. Note that on Windows,paths in this variable must be separated by semicolons, to distinguish themfrom the colon used in drive identifiers (C: etc.).

• Additional “application paths” can be added in the registry as subkeys ofSOFTWAREPythonPythonCore{version}PythonPath under both theHKEY_CURRENT_USER and HKEY_LOCAL_MACHINE hives. Subkeys which havesemicolon-delimited path strings as their default value will cause each pathto be added to sys.path. (Note that all known installers only useHKLM, so HKCU is typically empty.)

• If the environment variable PYTHONHOME is set, it is assumed as“Python Home”. Otherwise, the path of the main Python executable is used tolocate a “landmark file” (either Libos.py or pythonXY.zip) to deducethe “Python Home”. If a Python home is found, the relevant sub-directoriesadded to sys.path (Lib, plat-win, etc) are based on thatfolder. Otherwise, the core Python path is constructed from the PythonPathstored in the registry.

• If the Python Home cannot be located, no PYTHONPATH is specified inthe environment, and no registry entries can be found, a default path withrelative entries is used (e.g. .Lib;.plat-win, etc).

If a pyvenv.cfg file is found alongside the main executable or in thedirectory one level above the executable, the following variations apply:

• If home is an absolute path and PYTHONHOME is not set, thispath is used instead of the path to the main executable when deducing thehome location.

The end result of all this is:

• When running python.exe, or any other .exe in the main Pythondirectory (either an installed version, or directly from the PCbuilddirectory), the core path is deduced, and the core paths in the registry areignored. Other “application paths” in the registry are always read.

• When Python is hosted in another .exe (different directory, embedded via COM,etc), the “Python Home” will not be deduced, so the core path from theregistry is used. Other “application paths” in the registry are always read.

• If Python can’t find its home and there are no registry value (frozen .exe,some very strange installation setup) you get a path with some default, butrelative, paths.

For those who want to bundle Python into their application or distribution, thefollowing advice will prevent conflicts with other installations:

• Include a ._pth file alongside your executable containing thedirectories to include. This will ignore paths listed in the registry andenvironment variables, and also ignore site unless importsite islisted.

• If you are loading python3.dll or python37.dll in your ownexecutable, explicitly call Py_SetPath() or (at least)Py_SetProgramName() before Py_Initialize().

• Clear and/or overwrite PYTHONPATH and set PYTHONHOMEbefore launching python.exe from your application.

• If you cannot use the previous suggestions (for example, you are adistribution that allows people to run python.exe directly), ensurethat the landmark file (Libos.py) exists in your install directory.(Note that it will not be detected inside a ZIP file, but a correctly namedZIP file will be detected instead.)

These will ensure that the files in a system-wide installation will not takeprecedence over the copy of the standard library bundled with your application.Otherwise, your users may experience problems using your application. Note thatthe first suggestion is the best, as the others may still be susceptible tonon-standard paths in the registry and user site-packages.

• Adds ._pth file support and removes applocal option frompyvenv.cfg.

• Adds pythonXX.zip as a potential landmark when directly adjacentto the executable.

Modules specified in the registry under Modules (not PythonPath)may be imported by importlib.machinery.WindowsRegistryFinder.This finder is enabled on Windows in 3.6.0 and earlier, but may need tobe explicitly added to sys.meta_path in the future.

3.10. Additional modules¶

Even though Python aims to be portable among all platforms, there are featuresthat are unique to Windows. A couple of modules, both in the standard libraryand external, and snippets exist to use these features.

The Windows-specific standard modules are documented inMS Windows Specific Services.

3.10.1. PyWin32¶

The PyWin32 module by Mark Hammondis a collection of modules for advanced Windows-specific support. This includesutilities for:

• Component Object Model(COM)

• Win32 API calls

• Registry

• Event log

• Microsoft Foundation Classes (MFC)user interfaces

PythonWin is a sample MFC applicationshipped with PyWin32. It is an embeddable IDE with a built-in debugger.

See also

by Tim Golden

by David and Paul Boddie

3.10.2. cx_Freeze¶

cx_Freeze is a distutilsextension (see Extending Distutils) which wraps Python scripts intoexecutable Windows programs (*.exe files). When you have done this,you can distribute your application without requiring your users to installPython.

3.10.3. WConio¶

Since Python’s advanced terminal handling layer, curses, is restricted toUnix-like systems, there is a library exclusive to Windows as well: WindowsConsole I/O for Python.

WConio is a wrapper forTurbo-C’s CONIO.H, used to create text user interfaces.

3.11. Compiling Python on Windows¶

If you want to compile CPython yourself, first thing you should do is get thesource. You can download either thelatest release’s source or just grab a fresh checkout.

The source tree contains a build solution and project files for MicrosoftVisual Studio 2015, which is the compiler used to build the official Pythonreleases. 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

or “Creating Python extensions in C/C++ with SWIG and compiling them withMinGW gcc under Windows” or “Installing Python extension with distutilsand without Microsoft Visual C++” by Sébastien Sauvage, 2003

3.12. Other Platforms¶

With ongoing development of Python, some platforms that used to be supportedearlier are no longer supported (due to the lack of users or developers).Check PEP 11 for details on all unsupported platforms.

• Windows CE is still supported.

• The Cygwin installer offers to install the Pythoninterpreter as well (cf. Cygwin package source, Maintainer releases)

See Python for Windowsfor detailed information about platforms with pre-compiled installers.