Python Windows Launcher Usage
Python Windows Launcher Usage
On Windows (only), Python 3.3 and later install a script launcher, also available separately for earlier versions. This launcher consists of the executables py.exe (console) and pyw.exe (nonconsole), which can be invoked without PATH settings; are registered to run Python files via filename associations; and allow Python versions to be selected in three ways — with “#!” Unix-like directives at the top of scripts, with command-line arguments, and with configurable defaults.
Launcher File Directives
The launcher recognizes “#!” lines at the top of script files that name Python versions in one of the following forms, in which * is either: empty to use the default version (currently 2 if installed and similar to omitting a “#!” line); a major version number (e.g., 3) to launch the latest version in that line installed; or a complete major.minor specification, optionally suffixed by −32 to prefer a 32-bit install (e.g., 3.1–32):
- !/usr/bin/env python* #!/usr/bin/python* #!/usr/local/bin/python* #!python*
Any Python (python.exe) arguments may be given at the end of the line, and Python 3.4 and later may consult PATH for “#!” lines that give just python with no explicit version number.
Launcher Command Lines
The launcher may also be invoked from a system shell with command lines of the following form:
py [pyarg] [pythonarg*] script.py [scriptarg*]
More generally, anything that may appear in a python command after its python component may also appear after the optional pyarg in a py command, and is passed on to the spawned Python verbatim. This includes the -m, -c, and - program specification forms; see Python Command-Line Usage.
The launcher accepts the following argument forms for its optional pyarg, which mirror the * part at the end of a file’s “#!” line:
−2 Launch latest 2.X version installed -3 Launch latest 3.X version installed -X.Y Launch specified version (X is 2 or 3) -X.Y−32 Launch the specified 32-bit version
If both are present, command-line arguments have precedence over values given in “#!” lines. As installed, “#!” lines may be applied in more contexts (e.g., icon clicks).
Launcher Environment Variables
The launcher also recognizes optional environment variable settings, which may be used to customize version selection in default or partial cases (e.g., missing or major-only “#!” or py command argument):
PY_PYTHON Version to use in default cases (else 2) PY_PYTHON3 Version to use in 3 partials (e.g., 3.2) PY_PYTHON2 Version to use in 2 partials (e.g., 2.6)
These settings are used only by launcher executables, not when python is invoked directly.