Invoking Python

While this is not a change in Python 3, the transition increased the number of systems that have more than one Python interpreter installed: it is not uncommon for python, python2, python3, python3.6 and python3.9 to all be valid system commands; other interpreters may be installed in non-standard locations.

This makes it important to use the correct command for each situation.

Current interpreter

The current Python interpreter should be invoked via sys.executable.

Python provides the path of the currently running interpreter as sys.executable. This variable should be preferred over python or other hard-coded commands.

For example, rather than:

subprocess.Popen('python', '')


subprocess.Popen(sys.executable, '')

The assumption that 'python' is correct is only valid in tightly controlled environments; however, even in those environments sys.executable is likely to be correct.

The documentation does include a warning:

If Python is unable to retrieve the real path to its executable, sys.executable will be an empty string or None.

In practice, this does not apply to mainstream platforms. If sys.executable is unusable, then either your platform’s concept of launching a process via filename is somehow unusual (and in this case you should know what to do), or there’s an issue in Python itself.

Unix shebangs

On Unix, executables written in Python must have a shebang line identifying the interpreter. The correct shebang to use will depend on the environment you are targeting and on the version compatibility of the project.

General recommendations for Python shebangs are listed in the For Python script publishers section of PEP 394.