Which Python implementation am I using?

Steven D'Aprano steve at pearwood.info
Fri Aug 7 19:27:04 CEST 2015

I have a need to determine which Python implementation is running. Starting
from Python 2.6, we have platform.python_implemention() which (at least in
principle) will do the right thing.

However, for my sins, I also need to support 2.4 and 2.5.

I have come up with this function to determine the Python implementation,
using platform.python_implementation when available, and if not, by trying
to detect the implementation indirectly.

import sys, platform

def implementation():
    """Return the Python implementation."""

    def jython():
        t = platform.java_ver()
        return (t and t[0]) or ('java' in sys.platform.lower())

    def ironpython():
        if sys.platform == 'cli':
            # Common Language Infrastructure == .Net or Mono.
            return True
        return 'ironpython' in sys.version.lower()

        return platform.python_implementation()
    except ValueError:
        # Work around a bug in some versions of IronPython.
        if ironpython():
            return 'ironpython'
    except AttributeError:
        # Python is too old! Probably 2.4 or 2.5.
        for func in (jython, ironpython):
            if func():
                return func.__name__
        # Otherwise, it's too hard to tell. Return a default.
        return 'python'

Is this the best way to detect Jython and IronPython when
python_implementation isn't available?

How about PyPy, Stackless, or others?

Is there a definitive test (other than python_implementation) for CPython
itself? I'd like to detect that specifically, and leave the default
python-with-no-c for those cases where I really am running some unknown
Python implementation.

Tests should be relatively lightweight, if possible.

Thanks in advance,


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