How to know if python script was run using interpreter's -m option?

Question

I couldn't find answer after having read all the following:

• PEP 338 Executing modules as scripts
• documentation of runpy standard module
• description of Python interpreter's -m option

Rationale:
When a test script which uses relative imports is being run without -m option I could print a warning message instead of leaving user with standard traceback leading to ValueError: Attempted relative import in non-package exception. Without knowing this I can catch this exception and only suggest lack of -m option could be the reason of error.

Answer 1

Disclaimer: this is just an observation, I have not seen it in the docs so it is probably implementation dependent and might not be consistent across different Python versions.

I have noticed that when calling a script using a -m option a variable called __loader__ is added to the namespace, so at the top of your script you could check for existence of that variable:

if '__loader__' in globals():
    # called with -m

For some extra safety you could check to see if __loader__ is an instance of pkgutil.ImpLoader:

if '__loader__' in globals() and __loader__.__class__.__name__ == 'ImpLoader':

Answer 2

Another observation is that __package__ is set to None when executing the script directly and to the package name when using -m (using the empty string when the module isn't included in any package, so it's still different from None).