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.

What do you need it for? Could you give some example or a test case in which this need becomes apparent? — jsalonen
– jsalonen, Commented Dec 1, 2011 at 21:46

Andrew Clark · Accepted Answer · 2011-12-01 22:02:29Z

3

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':

edited Dec 1, 2011 at 22:02

answered Dec 1, 2011 at 21:57

Andrew Clark

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5 Comments

Piotr Dobrogost Over a year ago

This is in the docs of runpy module; The special global variables __name__, __file__, __loader__ and __package__ are set in the globals dictionary before the module code is executed (Note that this is a minimal set of variables - other variables may be set implicitly as an interpreter implementation detail). However __loader__ is kind of generic and we never know what else might set it someday...

Piotr Dobrogost Over a year ago

@RaymondHettinger Is there any reason there's no really unique global variable set in this situation?

Raymond Hettinger Over a year ago

The only guaranteed unique variables are None and __debug__; however, any double underscored name is treated as reserved for creation by core developers, so __loader__ is somewhat safe (or at least as safe as __name__=='__main__'). If it keeps you up at night, it is alway possible to introspect the object to validate the match.

Piotr Dobrogost Over a year ago

I'm curious why you didn't use isinstance?

Andrew Clark Over a year ago

@PiotrDobrogost - pkgutil.ImpLoader is not a part of the namespace, it would need to be imported first. That is another option though.

jcollado · Accepted Answer · 2011-12-01 22:00:48Z

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).

answered Dec 1, 2011 at 22:00

jcollado

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1 Comment

Piotr Dobrogost Over a year ago

Nice! It's not only observation as this is mandated by PEP 366 Main module explicit relative imports.

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