Why does inspect fail to get source file for classes in a dynamically-imported module?

Question

inspect.getsource() and inspect.getsourcefile() can access source info for a function, but not for a class, when they are in a module that is imported dynamically with importlib.

Here are two files, thing1.py and thing2.py:

thing1.py

import inspect
import os
import importlib.util

dir_here = os.path.dirname(__file__)
spec = importlib.util.spec_from_file_location("thing2",
                                              os.path.join(dir_here, "thing2.py"))
module = importlib.util.module_from_spec(spec)
spec.loader.exec_module(module)

print(module.foo(3))
print(module.Bar().inc(3))

print("module source file:", inspect.getsourcefile(module))
for attr in ['foo','Bar']:
    print("%s source: %s" % (attr, inspect.getsourcefile(getattr(module, attr))))
    print(inspect.getsource(getattr(module, attr)))

thing2.py

def foo(x):
    return x+1

class Bar(object):
    def inc(self, x):
        return x+1

If I run test1.py here's what I get:

> python c:\tmp\python\test2\thing1.py
4
4
module source file: c:\tmp\python\test2\thing2.py
foo source: c:\tmp\python\test2\thing2.py
def foo(x):
    return x+1

Traceback (most recent call last):
  File "c:\tmp\python\test2\thing1.py", line 16, in <module>
    print("%s source: %s" % (attr, inspect.getsourcefile(getattr(module, attr))))
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jason\.conda\envs\py3datalab\Lib\inspect.py", line 940, in getsourcefile
    filename = getfile(object)
               ^^^^^^^^^^^^^^^
  File "C:\Users\jason\.conda\envs\py3datalab\Lib\inspect.py", line 909, in getfile
    raise TypeError('{!r} is a built-in class'.format(object))
TypeError: <class 'thing2.Bar'> is a built-in class

I'm using Python 3.11.4.

Am I missing something during my import step that tells Python how to get source info for classes?

jonrsharpe · Accepted Answer · 2025-05-30 00:05:46Z

1

The logic for getting the source file for a class looks like this:

    if isclass(object):
        if hasattr(object, '__module__'):
            module = sys.modules.get(object.__module__)
            if getattr(module, '__file__', None):
                return module.__file__
            if object.__module__ == '__main__':
                raise OSError('source code not available')
        raise TypeError('{!r} is a built-in class'.format(object))

In your case module.Bar.__module__ is 'thing2', which has not been added to sys.modules. Hence this machinery concludes (incorrectly) that it must be built-in, and raises an error claiming as much.

Am I missing something during my import step that tells Python how to get source info for classes?

Yes; note that the recipe in the importlib docs includes an explicit step to update sys.modules, as exec_module doesn't do it:

import importlib.util
import sys


def import_from_path(module_name, file_path):
    spec = importlib.util.spec_from_file_location(module_name, file_path)
    module = importlib.util.module_from_spec(spec)
    sys.modules[module_name] = module
    spec.loader.exec_module(module)
    return module

Similarly adding sys.modules["thing2"] = module into thing1.py would allow it to show the class implementation.

edited May 30 at 0:05

answered May 29 at 23:50

jonrsharpe

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2 Comments

Jason S May 29 at 23:54

ah, thanks. I got partway towards the same conclusion but you beat me to it. Is there any reason inspect requires a module to be added to sys.modules? I realize I can make the module name anything I want (__gesundheit__.thing2 for example), to avoid collisions, but it seems funny to put this into the global sys.modules if I'm just inspecting objects from a module loaded dynamically by path.

jonrsharpe May 30 at 0:16

It has to get back from the class to the module it came from somehow, and given that the only thing available on Bar is the name of the module, as a string, sys.modules is generally where you'd do that. I don't think inspect offers an API where you provide the module, and inspect.getmodule(module.Bar) is None. It's unfortunate that this is showing up as a "built-in class", though; even the OSError('source code not available') branch would be less confusing.

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