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

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?

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.