[ins] In [63]: class A:pass

[ins] In [64]: import inspect

[ins] In [65]: inspect.getsource(A)

[snip]

/usr/local/Cellar/python@3.9/3.9.1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/inspect.py in getfile(object)
    664             if getattr(module, '__file__', None):
    665                 return module.__file__
--> 666         raise TypeError('{!r} is a built-in class'.format(object))

The error is 'X is a built-in class', instead it should be OSError, source code is not available.