Inspecting function annotations in Python

Feb 2, 2020

For this article, I'm just going to spend some time in the python shell. Please let me know if this is helpful or terrible 😄.

>>> import inspect
>>> # Let's learn a little about the inspect module
>>> inspect.__doc__.splitlines()[0]
'Get useful information from live Python objects.'

>>> inspect.signature.__doc__
'Get a signature object for the passed callable.'

>>> inspect.Signature.__doc__.splitlines()[0]
'A Signature object represents the overall signature of a function.'

>>> # Let's inspect a simple function
>>> def add(x, y):
...     return x + y

>>> add_sig = inspect.signature(add)
>>> add_sig
<Signature (x, y)>

>>> add_sig.parameters['x']
<Parameter "x">

>>> add_sig.parameters['x'].annotation
<class 'inspect._empty'>

>>> add_sig.return_annotation
<class 'inspect._empty'>

>>> # If the function has more information
>>> # we can inspect more information
>>> def typed_add(x: int, y: float) -> float:
...     return x + y

>>> typed_add_sig = inspect.signature(typed_add)
>>> typed_add_sig.parameters['x'].annotation
<class 'int'>

>>> typed_add_sig.return_annotation
<class 'float'>

>>> # Let's pretend that we're pytest
>>> # and implement a baby version of fixtures
>>> def test_42_is_the_answer(the_answer):
...     assert the_answer == 42

>>> fixtures = {"the answer": 42}

>>> def fake_test_run(test_function):
...     test_function_parameters = inspect.signature(test_function).parameters.values()
...
...     if any(parameter.name not in fixtures for parameter in test_function_parameters):
...         raise Exception(f"Test function expected expected fixture named {name}")
...
...     arguments_to_pass = {
...         parameter.name: fixtures[parameter.name]
...         for parameter in test_function_parameters
...     }
...
...     try:
...         test_function(**arguments_to_pass)
...     except AssertionError as ex:
...         print("Fail: Test Failed")
...         print(ex)
...     except Exception as ex:
...         print("Error: Something bad happened")
...         print(ex)
...     else:
...         print("Pass!")

>>> fake_test_run(test_42_is_the_answer)
Pass!

>>> fixtures["the_answer"] = 2345

>>> fake_test_run(test_42_is_the_answer)
Fail: Test Failed

>>> # Goal!
>>> # Our test function is *magically* getting passed an argument
>>> # based on our function signature.