Is there any way to tell if a function object was a lambda or a def?

Consider the two functions below:

def f1():
    return "potato"

f2 = lambda: "potato"
f2.__name__ = f2.__qualname__ = "f2"

Short of introspecting the original source code, is there any way to detect that f1 was a def and f2 was a lambda?

>>> black_magic(f1)
"def"
>>> black_magic(f2)
"lambda"

Solution

You could check the code object's name. Unlike the function's name, the code object's name cannot be reassigned. A lambda's code object's name will still be '<lambda>':

>>> x = lambda: 5
>>> x.__name__ = 'foo'
>>> x.__name__
'foo'
>>> x.__code__.co_name
'<lambda>'
>>> x.__code__.co_name = 'foo'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: readonly attribute

It is impossible for a def statement to define a function whose code object's name is '<lambda>'. It is possible to replace a function's code object after creation, but doing so is rare and weird enough that it's probably not worth handling. Similarly, this won't handle functions or code objects created by manually calling types.FunctionType or types.CodeType. I don't see any good way to handle __code__ reassignment or manually-created functions and code objects.