Consider the following doc containing doctests.
def f(x):
"""
Increments by one.
>>> f(2)
3
"""
return x + 1
PyCharm's test runner (as seen here) picks the doctest up.
But the following is not seen by PyCharm as a doctest (though it is seen by pytest --doctest-modules -v for example).
f = lambda x: x + 1
f.__doc__ = """
Increments by one.
>>> f(2)
3
"""
Per comments, it looks like PyCharm expects all the tests to be indented at least one level (even in cases where it's the module level docstring, which need not be indented at all, it seems to rely on that level of indentation). This is not a requirement of regular doctests (python3 -mdoctest -v modulename.py works just fine), but PyCharm's gotta be difficult.
Your manually assigned docstring has no indentation, and PyCharm appears to ignore it as a result. Changing it to:
f = lambda x: x + 1
f.__doc__ = """
Increments by one.
>>> f(2)
3
"""
to make the string include one level of indentation seems to be enough to get PyCharm on board.
