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Currently mypy complains about missing return here and adding
return None
in the end of the function fixes that. However I think that's undesirable:
It's not actually catching a bug: it's a false positive. False positives are bad as they lead to lost time and confusion
It motivates people to put a useless
return None
line (even
return
doesn't work) which is just noise in the program code in this case
It invalidates core Python behavior: since the dawn of time, no return,
return
and
return None
mean absolutely the same in each function, but mypy only recognizes one of those forms in this case.
Obviously that seems like a simple example, but I have a longer if/elif function where mypy just says
missing return on <first line of function>
which has two issues : it's not a type bug, and mypy doesn't the invalid branch
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Potential false positive error of "Missing return statement" with Optional[NoReturn] typehint
#9312
For anyone looking at this later, I think this is what they were talking about:
Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as
return None
, and an explicit return statement should be present at the end of the function (if reachable):
# Correct:
def foo(x):
if x >= 0:
return math.sqrt(x)
else:
return None
def bar(x):
if x < 0:
return None
return math.sqrt(x)
# Wrong:
def foo(x):
if x >= 0:
return math.sqrt(x)
def bar(x):
if x < 0:
return
return math.sqrt(x)
If this behavior is explicitly desired, then there should be a clearer error message. For example instead of
Missing return statement
it should say:
Add explicit return statement to all code paths
