I have class member which is either None
or a dict
.
I have a method for the lengths of that member that returns None
if the
member is None
or the size of the dictionary if it exists:
class A:
def __init__(self, a: bool = False) -> None:
self.has_a = a
self.prop_a = {"a" : 1} if a else None
def length_prop(self) -> int:
length_prop = None
if self.has_a:
length_prop = len(self.prop_a)
return length_prop
If I check the typing with mypy
:
test.py:10: error: Argument 1 to "len" has incompatible type "Optional[Dict[str, int]]"; expected "Sized"
length_prop = len(self.prop_a)
^
test.py:11: error: Incompatible return value type (got "Optional[int]", expected "int")
return length_prop
Very understandable, since it does not know if self.prop_a
is really dict
.
How can I design my code to get around this and get the right typing?
There are two issues:
mypy
can't and shouldn't infer the relationship between has_a
and prop_a
across the functions. You need to include an explicit check for None
inside the length_prop
function. This error is not spurious -- for example, what happens if some external code modifies the value of has_a
but not prop_a
?
The simplest solution is to do away with the has_a
field, since all the information it encodes can be retrieved using a check on prop_a
(for example, prop_a is not None
) instead.
The return value of length_prop
can be None
, so use Optional[int]
rather than int
for the length_prop
method.
The following code snippet resolves both of these issues. mypy
does not find any errors in this class definition:
from typing import Optional
class A:
def __init__(self, a: bool = False) -> None:
self.prop_a = {"a" : 1} if a else None
def length_prop(self) -> Optional[int]:
length_prop = None
if self.prop_a is not None:
length_prop = len(self.prop_a)
return length_prop