I've got the following function, which given a string of the form 'a-02/b-03/foobarbaz_c-04'
, will extract the digits after a, b and c. The issue is that, for my use case, the input strings may not contain c, such that there will be no digits to extract.
Here's the code:
from typing import Tuple, Optional
def regex_a_b_c(name: str) -> Tuple[int, int, Optional[int]]:
a_b_info = re.search('a-(\d\d)/b-(\d\d)/', name)
a, b = [int(a_b_info.group(x)) for x in range(1, 3)]
c_info = re.search('c-(\d\d)', name)
if c_info:
c = int(c_info.group(1))
else:
c = None
return a, b, c
The issue I have is that, despite trying to make it clear that the last return argument is an Optional[int]
, I can't get my linter to stop complaining about the variable c.
I get a warning at the line c = None
that says:
Incompatible types in assignment (expression has type None, variable has type int)
How can I solve the issue?
If you don't annotate a variable, mypy will infer its type based on the very first assignment it sees.
So in this case, the line c = int(_info.group(1))
appears first, so mypy decides that the type must be int
. It then subsequently complains when it sees c = None
.
One way of working around this limitation is to just forward-declare the variable with the expected type. If you are using Python 3.6+ and can use variable annotations, you can do so like this:
c: Optional[int]
if c_info:
c = int(c_info.group(1))
else:
c = None
Or perhaps more concisely, like this:
c: Optional[int] = None
if c_info:
c = int(c_info.group(1))
If you need to support older versions of Python, you can annotate the type using the comment-based syntax, like so:
c = None # type: Optional[int]
if c_info:
c = int(c_info.group(1))
rje's suggestion of doing:
if c_info:
c = int(c_info.group(1))
return a, b, c
else:
return a, b, None
...is also a reasonable one.