Correct typing for list of ints-CodePudding

Consider the following code in file x.py

from typing import List, Optional

my_list: List[Optional[int]] = list()
for i in range(7):
    my_list.append(i)
my_list = sorted(my_list)
if (len(my_list) > 0):
    my_list.append(None)
# do something with my_list

The command mypy x.py gives the following error:

x.py:6: error: Value of type variable "SupportsRichComparisonT" of "sorted" cannot be "Optional[int]"

So I changed x.py to the following:

from typing import List

my_list: List[int] = list()
for i in range(7):
    my_list.append(i)
my_list = sorted(my_list)
if (len(my_list) > 0):
    my_list.append(None)
# do something with my_list

But now mypy x.py outputs

x.py:8: error: Argument 1 to "append" of "list" has incompatible type "None"; expected "int"

So how do I annotate the code correctly for mypy to not complain (of course both versions behave exactly the same during runtime and work as expected)?

The "None"-element at the end of the list is needed in the "do something"-part of the script (which is not relevant for the problem and therefore not shown here)

CodePudding user response：

Instead of assigning the sorted list back to the original variable my_list, assign it to a new variable which mypy will allow to hold different types.

my_list: List[int] = list()
for i in range(7):
    my_list.append(i)
# change here
sorted_list: List[Any] = sorted(my_list)
if len(sorted_list) > 0:
    sorted_list.append(None)

CodePudding user response：

Based on crunker99's answer that correctly pointed out the new list I was able to come up with this code that satisfies mypy:

from typing import List, Optional

my_list: List[int] = list()
for i in range(7):
    my_list.append(i)
my_sorted_list: List[Optional[int]] = list(sorted(my_list))
if (len(my_sorted_list) > 0):
    my_sorted_list.append(None)
# do something with my_sorted_list