[Python] mypy: Why is "int" a subtype of "float"?

Why does "mypy" consider "int" as a subtype of "float"? A subtype shall support all methods of its supertype, but "float" has methods, which "int" does not support:

test.py:

def f(x : float) -> bool:
return x.is_integer()

print(f(123.0))
print(f(123))


The static type checker accepts passing an "int" argument for a "float" parameter:

(3.8.1) myhost% mypy test.py
Success: no issues found in 1 source file


But this does not guarantee, that there are no errors at runtime:

(3.8.1) myhost% python test.py
True
Traceback (most recent call last):
File "test.py", line 5, in <module>
print(f(123))
File "test.py", line 2, in f
return x.is_integer()
AttributeError: 'int' object has no attribute 'is_integer'


because "float" has additional methods, which "int" does not have.

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