incremental typing

Concise and very good explanation

code:python

def foo(x: int):

y: str = x # NG

error: Incompatible types in assignment (expression has type "int", variable has type "str") [assignment]

Explicitly state implicit Any

code:python

import typing

def foo(x: typing.Any):

y: str = x # OK

code:python

def foo(x: int):

y: typing.Any = x # OK

Difference between Any and object

code:python

def foo(x: int):

y: object = x # OK

code:python

def foo(x: object):

y: str = x # NG

error: Incompatible types in assignment (expression has type "object", variable has type "str") [assignment]

Unlike Any, object is just a top type, so trying to assign a value of type object to a value of type str (implicit downcast) is not allowed.

This is normal type behavior.

That Any is a special type that can implicitly perform both upcasts and downcasts.

No guessing the return type.

code:python

def foo(x: int):

return x

print(typing.reveal_type(foo))

note: Revealed type is "def (x: builtins.int) -> Any"

This is a different behavior from TypeScript

https://gyazo.com/485b1234b2e40fa0839bf9379e8ddce3

code:python

def foo(x: int) -> int:

return x

print(typing.reveal_type(foo))

note: Revealed type is "def (x: builtins.int) -> builtins.int"

It is expected that humans will explicitly state in this way

https://gyazo.com/9ea5aa1dec79c546747debfa28db2621

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