incremental typing

Gradual Typing Concise and very good explanation

• What is Gradual Typing | Jeremy Siek
• Japanese translation What is Gradual Typing: Gradual Typing - Qiita.

https://mypy-play.net/?mypy=latest&python=3.11 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 python

import typing

def foo(x: typing.Any):
    y: str = x  # OK

python

def foo(x: int):
    y: typing.Any = x  # OK

• Difference between Any and object python

def foo(x: int):
    y: object = x  # OK

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. 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
  • 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

The Expression Problem

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