Pylance didn't honor return type from __new__ in special cases

[Brainor]

Environment data

• Language Server version: v2023.10.50
• OS and version:
  1. Windows 11 Pro, 22H2, 22621.2506
  2. Ubuntu 22.04.2 LTS
• Python version & distribution:
  1. 3.11.6, Anaconda
  2. 3.10.11, Anaconda

Code Snippet

class TestClass:
    def __new__(cls, message="") -> str:
        return "stub"


a1 = TestClass(12)  # typing: (variable) a1: TestClass
a2 = TestClass()  # typing: (variable) a2: str

Expected behavior

a1 and a2 are both string stub, and a1 and a2 should both be typed as (variable) a2: str

Actual behavior

a1 is typed as typing: (variable) a1: TestClass
a2 is typed as (variable) a2: str

According to #1092 , pyright checks return type from __new__ since version 2021.3.4, so I guess it might be a pylance issue.

[erictraut]

Pyright is working as intended here, so I don't consider it a bug.

The call expression TestClass(12) results in a type error because the message parameter has an inferred type of str based on the default argument value of "". Since pyright couldn't find any constructor signature that was compatible with the provided arguments, it emitted a type error and assumed that the return type of the constructor was TestClass.

If you want your constructor to support both str and int arguments for the message parameter, you can provide an explicit type annotation.

class TestClass:
    def __new__(cls, message: str | int="") -> str:
        return "stub"

[Brainor]

The call expression TestClass(12) results in a type error because the message parameter has an inferred type of str based on the default argument value of "". Since pyright couldn't find any constructor signature that was compatible with the provided arguments, it emitted a type error and assumed that the return type of the constructor was TestClass.

Thank you for your detailed explanation, and you are right, default argument value causes inferred type.

class TestClass:
    def __new__(cls, message: str | int="") -> str:
        return "stub"


a1 = TestClass('12')  # typing: (variable) a1: str
a2 = TestClass()  # typing: (variable) a2: str
a3 = TestClass(12) # typing: (variable) a3: str

gets the expected typing result. Lesson learned here. Closing this.