[attr-defined] for attribute defined in __new__

[Hocnonsense]

Bug Report

This bug caused by defind an attribute in new for a class. This bug had been reported in #982.

To Reproduce

class MyObject:
    """
    A file that is either input or output of a rule.
    """

    __slots__ = ["a"]

    def __new__(cls, text):
        obj = object.__new__(cls)
        obj.a = text

        return obj


mo = MyObject("")
mo.a

Expected Behavior

Success: no issues found in 1 source file

Actual Behavior

2023/08/03.py:16: error: "MyObject" has no attribute "a"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.4.1 (compiled: yes)
• Mypy command-line flags: mypy 2023/08/03.py
• Mypy configuration options from mypy.ini (and other config files): None (default)
• Python version used: Python 3.10.8

[SanjayManjunath]

Hey @Hocnonsense, I found out that 'a' should be initialized in the 'init' method since it's an instance attribute. Can you take a look at this and let me know if it's alright? https://mypy-play.net/?mypy=latest&python=3.10&gist=3e32e9112f3e5425c6aad2f8330e7ef8

[erictraut]

I think this is a duplicate of #1021.