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[red-knot] Add redundant-cast error (#17100 )

## Summary

Following up from earlier discussion on Discord, this PR adds logic to
flag casts as redundant when the inferred type of the expression is the
same as the target type. It should follow the semantics from
[mypy](https://github.com/python/mypy/pull/1705).

Example:

```python
def f() -> int:
    return 10

# error: [redundant-cast] "Value is already of type `int`"
cast(int, f())
```

2025-04-01 00:37:25 +00:00

1.2 KiB

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`cast`

cast() takes two arguments, one type and one value, and returns a value of the given type.

The (inferred) type of the value and the given type do not need to have any correlation.

from typing import Literal, cast, Any

reveal_type(True)  # revealed: Literal[True]
reveal_type(cast(str, True))  # revealed: str
reveal_type(cast("str", True))  # revealed: str

reveal_type(cast(int | str, 1))  # revealed: int | str

reveal_type(cast(val="foo", typ=int))  # revealed: int

# error: [invalid-type-form]
reveal_type(cast(Literal, True))  # revealed: Unknown

# error: [invalid-type-form]
reveal_type(cast(1, True))  # revealed: Unknown

# error: [missing-argument] "No argument provided for required parameter `val` of function `cast`"
cast(str)
# error: [too-many-positional-arguments] "Too many positional arguments to function `cast`: expected 2, got 3"
cast(str, b"ar", "foo")

def function_returning_int() -> int:
    return 10

# error: [redundant-cast] "Value is already of type `int`"
cast(int, function_returning_int())

def function_returning_any() -> Any:
    return "blah"

# error: [redundant-cast] "Value is already of type `Any`"
cast(Any, function_returning_any())

1.2 KiB Raw Blame History

cast

1.2 KiB

Raw Blame History

`cast`