Better error message for incompatible type assignment when using Generics

[rahul-kamat]

• Are you reporting a bug, or opening a feature request?

Feature Request

• Please insert below the code you are checking with mypy.

from typing import Generic, TypeVar


T = TypeVar("T", int, float)


class Foo(Generic[T]):
    def __init__(self, val: T):
        self.data: T = val 


def bar(x: T) -> Foo[T]:
    return Foo(x)


x = bar(1)
x = bar(2.0) # error thrown by mypy

• What is the actual behavior/output?
  mypy throws this error:

error: Argument 1 to "bar" has incompatible type "float"; expected "int"

• What is the behavior/output you expect?

This error message is thrown because of the combination of assigning a different type to x and using Generic. The error message is unclear because it indicates a type error on the argument passed to bar and can be interpreted as bar not accepting arguments of type float.

Having a different error message that mentions "Incompatible types in assignment" would be clearer.
A message to make it clearer that expression has type Foo[float], variable has type Foo[int]

• What are the versions of mypy and Python you are using?

mypy 0.770 and Python 3.7.6

• What are the mypy flags you are using? (For example --strict-optional)

None

[ethanhs]

I do think the diagnostic could be better. Perhaps something like:

error: Argument 1 to "bar" has incompatible type "float"; expected "int" due to target type "Foo[int]"

[sanlf]

Hi, I'm new to open source contribution. Could I try to solve this issue with the message @ethanhs mentioned?

[ethanhs]

@sanlf Welcome! I think the specifics of the output of the diagnostic need more discussion. I would recommend maybe trying to tackle another issue.

[sanlf]

@ethanhs Understood. I will try to find another good-first-issue.

[mental32]

@eternal861 How about something more conversational?

error: expected "{expected_type}" due to the target type "{name}: {target_type}"; {failure_reason}

Would expand to

error: expected "int" due to the target type "x: Foo[int]"; Argument 1 to "bar" produces incompatible type "Foo[float]"

Disclaimer: I'm new to the codebase/conventions used in mypy but looking to contribute where I can

[himanshu007-creator]

@eternal861 Sir, i would like to suggest
error: In line (eg 52) bar(Argument), Argument expected as "int" got "float"

[himanshu007-creator]

• Are you reporting a bug, or opening a feature request?

Feature Request

• Please insert below the code you are checking with mypy.

from typing import Generic, TypeVar


T = TypeVar("T", int, float)


class Foo(Generic[T]):
    def __init__(self, val: T):
        self.data: T = val 


def bar(x: T) -> Foo[T]:
    return Foo(x)


x = bar(1)
x = bar(2.0) # error thrown by mypy

• What is the actual behavior/output?
  mypy throws this error:

error: Argument 1 to "bar" has incompatible type "float"; expected "int"

• What is the behavior/output you expect?

This error message is thrown because of the combination of assigning a different type to x and using Generic. The error message is unclear because it indicates a type error on the argument passed to bar and can be interpreted as bar not accepting arguments of type float.

Having a different error message that mentions "Incompatible types in assignment" would be clearer.
A message to make it clearer that expression has type Foo[float], variable has type Foo[int]

• What are the versions of mypy and Python you are using?

mypy 0.770 and Python 3.7.6

• What are the mypy flags you are using? (For example --strict-optional)

None

can you tell the exact file location to be modified?

[srbhss]

Interested in working on this issue. @ethanhs can you please assign me to this issue.

[SwagatSBhuyan]

Hi, I'd like to tackle this issue. I am still new to the whereabouts of different scripts in the repository, but I guess the error message can be modified in the messages.py script. It would be really great if someone guided me as to where the rest of the important function definitions are, such as isinstance, outer_context, etc. @ethanhs