python / python/mypy

False "type assignment error" when assigning an instance of `Field` class or classes extended from it.

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bug
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Descripción

Bug Report
1- As you must know, there is no standard way in python to use decorators for defining metadata for a field of a class and the only possible approach is assigning an instance of Field class from dataclass module to the field. I believe it's weird and irrational but anyway it's now the standard defined in pep 0557 so we expect mypy to follow it.

2- there was an issue that describe mypy raising a false "Incompatible types in assignment" error in the simple following code:

@dataclass
class A:
    x: Optional[List[int]] = field(default_factory=list) 

3- It seems Sometime around Oct 2023, the problem solved but unfortunately as I can see it solved just in an special condition. in the other words, just when using the field factory method.

To Reproduce
If you remove the # type: ignore[assignment] directive at the end of the following code:

@dataclass
class TerminalArgs(ArgsDataClass):
    tab_width: int = ArgField(
        cmd_name=["--tab-width"],
        type_parser=argtypes.positive_int,
        default=4,
        help="Number of spaces to use for a tab character.",
    )# type: ignore[assignment]

(Also ArgField is accessible here)

mypy will raise errors like:

terminaltexteffects/utils/terminal.py:21: error: Incompatible types in assignment (expression has type "ArgField", variable has type "int")  [assignment]
terminaltexteffects/utils/terminal.py:27: error: Incompatible types in assignment (expression has type "ArgField", variable has type "bool")  [assignment]
terminaltexteffects/utils/terminal.py:33: error: Incompatible types in assignment (expression has type "ArgField", variable has type "bool")  [assignment]
terminaltexteffects/utils/terminal.py:39: error: Incompatible types in assignment (expression has type "ArgField", variable has type "int")  [assignment]
terminaltexteffects/utils/terminal.py:45: error: Incompatible types in assignment (expression has type "ArgField", variable has type "float")  [assignment]

Expected Behavior
Allow assigning instance of Field class or instance of any class extended from it to fields of a dataclass.

Actual Behavior
Raising a false Incompatible types in assignment error

Your Environment

  • Mypy version used: mypy 1.9.0 (compiled: yes)
  • Mypy command-line flags: nothing special
  • Mypy configuration options from mypy.ini (and other config files): nothing special
  • Python version used: Python 3.11.2

Guía de contribución

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  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

Línea de trabajo

Comienza con el uso de dataclass en terminaltexteffects/utils/terminal.py y la definición de ArgField en terminaltexteffects/utils/argsdataclass.py, y reproduce los errores de asignación con mypy. Rastrea el manejo que hace mypy de dataclasses.Field y sus subclases; el trabajo estará terminado cuando las asignaciones mostradas de Field y ArgField ya no produzcan errores de asignación falsos.

Escrito por el modelo de indexación a partir del texto del issue.

Evaluación

Stack tecnológico
python
Área
devtools
Tipo de issue
Error
Dificultad
4/5
Tiempo estimado
3-5 días
Estado de actividad
Estancado
Claridad
Bastante claro
Aptitud para principiantes
42/100

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