False "type assignment error" when assigning an instance of `Field` class or classes extended from it.
Nobody has claimed this yet.
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
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
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the dataclass usage in terminaltexteffects/utils/terminal.py and the ArgField definition in terminaltexteffects/utils/argsdataclass.py, then reproduce the assignment errors with mypy. Trace mypy's dataclass handling for dataclasses.Field and subclasses; done means the shown Field and ArgField assignments no longer produce false assignment errors.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100