Description
- Are you reporting a bug, or opening a feature request? Bug
The basic context here is I need to map data into different shapes using various Mappings, and then write those shapes to different targets using various Writers. I want to ensure that when I define a job configuration, I get a Mapping and Writer that are compatible with one another (i.e. they map to and write the same shape).
Initially I tried returning a Tuple[Mapping[E], Writer[E]], but found that mypy does not report an error when I deliberately mismatch a Mapping and Writer. Fortunately, however, an error is raised when I define a (data)class to hold these two objects instead of just a tuple. I think using a dataclass here is probably a good idea regardless (for reader comprehension purposes), but I would nevertheless expect the tuple implementation to behave the same way. It seems like someone could try the tuple implementation, not see any errors, and incorrectly assume they have a typechecking safety net that isn't actually there.
from dataclasses import dataclass
from typing import Generic, Tuple, TypeVar
from typing_extensions import Protocol
M = TypeVar("M", covariant=True)
class Mapping(Protocol[M]):
def map(self) -> M:
...
W = TypeVar("W", contravariant=True)
class Writer(Protocol[W]):
def write(self, shape: W):
...
class Circle:
pass
class CircleMapping:
def map(self) -> Circle:
return Circle()
class CircleWriter:
def write(self, shape: Circle):
print("in CircleWriter.write")
class Square:
pass
class SquareMapping:
def map(self) -> Square:
return Square()
class SquareWriter:
def write(self, shape: Square):
print("in SquareWriter.write")
E = TypeVar("E")
## Tuple strategy
Tools = Tuple[Mapping[E], Writer[E]]
def get_tools(shape_name: str) -> Tools:
if shape_name == "square":
return (SquareMapping(), CircleWriter()) # mypy does not flag an error here!
else:
return (CircleMapping(), CircleWriter())
def run_1(tools: Tools):
(mapping, writer) = tools
shape = mapping.map()
writer.write(shape)
## Dataclass strategy
@dataclass
class EtlTools(Generic[E]):
mapping: Mapping[E]
writer: Writer[E]
def get_etl_tools(shape_name: str) -> EtlTools:
if shape_name == "square":
return EtlTools(mapping=SquareMapping(), writer=CircleWriter()) # mypy error, cannot infer type argument 1 of EtlTools
else:
return EtlTools(mapping=CircleMapping(), writer=CircleWriter())
def run_2(etl_tools: EtlTools):
shape = etl_tools.mapping.map()
etl_tools.writer.write(shape)-
What is the actual behavior/output?
Mypy only errors on the dataclass implementation, not the tuple implementation -
What is the behavior/output you expect?
The tuple implementation should report an error as well -
What are the versions of mypy and Python you are using?
mypy = "==0.720"
python version 3.7.4 -
What are the mypy flags you are using? (For example --strict-optional)
None