问题
I have a subclass of queue.Queue like so:
class SetQueue(queue.Queue):
"""Queue which will allow a given object to be put once only.
Objects are considered identical if hash(object) are identical.
"""
def __init__(self, maxsize=0):
"""Initialise queue with maximum number of items.
0 for infinite queue
"""
super().__init__(maxsize)
self.all_items = set()
def _put(self):
if item not in self.all_items:
super()._put(item)
self.all_items.add(item)
I am trying to use mypy for static type checking. In this case, the SetQueue should take a generic object T. This is my attempt so far:
from typing import Generic, Iterable, Set, TypeVar
# Type for mypy generics
T = TypeVar('T')
class SetQueue(queue.Queue):
"""Queue which will allow a given object to be put once only.
Objects are considered identical if hash(object) are identical.
"""
def __init__(self, maxsize: int=0) -> None:
"""Initialise queue with maximum number of items.
0 for infinite queue
"""
super().__init__(maxsize)
self.all_items = set() # type: Set[T]
def _put(self, item: T) -> None:
if item not in self.all_items:
super()._put(item)
self.all_items.add(item)
mypy throws a warning on the class definition line saying "Missing type parameters for generic type".
I think that I need a Generic[T] somewhere but every attempt that I have made throws a syntax error. All of the examples in the docs show subclassing from Generic[T] but don't subclass from any other object.
Does anyone know how to define the generic type for SetQueue?
回答1:
The problem here is that queue.Queue does not actually not inherit from typing.Generic, but the typeshed stubs for it says that it does. This is a bit of a necessary evil until the stdlib fully buys into typing, if ever. As a result, the actual queue.Queue does not have the typing.GenericMeta metaclass that gives generic classes their __getitem__ ability at runtime:
For example, this code type-checks ok in mypy, but fails at runtime:
from typing import Generic, Iterable, Set, TypeVar, TYPE_CHECKING
import queue
# Type for mypy generics
T = TypeVar('T')
class SetQueue(queue.Queue[T]):
"""Queue which will allow a given object to be put once only.
Objects are considered identical if hash(object) are identical.
"""
def __init__(self, maxsize: int=0) -> None:
"""Initialise queue with maximum number of items.
0 for infinite queue
"""
super().__init__(maxsize)
self.all_items = set() # type: Set[T]
def _put(self, item: T) -> None:
if item not in self.all_items:
super()._put(item)
self.all_items.add(item)
my_queue = queue.Queue() # type: queue.Queue[int]
my_queue.put(1)
my_queue.put('foo') # error
my_set_queue = SetQueue() # type: SetQueue[int]
my_set_queue.put(1)
my_set_queue.put('foo') # error
The error raised is TypeError: 'type' object is not subscriptable, meaning that queue.Queue[T] (i.e. queue.Queue.__getitem__) is not supported.
Here's a hack to make it work at runtime as well:
from typing import Generic, Iterable, Set, TypeVar, TYPE_CHECKING
import queue
# Type for mypy generics
T = TypeVar('T')
if TYPE_CHECKING:
Queue = queue.Queue
else:
class FakeGenericMeta(type):
def __getitem__(self, item):
return self
class Queue(queue.Queue, metaclass=FakeGenericMeta):
pass
class SetQueue(Queue[T]):
"""Queue which will allow a given object to be put once only.
Objects are considered identical if hash(object) are identical.
"""
def __init__(self, maxsize: int=0) -> None:
"""Initialise queue with maximum number of items.
0 for infinite queue
"""
super().__init__(maxsize)
self.all_items = set() # type: Set[T]
def _put(self, item: T) -> None:
if item not in self.all_items:
super()._put(item)
self.all_items.add(item)
my_queue = queue.Queue() # type: queue.Queue[int]
my_queue.put(1)
my_queue.put('foo') # error
my_set_queue = SetQueue() # type: SetQueue[int]
my_set_queue.put(1)
my_set_queue.put('foo') # error
There may be a better way to patch in the metaclass. I'm curious to know if anyone comes up with a more elegant solution.
Edit: I should note that multiple inheritance did not work because class SetQueue(queue.Queue, Generic[T]) fails to relate SetQueue's T to queue.Queue's
来源：https://stackoverflow.com/questions/45414066/mypy-how-to-define-a-generic-subclass