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?
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
