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ViewsHow to type hint a generator in Python 3?
According to PEP-484, we should be able to type hinting a generator function as follows:
from typing import Generator
def generate() -> Generator[int, None, None]:
for i in range(10):
yield i
for i in generate():
print(i)
However, the list comprehension gives the following error in PyCharm.
Expected 'collections.Iterable', got 'Generator[int, None, None]' instead less... (⌘F1)
Any idea why PyCharm is considering this as error?
A few clarification after reading some answers. I am using PyCharm Community Edition 2016.3.2 (the latest version) and have imported the typing.Generator (updated in the code). The above code runs just fine, but PyCharm considers this an error:
So, I'm wondering if this is actually an error or an unsupported feature in PyCharm.
2 answers
Answer question0
You need to import the typing module. As per docs:
The return type of generator functions can be annotated by the generic type
Generator[yield_type, send_type, return_type]provided bytyping.pymodule
Try this way instead:
from typing import Generator
def generate() -> Generator[int, None, None]:
for i in range(10):
yield i
The above will have the desired result:
l = [i for i in generate()]
Output:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
As pointed out in the comments, you might not use the last version of PyCharm. Try switching to version 2016.3.2 and you might be fine. Unfortunately this is a well-known bug, as per @AshwiniChaudhary comment.
More, the reported issue (for the last version of PyCharm) was submitted on December, last year. They probably fixed it and pushed the modifications into the same version.
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This isn't a direct answer to the question, but I think it is a better solution.
I'm using the typing specification below, using Iterator[int] instead of Generator. The validation is OK. I think it is a lot clearer. It better describes the code intention and is recommended by Python docs.
from typing import Iterator
def generate() -> Iterator[int]:
for i in range(10):
yield i
It would also allow future refactorings if you change your Generator for a list or other iterable.
I'm using Visual Studio Code with PyLance for typing validation. PyCharm mypy should have the same behavior.