Non-optional send types and the Generator.send() parameter

Bug Report

Python doesn’t allow you to send a non-None value to a just-started generator, but if the send type of a generator is not annotated Optional then .send(None) can’t be used.

You could just use next() at the start, but a situation can be created where Python and Mypy don’t agree on a type error. Pyright behaves similarly.

I’m not sure what the solution is here. This issue might require discussion.

To Reproduce

$ cat file1.py
from collections.abc import Generator

def g() -> Generator[int, int, None]:
    sent = 0
    while True:
        sent = yield sent

gen = g()
print(gen.send(0))  # <--
print(gen.send(1))
print(gen.send(2))
$ mypy --strict file1.py
Success: no issues found in 1 source file
$ python3 file1.py
Traceback (most recent call last):
  File "/Users/danpro/Desktop/fold/file1.py", line 10, in <module>
    print(gen.send(0))  # <--
TypeError: can't send non-None value to a just-started generator
$ 
$ 
$ cat file2.py
from collections.abc import Generator

def g() -> Generator[int, int, None]:
    sent = 0
    while True:
        sent = yield sent

gen = g()
print(gen.send(None))  # <--
print(gen.send(1))
print(gen.send(2))
$ mypy --strict file2.py
file2.py:10: error: Argument 1 to "send" of "Generator" has incompatible type "None"; expected "int"
Found 1 error in 1 file (checked 1 source file)
$ python3 file2.py
0
1
2

Expected Behavior

A TypeError should not occur at runtime when Mypy reports successful type checking.

Actual Behavior

A TypeError occurs even though Mypy reports successful type checking.

My Environment

• Mypy version used: 0.910
• Python version used: 3.9.6
• Pyright version used: 1.1.163

2つの再現例 file1.py と file2.py に記述されている Generator.send() の型付け動作から始め、mypy の結果を Python の実行時 TypeError と比較します。Generator の send 型が初期の None 値をどのように扱うかを確認し、同等の Pyright の動作との相互作用を検討します。両方の例について意図した動作が確立され、実行時に直ちに失敗する可能性のあるコードを型チェッカーが受け入れなくなれば完了です。