Local type inference of numeric variables

I have a very simple example program that produces surprising and (I think...) wrong behavior:

foo = 0

foo = 1.0

Running mypy against this file yields

test.py:3: error: Incompatible types in assignment (expression has type "float", variable has type "int")

But foo could(/should) be inferred as a float in this environment. I draw this conclusion from PEP-484

when an argument is annotated as having type float, an argument of type int is acceptable

(although this is not about function arguments per se).

I know that some improvements have been recently been made to type inference in 0.770 to use information from second (or subsequent) assignments, rather than just the first assignment. So maybe a widening from int to float could be implemented in the same way...?

mypy を使って test.py の例で動作を再現し、次にバージョン 0.770 について参照されているローカルな型推論の変更と、関連する PEP-484 のガイダンスを調査します。int が代入された変数を後から float に拡張できるかどうかを明らかにし、その結果生じる代入動作をテストでカバーできれば完了です。