modified-iterating-dict / E4702

Message emitted:

Iterated dict '%s' is being modified inside for loop body, iterate through a copy of it instead.

Description:

Emitted when items are added or removed to a dict being iterated through. Doing so raises a RuntimeError.

Problematic code:

fruits = {"apple": 1, "orange": 2, "mango": 3}

i = 0
for fruit in fruits:
    fruits["apple"] = i  # [modified-iterating-dict]
    i += 1

Correct code:

fruits = {"apple": 1, "orange": 2, "mango": 3}

i = 0
for fruit in fruits.copy():
    fruits["apple"] = i
    i += 1

Created by the modified_iteration checker.