redefined-loop-name (PLW2901)

Derived from the Pylint linter.

What it does

Checks for variables defined in for loops and with statements that get overwritten within the body, for example by another for loop or with statement or by direct assignment.

Why is this bad?

Redefinition of a loop variable inside the loop's body causes its value to differ from the original loop iteration for the remainder of the block, in a way that will likely cause bugs.

In Python, unlike many other languages, for loops and with statements don't define their own scopes. Therefore, a nested loop that uses the same target variable name as an outer loop will reuse the same actual variable, and the value from the last iteration will "leak out" into the remainder of the enclosing loop.

While this mistake is easy to spot in small examples, it can be hidden in larger blocks of code, where the definition and redefinition of the variable may not be visible at the same time.

Example

for i in range(10):
    i = 9
    print(i)  # prints 9 every iteration

for i in range(10):
    for i in range(10):  # original value overwritten
        pass
    print(i)  # also prints 9 every iteration

with path1.open() as f:
    with path2.open() as f:
        f = path2.open()
    print(f.readline())  # prints a line from path2