No for i in range(len(x))

ID

python.no_range_loop_with_len

Severity

info

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Best Practice

Language

Python

Tags

best-practice, code-style

Description

Reports for i in range(len(x)): loops. The idiom is the C-style index loop ported to Python; enumerate(x) reads better, gives both the index and the value, and avoids the redundant len call. When only the value is needed, for item in x: is the right form.

for i in range(len(items)):                # FLAW
    do(items[i])

for i, item in enumerate(items):           # OK
    do(item)

for item in items:                         # OK
    do(item)

Rationale

The range(len(…​)) form keeps a Python beginner thinking in indices, which loses the speed and readability benefits of native iteration and risks off-by-one bugs. enumerate is also more amenable to sharing the loop body with other iteration patterns (e.g., zip(items, indices)).

Remediation

for i, item in enumerate(items):
    process(i, item)

# or, when the index is not actually used:
for item in items:
    process(item)

References