Empty Sequences Are Falsy

ID

python.empty_sequences_false

Severity

info

Remediation Complexity

medium

Remediation Risk

low

Remediation Effort

medium

Resource

Code Smell

Language

Python

Tags

best-practice, code-style

Description

Reports comparisons of len(x) against a numeric literal that are equivalent to a truthy / falsy check on the sequence itself. Empty sequences are falsy in Python and non-empty sequences are truthy, so if len(items) == 0: should be written if not items:, len(items) > 0 should be items, and so on.

if len(items) == 0:        # FLAW — use `if not items:`
    ...
if len(items) > 0:         # FLAW — use `if items:`
    ...
if len(items) != 0:        # FLAW — use `if items:`
    ...
if len(items) >= 1:        # FLAW — use `if items:`
    ...

if not items:              # OK
    ...
if items:                  # OK
    ...

Rationale

The shorter form is faster (no function call), reads more naturally for the cases the language was designed around, and is the PEP 8 recommendation. The longer len(…​) == 0 form invites typos (⇐ 0, < 1) that drift from the intent.

Remediation

Replace len(x) == 0 with not x and len(x) > 0 (and friends) with x:

if not items:
    return None
return items[0]

Keep len(…​) when the actual length matters (len(items) == expected_count).

References