Compare to None with is

ID

python.none_comparison_identity

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Best Practice

Language

Python

Tags

best-practice, suspicious-comparison

Description

Reports x == None or x != None comparisons. Python’s idiom is identity comparison (x is None / x is not None); equality comparison routes through eq, which a custom class can override to return something surprising for the None case (most often a NumPy / pandas object whose eq returns an element-wise array).

if x == None:               # FLAW
    ...

if x != None:               # FLAW
    ...

if x is None:               # OK
    ...

Rationale

None is a singleton. There is exactly one None object in the running interpreter, so identity comparison is both correct and faster than the equality variant. Identity comparison also bypasses eq, which is what you almost always want when checking for None: a custom eq that decides "I’m equal to anything" or "I return an array" silently breaks the test.

Remediation

Replace == with is and != with is not:

if x is None:
    ...
if x is not None:
    ...

References