Comparison of identical values¶
ID: py/comparison-of-identical-expressions
Kind: problem
Security severity:
Severity: warning
Precision: very-high
Tags:
- reliability
- correctness
- readability
- convention
- external/cwe/cwe-570
- external/cwe/cwe-571
Query suites:
- python-security-and-quality.qls
Click to see the query in the CodeQL repository
When two identical expressions are compared it is typically an indication of a mistake, since the Boolean value of the comparison will always be the same, unless the value is the floating point value float('nan').
Recommendation¶
It is not good practice to compare a value with itself, as it makes the code hard to read and can hide errors with classes that do not correctly implement equality. If testing whether a floating-point value is not-a-number, then use math.isnan(). If the value may be a complex number, then use cmath.isnan() instead.
Example¶
In this example f == f is used to check for float('nan'). This makes the code difficult to understand as the reader may not be immediately familiar with this pattern.
#Using 'x == x' to check that 'x' is not a float('nan').
def is_normal(f):
return not cmath.isinf(f) and f == f
#Improved version; intention is explicit.
def is_normal(f):
return not cmath.isinf(f) and not cmath.isnan(f)
References¶
Python Language Reference: Comparisons.
Python Library Reference: math.isnan().
Python Library Reference: cmath.isnan().
Common Weakness Enumeration: CWE-570.
Common Weakness Enumeration: CWE-571.