CodeQL documentation

Comparison of identical values

ID: py/comparison-of-identical-expressions
Kind: problem
Severity: warning
Precision: very-high
   - 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').


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.


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)