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.

Python Language Reference: Comparisons.

Python Library Reference: math.isnan().

Python Library Reference: cmath.isnan().