all Python 3mathterm

math.isnan

checking

Returns True if the number is NaN (Not a Number). It cannot be tested with ==, because nan != nan.

Syntax

math.isnan(x)

nan does not come from division by zero (that raises ZeroDivisionError) but from float('nan'), from parsing the text 'nan' out of input, and from operations like math.inf - math.inf. A separate trap: the in operator and list.index compare by identity first, so the very same nan object is reported as present in a list even though nan == nan is false — the only reliable way to find or filter nan is math.isnan.

Examples

import math
print(math.isnan(math.nan))  # → True
print(math.isnan(float('nan')))  # → True
print(math.isnan(42.0))  # → False
print(math.isnan(math.inf - math.inf))  # → True
print(math.isnan(0.0))  # → False
print(math.isnan(math.nan * 0))  # → True

See also

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