smallest float that can increment a given value

[casevh]

Not exactly, but the math module provides the function nextafter(x,y) that returns the next floating-point value after x towards y.

>>> math.nextafter(12,0) # The next float towards 0.
11.999999999999998
>>> 12-math.nextafter(12,0)
1.7763568394002505e-15
>>> _.hex()
'0x1.0000000000000p-49'
>>> 

Naming isn't really consistent.

The decimal provides context methods called next_plus, 'next_minus, and next_towards`.

gmpy2 uses the names next_above and next_below.