Python floating point is divisible by another floating point

Question

Does anyone know a good way in Python to check if a number is divisible by another in floating point in python?

The first thing I tried was ...

3.5 % 0.1 == 0.0

But this returns False so then maybe

3.5 % 0.1 >= 1e-6 

But also False ... bummer ... it turns out that

3.5 % 0.1
>> 0.099999999924

So then this works:

LAMBDA = 1e-9
def is_divisible_by(x, y):
   m = x % y
   dy = abs(y - m)
   return m < LAMBDA or dy < LAMBDA

is_divisible_by(3.5, 0.1)

But this seems dangerous because I have to pick a LAMBDA. What about if y = LAMBDA / 2...

is_divisible_by(LAMBDA/2, (LAMBDA/2) + 1e-10)
>>> True

So then

  def is_divisible_by(x, y):
      l = y * 1e-2
      m = x % y
      dy = abs(y - m)
      return m < l or dy < l

  is_divisible_by(3.5 * 1e-10, 0.1 * 1e-10)
  >>> True

  is_divisible_by(0.21, 0.211)
  >>> True
  

Bummer.

Is there anyway to solve this without going down a massive rabbit hole?

Answer 1

Depending on the source of your floating point numbers, the decimal module might be useful.

>>> import decimal
>>> decimal.Decimal("3.5") % decimal.Decimal("0.1")
Decimal('0.0')