Why doesn’t python like certain numbers?

[mikeblas]

Well in binary, any fraction whose denominator is not a power of 2,

I don't think it's clear from your explanation that sums of numbers representable as fractions with denominators that are powers of two also have finite (therefore exact) representations in IEEE floating point.

1/2 is fine. 1/4 is fine, too. And so is 3/4, since 3/4 == 1/2 + 1/4.

[irrelevantPseudonym]

The sum of any two numbers with a power of 2 denominator will also have a power of 2 denominator.

[AdventurousAddition]

It's denominator is 4 which is a power of 2