Why round(4.5) == 4 and round(5.5) == 6 in Python

Question

This question already has answers here:

Python 3.x rounding behavior (11 answers)

Closed 2 years ago.

Looks like both 4.5 and 5.5 have exact float representations in Python 3.5:

>>> from decimal import Decimal
>>> Decimal(4.5)
Decimal('4.5')
>>> Decimal(5.5)
Decimal('5.5')

If this is the case, then why

>>> round(4.5)
4
>>> round(5.5)
6

?

Answer 1

Python 3 uses Bankers Rounding, which rounds .5 values to the closest even number.

Answer 2

In Python 3, exact half way numbers are rounded to the nearest even result. This behavior changed in Python 3

The round() function rounding strategy and return type have changed. Exact halfway cases are now rounded to the nearest even result instead of away from zero. (For example, round(2.5) now returns 2 rather than 3.) round(x[, n]) now delegates to x.round([n]) instead of always returning a float. It generally returns an integer when called with a single argument and a value of the same type as x when called with two arguments.