Why round off of 0.500000 in python differs from 45.500000 using '%.0f'?

Question

Recently, I learned art of string formatting in Python 2.7. I decided to play with floating point numbers.
Came across an awkward looking solution, as written below.

print "%.0f"%45.5000000 #46
print "%.0f"%0.5000000 #0

#Why??

BUT

print int(round(45.5000000)) #46
print int(round(0.5000000)) #1

Please help me understand, why this behavior is shown by %f.

Answer 1

The internal implementation for the %.0f string format uses a round-half-even rounding mode.

In Python 2, the round() function uses round-away-from-zero. In Python 3, that was changed to round-half-even making it consistent with string formatting.

FWIW, the decimal module offers you a choice of rounding modes if you want more control than afforded by round() or by string formatting. The decimal rounding modes are: ROUND_05UP ROUND_CEILING ROUND_DOWN ROUND_FLOOR ROUND_HALF_DOWN ROUND_HALF_EVEN ROUND_HALF_UP ROUND_UP.