Print either an integer or a float with n decimals

With Python 3*, you can just use round() because in addition to rounding floats, when applied to an integer it will always return an int:

>>> num = 1.2345
>>> round(num,3)
1.234
>>> num = 1
>>> round(num,3)
1

This behavior is documented in help(float.__round__):

Help on method_descriptor:

__round__(...)
    Return the Integral closest to x, rounding half toward even.
    When an argument is passed, work like built-in round(x, ndigits).

And help(int.__round__):

Help on method_descriptor:

__round__(...)
    Rounding an Integral returns itself.
    Rounding with an ndigits argument also returns an integer.

* With Python 2, round() always returns a float.

If you need to maintain a fixed-width for float values, you could use the printf-style formatting, like this:

>>> num = 1
>>> print('%0.*f' % (isinstance(num, float) * 3, num))
1
>>> num = 1.2345
>>> print('%0.*f' % (isinstance(num, float) * 3, num))
1.234
>>> num = 1.2
>>> print('%0.*f' % (isinstance(num, float) * 3, num))
1.200