#Round is unexpectedly wrong
>>> z = complex(1,1)
>>> s=abs(z)
>>> round(s,2)
1.4099999999999999

Please see
http://www.python.org/doc/faq/general/#why-are-floating-point-calculations-so-inaccurate

>>> def my_round(n,i):
...     t = n * (10**i)
...     s = round(t)
...     r = s / (10**i)
...     return r
... 
>>> print my_round(s,2)
1.41

Your function is not better:

>>> print my_round(s,2)
1.41
>>> my_round(s,2)
1.4099999999999999

print uses str(), and restricts itself to 12 significant digits.
the direct call uses repr(), and display the most precise number.

The problem is not in the computation, but is inherent to the 'float'
type which cannot represent 1.41 exactly. Please carefully read the faq
entry above.