About indentation

Python uses indentation to delimit blocks of code. A block starts with a line ending in colon, and continues for all lines that have a similar or higher indentation as the next line. For example:

>>> i = 0
>>> while i < 3:
...     print i
...     i = i + 1
... 
0
1
2

It is common to use four spaces for each level of indentation. It is a good policy not to mix tabs with spaces, which can result in (invisible) confusion.