Wrong math with Python?

Python's 'leading zero' syntax for octal literals is a common gotcha:

Python 2.7.3
>>> 010
8

The syntax was changed in Python 3.x http://docs.python.org/3.0/whatsnew/3.0.html#integers

Because of octal arithmetic, 013 is actually the integer 11.

>>> 013
11

With a leading zero, 013 is interpreted as a base-8 number and 1*8¹ + 3*8⁰ = 11.

Note: this behaviour was changed in python 3. Here is a particularly appropriate quote from PEP 3127

The default octal representation of integers is silently confusing to people unfamiliar with C-like languages. It is extremely easy to inadvertently create an integer object with the wrong value, because '013' means 'decimal 11', not 'decimal 13', to the Python language itself, which is not the meaning that most humans would assign to this literal.

013 is an octal integer literal (equivalent to the decimal integer literal 11), due to the leading 0.

>>> 2013*013
22143
>>> 2013*11
22143
>>> 2013*13
26169

It is very common (certainly in most of the languages I'm familiar with) to have octal integer literals start with 0 and hexadecimal integer literals start with 0x. Due to the exact confusion you experienced, Python 3 raises a SyntaxError:

>>> 2013*013
  File "<stdin>", line 1
    2013*013
           ^
SyntaxError: invalid token

and requires either 0o or 0O instead:

>>> 2013*0o13
22143
>>> 2013*0O13
22143