These are numbers represented in base 8 (octal numbers).
Some examples:
Python 2 (old format)
Note: these forms only work on Python 2.x.
011 is equal to 1⋅8¹ + 1⋅8⁰ = 9,
0100 is equal to 1⋅8² + 0⋅8¹ + 0⋅8⁰ = 64,
027 is equal to 2⋅8¹ + 7⋅8⁰ = 16 + 7 = 23.
Python 3 (new format)
In Python 3, one must use 0o instead of just 0 to indicate an octal constant, e.g. 0o11 or 0o27, etc. Python 2.x versions >= 2.6 supports both the new and the old format.
0o11 is equal to 1⋅8¹ + 1⋅8⁰ = 9,
0o100 is equal to 1⋅8² + 0⋅8¹ + 0⋅8⁰ = 64,
0o27 is equal to 2⋅8¹ + 7⋅8⁰ = 16 + 7 = 23.