Why Does Python 2.x Interpret Numbers with Leading Zeros as Octal?

Decoding the Mysterious "0" Leading Numbers in Python

When you type small integers with a leading zero into Python 2.7.3, they yield unexpected results, such as:

>>> 011
9
>>> 0100
64
>>> 027
23

This quirk stems from the way Python interprets numbers prefixed with "0". In Python 2.x, these numbers are considered octal (base 8) integers.

Unveiling the Octal Numbers

In the octal number system, each digit represents powers of 8. For instance:

• 011 represents 1 x 8¹ 1 x 8⁰ = 9
• 0100 represents 1 x 8² 0 x 8¹ 0 x 8⁰ = 64
• 027 represents 2 x 8¹ 7 x 8⁰ = 16 7 = 23

Python 3's Modern Approach

Unlike Python 2.7.3, Python 3.0 does not recognize numbers with leading "0" as octal integers and instead raises an error. To represent octal numbers in Python 3, one must use the "0o" prefix, e.g.:

>>> 0o11
9
>>> 0o100
64
>>> 0o27
23

Legacy Support in Python 2.x

Python 2.x versions >= 2.6 support both the old format (leading "0") and the new format (leading "0o") for octal numbers, providing backward compatibility.

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