Why Do Leading Zeros in Python 2.x Sometimes Produce Unexpected Integer Values?

Python's Curious Octal Constants

When using small integers with a leading zero in Python 2.x versions prior to 2.6, unexpected results may arise. These integers are interpreted as octal numbers, a different number base system than the standard decimal system.

Octal Notation Explained

In octal notation, the base is 8 instead of 10. Each digit represents a power of 8. For example, in the number 011:

• 1 represents 1 * 8¹ (8)
• 1 represents 1 * 8⁰ (1)

Thus, 011 is equal to 9 in decimal (8 1 = 9). Similarly:

• 0100 = 1 8² 0 8¹ 0 * 8⁰ = 64
• 027 = 2 8¹ 7 8⁰ = 23

Python 3 and Octal Constants

In Python 3, octal constants require an explicit prefix, 0o, to distinguish them from decimal integers. Thus, 011 from Python 2.x would be written as 0o11 in Python 3.

Type Verification

Despite their unusual behavior, these octal constants remain integers in Python:

>>> type(027)
<class 'int'>

Practicality of Octal Constants

While octal notation was common in the early days of computing, it is rarely used today. Decimal or hexadecimal notation is generally preferred for readability and compatibility. It is important to be aware of octal constants in Python 2.x, however, to understand their potential effects on code behavior.

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