This is an old copy of the Python FAQ. The information here may be outdated.

Why are Python strings immutable?

There are several advantages.

One is performance: knowing that a string is immutable makes it easy to lay it out at construction time — fixed and unchanging storage requirements. This is also one of the reasons for the distinction between tuples and lists. This also allows the implementation to safely reuse string objects. For example, the CPython implemenation uses pre-allocated objects for single-character strings, and usually returns the original string for string operations that doesn’t change the content.

The other is that strings in Python are considered as elemental as numbers. No amount of activity will change the value 8 to anything else, and in Python, no amount of activity will change the string “eight” to anything else.

CATEGORY: general design

 

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