How Does Python String Interning Work and When Does It Occur?

Understanding Python String Interning

Python performs string interning, a technique for optimizing memory usage by storing only one copy of string literals in memory. By interning identical strings, Python can avoid creating unnecessary duplicates.

The Mechanics of Interning

Python typically interns string literals during compilation. When a string literal is encountered in the code, Python checks if that string is already stored in the interned pool. If it is, the existing reference is returned. Otherwise, a new copy of the string is created and added to the interned pool.

Example with Compile-Time Constants

In this case, both string literals are present in the source code. Python recognizes them as compile-time constants and interns them during compilation. As a result, both variables point to the same string object.

Interning Run-Time Expressions

Python does not automatically intern the results of run-time expressions. For instance:

Here, the string concatenation is performed at run-time. The resulting string is not interned, so it's not compared to the interned string "string".

Explicit Interning

You can manually intern an existing string using the sys.intern() function. Interned strings are shared across the program, improving memory efficiency.

Implementation Details

The implementation of string interning is dependent on the Python interpreter. In CPython (the most common Python implementation), string interning is performed using a hash table. Strings with the same hash value are compared for equality to determine if they should be interned.

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