How Does Python's String Interning Mechanism Work?

Python's String Interning Mechanism

Python employs string interning as an optimization technique to conserve memory. Interning means storing identical string objects at the same memory location, avoiding redundant copies.

Compile-Time Interning

In Python, string interning is primarily applied to compile-time constant strings. When an expression involving string concatenation is evaluated at compile time, the interpreter attempts to intern the resulting string.

For instance:

"string" is "string" # True

Here, the expression "string" is internally replaced with the existing "string" object, resulting in a True comparison.

Conditional Interning

However, interning is not guaranteed for expressions evaluated at runtime. Consider the following:

s1 = "strin"
s2 = "string"
s1 + "g" is s2 # False

In this case, s1 "g" is evaluated at runtime and is not interned. Consequently, it occupies a new memory location, and s1 "g" is s2 evaluates to False.

Explicit Interning

Manually interning a string can be achieved using sys.intern(), which returns the existing interned string if it exists, or creates a new interned string otherwise. For example:

import sys
s3 = s1 + "g"
s3 is "string" # False
sys.intern(s3) is "string" # True

Implementation Details

The implementation of string interning in Python is language-specific and dependent on the specific interpreter. In CPython (the most popular Python implementation), compile-time constant strings are interned in a hash table.

By understanding Python's string interning mechanism, developers can optimize their code and improve performance in scenarios where string interning can be utilized effectively.

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