How Does Python Manage String IDs: Interning, Memory Reuse, and Optimization?

String IDs in Python: Interning and Memory Reuse

In Python, strings are immutable. However, as demonstrated by the initial examples, the ID of a string literal can change upon repeated evaluation. This behavior stems from a combination of Python's internal string handling mechanisms and memory optimization strategies.

CPython's Interning

While not explicitly defined in CPython's documentation, the interpreter often interns strings that are frequently used. This involves storing commonly referenced strings in a global table and reusing the same memory location for identical strings. So, if two string literals appear in the same code block or are bound to different variables, they may share the same ID if they are interned.

Memory Reuse and Garbage Collection

Once a string object is created, its memory location is not bound to it permanently. Python's garbage collector can reclaim memory from unused objects, including string objects. If a string is no longer referenced anywhere in the code, its memory location can be reused by a new string object. This can lead to different IDs for the same string literal when evaluated multiple times.

Compiler Optimizations

Python compilers often perform optimizations on code at runtime. For string literals that meet certain criteria (e.g., only containing ASCII letters, digits, or underscores), the compiler may intern them and optimize their creation. This means that subsequent evaluations of the same string literal within the optimized code may produce the same ID.

Dynamic Binding and String Interning

When a variable is bound to a string literal, Python creates a new object that references the interned string. If the variable's name meets the aforementioned criteria, it may be interned as well, resulting in the variable name and its value sharing the same ID.

Concatenation and Interning

Concatenating strings in Python can sometimes lead to interning if the resulting string fits within the compiler's optimization limit (4096 characters in Python 3.7 ). Therefore, two concatenated strings may have the same ID if they produce a valid identifier and fall within the limit.

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