python - CPython: Why does += for strings change the id of string variable

Question

Cpython optimizes string increment operations,When initializing memory for a string, the program leaves extra expansion space for it,so, when incrementing, the original string is not copied to the new location. my question is why the id of string variable changes.

>>> s = 'ab'
>>> id(s)
991736112104
>>> s += 'cd'
>>> id(s)
991736774080

why the id of string variable changes.

Answer 1

Strings are immutabale. Using += on a str is not an in-place operation; it creates a new object with a new memory address, which is what id() gives under CPython's implementation.

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For str specifically, __iadd__ is not defined, so the operation falls back to a either __add__ or __radd__. See the data model section of the Python docs for some detail.

>>> hasattr(s, '__iadd__')                                                                                                                                
False