Lerner Consulting BlogReuven Lerner1 min readintermediate
Python += calls __add__ and rebinds to a new object
Summary
A short blog post showing that Python's `+=` operator invokes `__add__` and rebinds the left‑hand variable to a new object rather than mutating it in place.
- When a class defines `__add__`, the `+=` operator will call that method.
- The result of `__add__` is assigned back to the left‑hand variable, so the variable now points to a new instance.
- This behavior is demonstrated with a minimal `MyClass` that prints when `__add__` runs.
Understanding that `+=` may not be in‑place is important for writing correct mutable/immutable semantics and avoiding subtle bugs in Python code.
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