Iadd

Iadd typically refers to the in-place addition operator, commonly written as the plus-equals symbol (+=). In many programming languages, including Python, iadd denotes the operation of adding a value to another and storing the result back in the original variable, potentially modifying the object in place.

In Python, the behavior of a += b is governed by the special method __iadd__ on the left-hand operand. If the left operand defines __iadd__, that method is called to perform the in-place update and its return value is assigned back to the left-hand variable. If __iadd__ is not defined or returns NotImplemented, Python falls back to using __add__ to compute a + b and then rebinds the result to the left-hand variable. This distinction means that mutability matters: mutable types can modify themselves in place, while immutable types will typically produce a new object and rebind the name.

Examples:

- For a list: lst = [1, 2, 3]; lst += [4, 5] results in lst being [1, 2, 3, 4, 5] because lists implement in-place extension.

- For a tuple: tup = (1, 2); tup += (3,) results in tup becoming (1, 2, 3) because the operation creates a new tuple and rebinds the name.

Not all types implement in-place addition. Some objects rely on immutable semantics, or provide alternative methods (such as update or union) for combining values. The exact behavior of iadd can thus vary by language and type, even though the general concept is the in-place addition and assignment in a single operation. See also __iadd__ and __add__ for implementation details.