When Does `i = x` Differ from `i = i x` in Python?

Python's Subtle Distinction: "i = x" vs. "i = i x"

In Python, programmers often use " =" as a shorthand for addition assignment, expecting it to behave identically to "i = i x." However, this assumption is not always correct.

"i = x" invokes the "__iadd__" method, while "i = i x" calls "__add__." In most cases, "__iadd__" and "__add__" return equivalent results. However, this can vary based on the object in question.

For mutable objects, "__iadd__" modifies the object in place, while "__add__" creates a new instance. For immutable objects, however, both methods return a new instance.

For mutable objects, "__iadd__" is useful for modifying the object in place, thus conserving memory. For example:

a = [1, 2, 3]
b = a
b += [1, 2, 3]

print(a)  # [1, 2, 3, 1, 2, 3]
print(b)  # [1, 2, 3, 1, 2, 3]

Contrast this with:

a = [1, 2, 3]
b = a
b = b + [1, 2, 3]

print(a)  # [1, 2, 3]
print(b)  # [1, 2, 3, 1, 2, 3]

In the latter example, a and b become different objects.

In certain scenarios, "__add__" can also call "__radd__." For example, if an object does not implement "__add__," the interpreter may attempt to use the "__radd__" method of the other object.

In summary, "i = x" and "i = i x" are usually equivalent for mutable objects. However, for immutable objects, "__iadd__" and "__add__" create distinct behaviors. Understanding these nuances is crucial for effective Python programming.

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