How are Python in-place operator functions different than the standard operator functions?

First, you need to understand the difference between __add__ and __iadd__.

An object's __add__ method is regular addition: it takes two parameters, returns their sum, and doesn't modify either parameter.

An object's __iadd__ method also takes two parameters, but makes the change in-place, modifying the contents of the first parameter. Because this requires object mutation, immutable types (like the standard number types) shouldn't have an __iadd__ method.

a + b uses __add__. a += b uses __iadd__ if it exists; if it doesn't, it emulates it via __add__, as in tmp = a + b; a = tmp. operator.add and operator.iadd differ in the same way.

To the other question: operator.iadd(x, y) isn't equivalent to z = x; z += y, because if no __iadd__ exists __add__ will be used instead. You need to assign the value to ensure that the result is stored in both cases: x = operator.iadd(x, y).

You can see this yourself easily enough:

import operator
a = 1
operator.iadd(a, 2)
# a is still 1, because ints don't have __iadd__; iadd returned 3

b = ['a']
operator.iadd(b, ['b'])
# lists do have __iadd__, so b is now ['a', 'b']