Python and +=

gistfile1.py

# -*- coding: utf-8 -*-

from collections import MutableSequence


def trace(f):
    def inner(*args, **kwargs):
        print(f.__name__)
        return f(*args, **kwargs)
    return inner


class Dummy(MutableSequence):
    def __init__(self, data=None):
        self.data = data or []

    def insert(self, n, value):
        self.data.insert(n, value)

    @trace
    def __getitem__(self, n):
        return self.data[n]

    @trace
    def __setitem__(self, n, value):
        self.data[n] = value

    def __delitem__(self, n):
        del self.data[n]

    def __repr__(self):
        return repr(self.data)

    def __iter__(self):
        return iter(self.data)

    def __len__(self):
        return len(self.data)


d = Dummy([[]])
print(d)
d[0] += [42]
print(d)

# [[]]
# __getitem__
# __setitem__
# [[42]]