Unexpected object assignment

Question

class TrafficData(object):
    def __init__(self):
        self.__data = {}
    def __getitem__(self, epoch):
        if not isinstance(epoch, int):
            raise TypeError()
        return self.__data.setdefault(epoch, ProcessTraffic())
    def __iadd__(self, other):
        for epoch, traffic in other.iteritems():

            # these work
            #existing = self[epoch]
            #existing += traffic

            # this does not
            self[epoch] += traffic # here the exception is thrown

        return self

In the above trimmed down code, I do not expect an item assignment, yet apparently one is occurring on the marked line, and throwing the following exception:

  File "nethogs2.py", line 130, in __iadd__
    self[epoch] += traffic
TypeError: 'TrafficData' object does not support item assignment

However if I instead use the preceding 2 commented out lines, no exception is thrown.

As I see it, the 2 should behave in the same way. self[epoch] returns a reference to an object, and it's modified in place through that objects __iadd__. What am I misunderstanding here? I frequently run into this problem when using dictionaries.

Update0

It's probably worth pointing out that the values in self.__data have __iadd__ defined, but not __add__, and I'd much prefer to modify the value in place if possible. I would also like to avoid creating a __setitem__ method.

Update1

Below is a test case demonstrating the problem, I've left the code above for existing answers.

class Value(object):
    def __init__(self, initial=0):
        self.a = initial
    def __iadd__(self, other):
        self.a += other
        return self
    def __str__(self):
        return str(self.a)

class Blah(object):
    def __init__(self):
        self.__data = {}
    def __getitem__(self, key):
        return self.__data.setdefault(key, Value())

a = Blah()
b = a[1]
b += 1
print a[1]
a[1] += 2
print a[1]

Answer 1

What you are exactly doing in:

self[epoch] += traffic

is:

self[epoch] = self[epoch] + traffic

But you haven't defined __setitem__ method, so you can do that on self.

You also need:

def __setitem__(self, epoch, value):
        self.__data[epoch] = value

or something similar.

Answer 2

The code:

self[epoch] += traffic

Is syntactic sugar for:

self[epoch] = self[epoch] + traffic

So the assignment is not unexpected it is the assignment in +=. So you also need to override the __setitem__() method.

Answer 3

It's probably worth pointing out that the values in self.__data have __iadd__ defined, but not __add__, and I'd much prefer to modify the value in place if possible.

To add some precision to previous answers, under the circumstances you describe, self[epoch] += traffic translates exactly to:

self[epoch] = self[epoch].__iadd__(traffic)

So if all you want are the side effects of __iadd__, without the item-assignment part, your choices are limited to the workaround that you've already identified in the comments in the code you've posted, or calling __iadd__ yourself -- possibly through the operator module, though I believe operator.__iadd__(self[epoch], traffic) has no added value compared to the simpler self[epoch].__iadd__(traffic) (when self[epoch] does have a __iadd__ method).