Indirect __iadd__ method combined with __setitem__

Question

I have something like this:

 class Foo:
    def __init__(self):
       self.bar = ...

    def __setitem__(self, key, value=None) :
       self.bar[key] = value

Now I want:

  f = Foo()
  f['shmoo'] += 5

to do:

  f.bar['shmoo'] += 5

there is:

  def __iadd__(self,other): .....

but there the 'key' is not available. Any way to solve this?

Answer 1

f.['shmoo'] += 5 does the augmented assignment operation on the value in the collection (f.['shmoo']), not the collection itself. The pseudocode is

val = f.__getitem__('shmoo')
val += 5
f.__setitem('shmoo', val)

Implement __getitem__ but there is no need to implememnt __iadd__ and friends.

We can demonstrate by adding some prints to Foo and creating a new class for the value.

class Foo:

    def __init__(self):
        self.bar = {}

    def __getitem__(self, name):
        print("foo get", name)
        return self.bar[name]

    def __setitem__(self, name, value):
        print('foo set', name, value)
        self.bar[name] = value

class Baz:

    def __init__(self):
        self.val = 0

    def __iadd__(self, other):
        self.val = self.val + other
        print("baz iadd", other, self.val)
        return self

    def __str__(self):
        return str(self.val)

Running, the sequence of events is

assign
foo set a 0
augmented addition
foo get a
baz iadd 2 2
foo set a 2