How to implement __iter__(self) for a container object (Python)

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I have written a custom container object.

According to this page, I need to implement this method on my object:

__iter__(self)

However, upon following up the link to Iterator Types in the Python reference manual, there are no examples given of how to implement your own.

Can someone post a snippet (or link to a resource), that shows how to do this?

The container I am writing, is a map (i.e. stores values by unique keys). dicts can be iterated like this:

for k, v in mydict.items()

In this case I need to be able to return two elements (a tuple?) in the iterator. It is still not clear how to implement such an iterator (despite the several answers that have been kindly provided). Could someone please shed some more light on how to implement an iterator for a map-like container object? (i.e. a custom class that acts like a dict)?

python
9 Answers

To answer the question about mappings: your provided __iter__ should iterate over the keys of the mapping. The following is a simple example that creates a mapping x -> x * x and works on Python3 extending the ABC mapping.

import collections.abc

class MyMap(collections.abc.Mapping):
    def __init__(self, n):
        self.n = n

    def __getitem__(self, key): # given a key, return it's value
        if 0 <= key < self.n:
            return key * key
        else:
            raise KeyError('Invalid key')

    def __iter__(self): # iterate over all keys
        for x in range(self.n):
            yield x

    def __len__(self):
        return self.n

m = MyMap(5)
for k, v in m.items():
    print(k, '->', v)
# 0 -> 0
# 1 -> 1
# 2 -> 4
# 3 -> 9
# 4 -> 16
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