how to customize implementation of the for in statement

Question

When we implement our own obj in python which support for i in obj statement, what magic method we need to create?

Is there a link to the official document that I can check?

During my reading of some other's source code, I found there's a __getitem__ method, it has a signatue of __getitem__(slef, index), then when we instantiate an object of this class, and call

for i in obj:
   print(i)

what method would python try to find and pass what argument?

Did you read https://stackoverflow.com/questions/926574/why-does-defining-getitem-on-a-class-make-it-iterable-in-python ? — azro, Sep 04 '22 at 11:44
The code passes ints starting at 0, until your code (in getitem) raises an IndexError — azro, Sep 04 '22 at 11:44
thanks, @zaro. I only know that __iter__ is the magic method for for in statement, now I get it — Allen Huang, Sep 04 '22 at 14:35

score 1 · Accepted Answer · answered Sep 04 '22 at 11:46

You can make a class iterable by implementing

__getitem, it'll receive ints starting at 0, your code needs to raise a IndexError to stop it

class Foo:
    def __getitem__(self, item):
        if isinstance(item, int) and item > 10:
            raise IndexError
        return item * 2

__iter__ and __next__

class Foo:

    def __init__(self):
        self.counter = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.counter > 10:
            raise StopIteration
        result = self.counter * 2
        self.counter += 1
        return result

Both leads to same output

for i in Foo():
    print(i)

0
2
4
6
8
10
12
14
16
18
20

how to customize implementation of the for in statement

1 Answers1