How to override FOR IN loop list behavior?

Question

I try to implement list which returns wrapped elements when accessing directly (by index) or via FOR IN loop statement. At present I have following code:

class ItemWrappedList(list):

    def __getitem__(self, y):
        result = super().__getitem__(y)
        return f'<item>{result}</item>'


lst = ItemWrappedList(['1', '2', '3'])

for pos in range(len(lst)):
    print(lst[pos])

#>>> <item>1</item>
#>>> <item>2</item>
#>>> <item>3</item>

for value in lst:
    print(value)
#>>> 1
#>>> 2
#>>> 3

As you can see first loop works correctly, as intended. But second loop calls list methods that I was unable to override. Are there any possibilities to force ItemWrappedList class to have same behavior in all situations?

P.S. I tried different ways with implementing/overriding __next__ and __iter__ methods, but seems that __next__ ever not called.

Asked By: Yuri

||

Source

Answer 1

Overriding the __iter__ method should do the trick:

class ItemWrappedList(list):

    def __getitem__(self, y):
        result = super().__getitem__(y)
        return f'<item>{result}</item>'

    def __iter__(self):
        return (f'<item>{x}</item>' for x in list.__iter__(self))


lst = ItemWrappedList(['1', '2', '3'])

for pos in range(len(lst)):
    print(lst[pos])


for value in lst:
    print(value)

<item>1</item>
<item>2</item>
<item>3</item>
<item>1</item>
<item>2</item>
<item>3</item>

Answered By: Tomer Ariel

Answer 2

That’s because when you say for i in lst Python first calls iter() on lst. If lst‘s class has implemented __iter__ method, it is the first thing that is considered. Otherwise it proceeds to see if it has implemented __getitem__. (Otherwise it raises TypeError: object is not iterable)

Since you inherit from list and the list class already defined __iter__, iter() always finds its __iter__ first not your __getitem__.

So just implement __iter__ for your class:

class ItemWrappedList(list):
    def __getitem__(self, y):
        result = super().__getitem__(y)
        return f"<item>{result}</item>"

    def __iter__(self):
        return (f"<item>{item}</item>" for item in super().__iter__())


lst = ItemWrappedList(["1", "2", "3"])

for pos in range(len(lst)):
    print(lst[pos])


for value in lst:
    print(value)

Answered By: S.B

How to override FOR IN loop list behavior?

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