What is the default __iter__ function? [duplicate]

I wrote a class which have init, __getitem__ and __len__ functions.
After making the __getitem__ function more complex I noticed an iteration over the object doesn't know when to stop (although __len__ working as intended).

So what is the default __iter__ doing then?

my code for example (although it shouldnt matter a lot):

class SubsetDataset(Dataset): def __init__(self, source_dataset: Dataset, desired_classes: list): self.source_dataset = source_dataset self.desired_classes = desired_classes self.index_to_sub = dict() i = 0 for j, d in enumerate(self.source_dataset): sampel, label = d if label in self.desired_classes: self.index_to_sub[i] = j i += 1 def __getitem__(self, index): sample, label = self.source_dataset[self.index_to_sub[index]] return sample, label def __len__(self): return len(self.index_to_sub) 

my __iter__ and __next__ function that solved the problem (which I didn't want to implement):

 def __iter__(self): self.n = 0 return self def __next__(self): if self.n >= len(self): raise StopIteration next = self.source_dataset[self.index_to_sub[self.n]] self.n += 1 return next