What is the purpose of the iter method in Python?

The __iter__ method in Python is responsible for returning an iterator for an object. If an object has this method, it is considered iterable. The iterator returned by __iter__ must implement the __next__ method, which returns the next element in the container or raises a StopIteration exception when there are no more elements.

Example:

class MyCollection:
    def __init__(self, data):
        self.data = data
    
    def __iter__(self): # Returns an iterator for the class instance
        return iter(self.data)

# Using the iterator:
collection = MyCollection([1, 2, 3])
iterator = iter(collection) # Calls collection.__iter__()

print(next(iterator)) # Calls iterator.__next__() -> 1
print(next(iterator)) # Calls iterator.__next__() -> 2
print(next(iterator)) # Calls iterator.__next__() -> 3

try:
    print(next(iterator))
except StopIteration:
    print("Iteration stopped.")

# The for loop implicitly uses __iter__ and __next__:
print("Using for loop:")
for item in collection: # First calls collection.__iter__(), then iteratively next()
    print(item)

Thus, __iter__ is key to supporting the iteration protocol in Python.