Как связаны конструкции for и next в Python?

The for loop construct in Python implicitly uses the next() function to iterate over elements of an iterable object.

When a for loop starts executing, it calls the iter() function on the iterable object to obtain an iterator. Then, in each iteration of the for loop, the next() function is called on this iterator. next() returns the next element in the sequence.

The iteration continues until next() raises a StopIteration exception, which signals the end of the iteration. The for loop automatically handles this exception and terminates.

Example of manual iteration using iter() and next():

# Create a list
my_list = [1, 2, 3]

# Get an iterator
my_iterator = iter(my_list)

# Manual iteration
try:
    print(next(my_iterator)) # Outputs 1
    print(next(my_iterator)) # Outputs 2
    print(next(my_iterator)) # Outputs 3
    print(next(my_iterator)) # Will raise StopIteration here
except StopIteration:
    print("Iteration finished")

Equivalent for loop:

# Using a for loop
my_list = [1, 2, 3]

for item in my_list:
    print(item)
# The for loop does the same as manual calls to iter() and next(),
# automatically handling StopIteration.

Thus, for is syntactic sugar for using iter() and next().