Specific use of iter function in Python

Because I came into contact with this function in jax code, I was not very familiar with it. Every time I saw the name, I only knew it was iterating, but I didn’t know how it iterated, so I wrote the following notes to remind myself.

def iter(source, sentinel=None): # known special case of iter
    """
    iter(iterable) -> iterator
    iter(callable, sentinel) -> iterator
    
    Get an iterator from an object.  In the first form, the argument must
    supply its own iterator, or be a sequence.
    In the second form, the callable is called until it returns the sentinel.
    """
    pass

Iter function in Python

In Python programming,iterFunctions are a very useful built-in function for creating iterator objects. An iterator is an object that allows you to iterate over elements in a collection (such as lists, tuples, dictionaries, etc.).iterThere are two main uses of functions:

iter(iterable) -> iterator：

This form accepts an iterable object (such as lists, tuples, dictionaries, etc.) and returns an iterator. Iterators can be used to iterate over elements of an iterable object.

For example:

my_list = [1, 2, 3, 4, 5]
iterator = iter(my_list)

print(next(iterator))  # Output: 1print(next(iterator))  # Output: 2

iter(callable, sentinel) -> iterator：

This form accepts a callable object (such as a function) and a sentinel value. It calls the callable object until the sentinel value is returned.

For example:

import random

def my_callable():
    return (1, 10)

iterator = iter(my_callable, 5)

print(next(iterator))  # Output: Random integers between 1 and 10print(next(iterator))  # Output: Random integers between 1 and 10

Assuming that the sequence of random numbers returned by the my_callable function is [3, 7, 5, 2, 8], the output of the code may be:

print(next(iterator))  # Output: 3print(next(iterator))  # Output: 7

When the my_callable function returns 5, the iterator stops because 5 is the sentinel value.

Custom iter function

To better understanditerHow the function works, we can implement a simple custom version:

def iter(source, sentinel=None):
    if sentinel is None:
        # Form 1: iter(iterable)
        return source.__iter__()
    else:
        # Form 2: iter(callable, sentinel)
        while True:
            value = source()
            if value == sentinel:
                break
            yield value

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