Python iter() Method

The Python iter() function is a built-in that returns an iterator object from an iterable. It accepts two parameters: the required object parameter, which must be a collection or an object implementing the __iter__() method, and an optional sentinel value that defines a termination condition when calling the object as a callable. When used with a single argument, iter() calls the object’s __iter__() method and returns the resulting iterator. When used with a sentinel, the first argument must be a callable, and the iterator will keep calling it until the sentinel value is returned. The function is fundamental to Python’s iteration protocol, which powers for loops, list comprehensions, and generator expressions. A common real-world use case is reading lines from a file or socket until a specific termination marker is encountered, using the two-argument sentinel form like iter(socket.recv, b'').

What does iter() return?

The iter() function returns an iterator object that produces successive items from the given iterable when passed to next().

When should you use iter()?

Use iter() when you need explicit control over iteration, such as manually stepping through items with next(), or when using the sentinel form to read data until a stop condition is met.

What is Python iter() Method?

The iter() is a Python built-in method that returns an iterator for the given object.

The syntax of the iter() method is:

iter(object, sentinel)

Python iter() Method Parameters

The iter() methods take two parameters as an argument:

• object - the name of the object whose iterator has to be returned.
• sentinel (optional) - A numeric value that is used to represent the end of the sequence.

Let’s check some examples of the python iter() method.

Example 1: How to use the iter() method in python?

cars = ['BMW','AUDI','FORD']

My_iter = iter(cars)

print(My_iter)

print(type(My_iter))

print(next(My_iter)) # BMW
print(next(My_iter)) # AUDI
print(next(My_iter)) # FORD

Output:

<list_iterator object at 0x7fddb59cb820>
<class 'list_iterator'>
BMW
AUDI
FORD

We can use the next() method to print the elements of iteration, and the iteration will remember the next count via the internal count variable. Once the iteration is complete, it raises a StopIteration exception, and the iteration count cannot be reassigned to 0.

We can also use __next__() method with ithe ter() method to print the iterator.

Example 2: Using __next__() method in python iter() method.

cars = ['BMW','AUDI','FORD']

My_iter = iter(cars)

print(My_iter.__next__()) # BMW
print(My_iter.__next__()) # AUDI
print(My_iter.__next__()) # FORD

The output will be as follows.

BMW
AUDI
FORD

Example 3: Using iter() with custom objects.

class PrintNumber:
    def __init__(self, max):
        self.max = max

    def __iter__(self):
        self.num = 0
        return self

    def __next__(self):
        if(self.num >= self.max):
            raise StopIteration
        self.num += 1
        return self.num

print_num = PrintNumber(3)

print_num_iter = iter(print_num)
print(next(print_num_iter))  # 1
print(next(print_num_iter))  # 2
print(next(print_num_iter))  # 3

# raises StopIteration
print(next(print_num_iter))

The output will be as follows.

1
2
3
Traceback (most recent call last):
  File "", line 23, in <module>
    print(next(print_num_iter))
  File "", line 11, in __next__
    raise StopIteration
StopIteration

Common Use Cases

A common use of iter() is implementing custom iteration in classes by defining __iter__() and __next__() methods, then using iter() to obtain the iterator for use in loops. Another practical scenario is the sentinel pattern for reading blocks of data from a file: iter(lambda: file.read(4096), b'') reads 4KB chunks until the file is exhausted, which is memory-efficient for processing large files. Developers also use iter() when they need to manually control iteration stepping, such as consuming items from an iterator at different rates in interleaved processing pipelines.

To advance an iterator by one step and retrieve the next value, use the Python next() method. If you need to create a list from an iterable instead of an iterator, see the Python list() method.

Rules of iter()

• If the user-defined object doesn’t implement __iter__(), and __next__() or __getitem()__, the TypeError exception is raised.
• If the sentinel parameter is also provided, iter() returns an iterator until the sentinel character isn’t found.