iter

Syntax

iter(object, sentinel)

Parameters

• object - object whose iterator has to be created (can be sets, tuples, etc.)

• sentinel (optional) - special value that is used to represent the end of a sequence. If the sentinel parameter is also provided, iter returns an iterator until the sentinel character isn’t found.

Return

The iter function returns an iterator object for the given object. If the user-defined object doesn’t implement __iter__, and __next__ or __getitem__, the TypeError exception is raised.

Tip

The most important thing to remember is __iter__ return a iterable object while __next__ return one element.

Example: next

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) # Call __iter__

>>> print(next(print_num_iter))  # Call __next__
1
>>> print(next(print_num_iter))
2
>>> print(next(print_num_iter))
3
>>> print(next(print_num_iter))
# raises StopIteration
# ---------------------------------------------------------------------------
# StopIteration                             Traceback (most recent call last)

Example: for loop

print_num = PrintNumber(3)
for i in print_num:
    print(i)

# Output
1
2
3


print_num = PrintNumber(3)
>>> print_num_iter = iter(print_num) # Call __iter__
for i in print_num:
    print(i)

# Output
1
2
3

Example: iter() with sentinel parameter

with open('mydata.txt') as fp:
    for line in iter(fp.readline, ''):
        processLine(line)

When you run the program, it will open the mydata.txt file in reading mode. Then, the iter(fp.readline, ‘’) in the for loop calls readline (which reads each line in the text file) until the sentinel character, ‘’ (empty string), is reached.