Understanding Iterators in Python

Python is a special language in terms of iterations and their implementation, in this article we will analyze in detail the device of iterable objects and the notorious loop for.

Features you may often encounter in daily activities

1. Using the generator twice

>>> numbers = [1,2,3,4,5]

>>> squared_numbers = (number**2 for number in numbers)

>>> list(squared_numbers)
[1, 4, 9, 16, 25]

>>> list(squared_numbers)
[]

As we see in this example, using the variable squared_numberstwice gave the expected result in the first case, and, for people unfamiliar with Python sufficiently, the unexpected result in the second.

2. Checking the entry of the element in the generator

Let's take the same variables:

>>> numbers = [1,2,3,4,5]
>>> squared_numbers = (number**2 for number in numbers)

, , :

>>> 4 in squared_numbers
True
>>> 4 in squared_numbers
False

.

3.

:

>>> fruits_amount = {'apples': 2, 'bananas': 5}

:

>>> x, y = fruits_amount

, , Python, " ":

>>> x
'apples'
>>> y
'bananas'

-, , , , .

, : , .

>>> numbers = [1,2,3,4,5]
>>> letters = ('a','b','c')
>>> characters = 'habristhebestsiteever'
>>> numbers[1]
2
>>> letters[2]
'c'
>>> characters[11]
's'
>>> characters[0:4]
'habr'

, , , , , , : for, , — .

# Can't be indexed
>>> unordered_numbers = {1,2,3}
>>> unordered_numbers[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'set' object is not subscriptable

>>> users = {'males': 23, 'females': 32}
>>> users[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 1

# Can be used as sequence
>>> [number**2 for number in unordered_numbers]
[1, 4, 9]
>>>
>>> for user in users:
...     print(user)
... 
males
females

for Python

, for, Python, , . for...each, for...of.

, for while, , :

>>> list_of_numbers = [1,2,3]
>>> index = 0
>>> while index < len(list_of_numbers):
...     print(list_of_numbers[index])
...     index += 1
... 
1
2
3

, , :

>>> set_of_numbers = {1,2,3}
>>> index = 0 
>>> while index < len(set_of_numbers):
...     print(set_of_numbers[index])
...     index += 1
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 'set' object is not subscriptable

index, enumerate:

>>> set_of_numbers = {1,2,3}
>>> for index, number in enumerate(set_of_numbers):
...     print(number, index)
... 
1 0
2 1
3 2

for

, for . .

— , , , :)
.

iter:

>>> set_of_numbers = {1,2,3}
>>> list_of_numbers = [1,2,3]
>>> string_of_numbers = '123'
>>> 
>>> iter(set_of_numbers)
<set_iterator object at 0x7fb192fa0480>
>>> iter(list_of_numbers)
<list_iterator object at 0x7fb193030780>
>>> iter(string_of_numbers)
<str_iterator object at 0x7fb19303d320>

, , next.

>>> set_of_numbers = {1,2,3}
>>> 
>>> numbers_iterator = iter(set_of_numbers)
>>> next(numbers_iterator)
1
>>> next(numbers_iterator)
2

, . , next StopIteration.

>>> next(numbers_iterator)
3
>>> next(numbers_iterator)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

-, , : next.
StopIteration, .

for while

, for, for. :)

, :

1. .
2. next.
3. ' '.
4. , StopIteration.

def for_loop(iterable, loop_body_func):
    iterator = iter(iterable)
    next_element_exist = True
    while next_element_exist:
        try:
            element_from_iterator = next(iterator)
        except StopIteration:
            next_element_exist = False
        else:
            loop_body_func(element_from_iterator)

, try-else. . , , .

.
, — , Python.
iter next . . for . "" :

coordinates = [1,2,3]
x, y, z = coordinates

numbers = [1,2,3,4,5]
a,b, *rest = numbers

print(*numbers)

—

:

>>> def custom_range(number):
...     index = 0 
...     while index < number:
...             yield index
...             index += 1
... 
>>> range_of_four = custom_range(4)
>>> next(range_of_four)
0
>>> next(range_of_four)
1
>>> next(range_of_four)
2
>>> next(range_of_four)
3
>>> next(range_of_four)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

, iter ,

>>> numbers = [1,2,3,4,5]
>>> iter1 = iter(numbers)
>>> iter2 = iter(iter1)
>>> next(iter1)
1
>>> next(iter2)
2
>>> iter1 is iter2
True

.

— -, .
— iter, .

.

:

1. iter .
2. next.
3. , StopIteration.

:

1. , iter TypeError — .
2. , next TypeError — .
3. , iter — .

:

1. "" (en. lazy). , - , , .

   
   

2. , CPU, .

   
   

Python.
, — .
.

, , enumerate:

>>> numbers = [1,2,3]
>>> enumerate_var = enumerate(numbers)
>>> enumerate_var
<enumerate object at 0x7ff975dfdd80>
>>> next(enumerate_var)
(0, 1)

zip:

>>> letters = ['a','b','c']
>>> z = zip(letters, numbers)
>>> z
<zip object at 0x7ff975e00588>
>>> next(z)
('a', 1)

open:

>>> f = open('foo.txt')
>>> next(f)
'bar\n'
>>> next(f)
'baz\n'
>>> 

Python , , , , next. , "" .

, , , .

, , , , :)

class InfiniteSquaring:
"""        ."""
    def __init__(self, initial_number):
        #    
        self.number_to_square = initial_number

    def __next__(self):
        #       
        self.number_to_square = self.number_to_square ** 2
        return self.number_to_square

    def __iter__(self):
        """       iter   ,       ."""
        return self

>>> squaring_of_six = InfiniteSquaring(6)
>>> next(squaring_of_six)
36
>>> next(squaring_of_six)
1296
>>> next(squaring_of_six)
1679616
>>> next(squaring_of_six)
2821109907456
>>> next(squaring_of_six)
7958661109946400884391936
>>> #    ...

:

>>>iter(squaring_of_six) is squaring_of_six
True

.
, .
, .

,

1.

>>> numbers = [1,2,3,4,5]

>>> squared_numbers = (number**2 for number in numbers)

>>> list(squared_numbers)
[1, 4, 9, 16, 25]

>>> list(squared_numbers)
[]

, , — , , — . .

2.

>>> numbers = [1,2,3,4,5]
>>> squared_numbers = (number**2 for number in numbers)

, :

>>> 4 in squared_numbers
True
>>> 4 in squared_numbers
False

, 1 , , , , . :

>>> 4 in squared_numbers
True
>>> list(squared_numbers)
[9, 16, 25]
>>> list(squared_numbers)
[]

, , , . , , .

3.

for, :

>>> fruits_amount = {'apples': 2, 'bananas': 5}

>>> for fruit_name in fruits_amount:
...     print(fruit_name)
... 
apples
bananas

, :

>>> x, y = fruits_amount
>>> x
'apples'
>>> y
'bananas'

— , — .

— Python.

Any iterable object implements an iterator protocol. Understanding this protocol is the key to understanding any iteration in Python.