Python ist eine spezielle Sprache in Bezug auf Iterationen und deren Implementierung. In diesem Artikel werden wir das Gerät iterierbarer Objekte und die berüchtigte Schleife detailliert analysieren for.
Funktionen, auf die Sie bei täglichen Aktivitäten häufig stoßen
1. Verwenden Sie den Generator zweimal
>>> 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)
[]
Wie wir in diesem Beispiel sehen, squared_numbersergab die zweimalige Verwendung der Variablen im ersten Fall das erwartete Ergebnis, und für Personen, die mit Python nicht ausreichend vertraut sind, im zweiten Fall das unerwartete Ergebnis.
2. Überprüfen Sie den Eintrag des Elements im Generator
Nehmen wir die gleichen Variablen:
>>> 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. :)
, :
- .
next.- ' '.
- ,
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, .
.
:
iter .next.- ,
StopIteration.
:
- ,
iter TypeError — . - ,
next TypeError — . - ,
iter — .
:
"" (en. lazy). , - , , .
, 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.
Jedes iterierbare Objekt implementiert ein Iteratorprotokoll. Das Verständnis dieses Protokolls ist der Schlüssel zum Verständnis jeder Iteration in Python.