[Python] Iterator

minsoo9506 included in Python

2022-02-21 447 words 3 minutes

Contents

Python iterator애 대해 알아보자.

개념

일단 Python docs에 있는 개념을 정리해보자.

iterable

정의
- An object capable of returning its members one at a time
종류
- include all sequence types (such as list, str, and tuple)
- some non-sequence types like dict and file
- objects of any classes you define with an __iter__() or __getitem__() method that implements sequence semantics
작동원리
- Iterables can be used in a for loop and in many other places where a sequence is needed (zip(), map(), …)
- When an iterable object is passed as an argument to the built-in function iter(), it returns an iterator for the object
  - 예들 들어, list는 iterable이지만 iterator는 아니다. 이를 iterator로 만들고 싶으면 iter(해당list)처럼 iter()로 감싸면 된다.
- it is usually not necessary to call iter() or deal with iterator objects yourself
- The for statement does that automatically for you, creating a temporary unnamed variable to hold the iterator for the duration of the loop
  - 위의 예시에서 우리는 iterator로 만드는 과정이 필요했다. 하지만 for문에서는 이를 자동으로 해준다는 의미이다.

iterator

정의
- An object representing a stream of data
작동원리
- Repeated calls to the iterator’s __next__() method (or passing it to the built-in function next()) return successive items in the stream
  - list, tuple, set, dict 등은 iterable이지만 iterator는 아니다 (next() 함수 씌우면 에러발생)
- When no more data are available a StopIteration exception is raised instead
- Iterators are required to have an __iter__() method that returns the iterator object itself so every iterator is also iterable

코드

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x = [1,2,3] # iterable
x_iter = iter(x) # iterator로 만들기
print(type(x_iter)) 
# <class 'list_iterator'>
print(next(x_iter))
# 1

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# iterator
# __next__ 가 없으면 iterable
class Counter:
    def __init__(self, stop):
        self.current = 0
        self.stop = stop

    def __iter__(self):
        return self # 스스로가 iterator인 상태

    def __next__(self):
        if self.current < self.stop:
            r = self.current
            self.current += 1
            return r
        else:
            raise StopIteration
        
for i in Counter(3):
    print(i)
# 0
# 1
# 2
c = Counter(3)
print(next(c))
# 0

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# iterable
# __getitem__ 만 구현되어 있으면 for문은 가능
class Counter:
    def __init__(self, stop):
        self.current = 0
        self.stop = stop

    def __getitem__(self, index):
        if index < self.stop:
            return index
        else:
            raise IndexError

c = Counter(3)
for i in c:
    print(i)
# 0
# 1
# 2

print(next(c))
# TypeError: 'Counter' object is not an iterator

c_iter  = iter(c)
print(next(c_iter))
# 0
print(next(c_iter))
# 1

Contents

[Python] Iterator

개념

iterable

iterator

코드

Reference