itertools 'previous' (opposite of next) python

问题

I'm currently using something like

>> import itertools
>> ABC = [a, b, c]
>> abc = itertools.cycle( ABC )
>> next( abc )
a
>> next( abc )
b    
>> next( abc )
c

I want my next call to be

>> previous( abc )
b

Is there a method in itertools that can accomplish this?

回答1:

No, there isn't.

Because of the way Python's iteration protocol works, it would be impossible to implement previous without keeping the entire history of the generated values. Python doesn't do this, and given the memory requirements you probably wouldn't want it to.

回答2:

You can use deque from collections module and rotate method, for example:

from collections import deque

alist=['a','b','c']
d=deque(alist)

current = d[0] 
print(current) # 'a'

d.rotate(1) # rotate one step to the right
current = d[0] 
print(current) # 'c'

d.rotate(-1) # rotate one step to the left
current = d[0] 
print(current) # 'a' again

回答3:

You can write your own class to emulate an iterable object with next and previous. This is the simplest implementation:

class cycle:
    def __init__(self, c):
        self._c = c
        self._index = -1

    def __next__(self):
        self._index += 1
        if self._index>=len(self._c):
            self._index = 0
        return self._c[self._index]

    def previous(self):
        self._index -= 1
        if self._index < 0:
            self._index = len(self._c)-1
        return self._c[self._index]

ABC = ['a', 'b', 'c']
abc = cycle(ABC)
print(next(abc))
print(next(abc))
print(next(abc))
print(abc.previous())

回答4:

Although deque is the way to go, here's another example:

Code

import itertools as it


class Cycle:
    """Wrap cycle."""
    def __init__(self, seq):
        self._container = it.cycle(seq)
        self._here = None
        self.prev = None

    def __iter__(self):
        return self._container

    def __next__(self):
        self.prev = self._here
        self._here = next(self._container)
        return self._here

Demo

c = Cycle("abc")
next(c)
# 'a'
next(c)
# 'b'
c.prev
# 'a'

来源：https://stackoverflow.com/questions/44611890/itertools-previous-opposite-of-next-python

标签

python

python-3.x

itertools