我知道您可以使用
c = cycle(['a', 'b', 'c'])
使用 c.next()
在元素之间循环,但是有没有办法获取迭代器的当前元素?
例如,如果
c.next()
返回 'c'
,则意味着迭代器之前位于 'b'
。有没有一种方法可以在不使用 'b'
的情况下获得 next()
?
迭代器/生成器没有任何方法来获取当前值。您应该保留对它的引用,或者创建一些为您保留它的包装器。
注意:这仅在没有重复元素的情况下有效。 这是否是一个严重的实际限制取决于每个人的使用情况。 就我而言,我合作过的大多数
itertools.cycle
都属于这一类。
实际上可以通过辅助函数获取
cycle
的当前状态以及其他信息。
它实际上使用 next
,但这对调用者来说是透明的。
import itertools
def get_cycle_props(cycle) :
"""Get the properties (elements, length, current state) of a cycle, without advancing it"""
# Get the current state
partial = []
n = 0
g = next(cycle)
while ( g not in partial ) :
partial.append(g)
g = next(cycle)
n += 1
# Cycle until the "current" (now previous) state
for i in range(n-1) :
g = next(cycle)
return (partial, n, partial[0])
def get_cycle_list(cycle) :
"""Get the elements of a cycle, without advancing it"""
return get_cycle_props(cycle)[0]
def get_cycle_state(cycle) :
"""Get the current state of a cycle, without advancing it"""
return get_cycle_props(cycle)[2]
def get_cycle_len(cycle) :
"""Get the length of a cycle, without advancing it"""
return get_cycle_props(cycle)[1]
# initialize list
test_list = [3, 4, 5, 7, 1]
c = itertools.cycle(test_list)
print('cycle state =', get_cycle_state(c))
print('cycle length =', get_cycle_len(c))
print('cycle list =', get_cycle_list(c))
next(c)
print('cycle state =', get_cycle_state(c))
print('cycle length =', get_cycle_len(c))
print('cycle list =', get_cycle_list(c))
产生以下输出
cycle state = 3
cycle length = 5
cycle list = [3, 4, 5, 7, 1]
cycle state = 4
cycle length = 5
cycle list = [4, 5, 7, 1, 3]
实际上可以利用该功能来“倒带”一个周期
def shift_cycle(cycle, npos=0) :
"""Shift a cycle, a given number of positions (can be negative)."""
(cycle_list, nelem, curr_state) = get_cycle_props(cycle)
for i in range(nelem+npos) :
g = next(cycle)
return
尝试
shift_cycle(c, -2)
print('cycle state =', get_cycle_state(c))
这是一个简单的包装:
from itertools import cycle
from typing import Generic, Iterator, TypeVar
_T = TypeVar("_T")
class Cycler(Generic[_T]):
def __init__(self, cycle: "cycle[_T]") -> None:
self._cycle = cycle
@property
def current(self):
if not hasattr(self, "_current"):
return next(self)
return self._current
def __iter__(self) -> Iterator[_T]:
return self
def __next__(self):
self._current = next(self._cycle)
return self._current
这个演示—
letters = ("a", "b", "c")
letters_cycler = Cycler(cycle(letters))
# The cycle can start by attempting to get the current element, or by calling next on it
print("Start cycle")
# print(next(letters_cycler))
print("Check:", letters_cycler.current)
print()
print("Next: ", next(letters_cycler))
print("Check:", letters_cycler.current)
print()
print("Next: ", next(letters_cycler))
print("Check:", letters_cycler.current)
print()
print("Next: ", next(letters_cycler))
print("Check:", letters_cycler.current)
print()
print("Next: ", next(letters_cycler))
print("Check:", letters_cycler.current)
print()
—打印:
Start cycle
Check: a
Next: b
Check: b
Next: c
Check: c
Next: a
Check: a
Next: b
Check: b
这个演示—
letters_cycler_new = Cycler(cycle(letters))
for i, letter in enumerate(letters_cycler_new):
print(letter)
assert letter == letters_cycler_new.current
if i == 6:
break
—打印:
a
b
c
a
b
c
a