在 Python 中只从单元素列表中获取元素？

当已知 Python 列表总是包含单个项目时，是否有其他方法访问它，而不是:

mylist[0]

你可能会问，‘你为什么要这么做?’？'.只是好奇。在 Python 中似乎有另一种做 一切的方法。

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最佳答案

Raises exception if not exactly one item:

Sequence unpacking:

singleitem, = mylist
# Identical in behavior (byte code produced is the same),
# but arguably more readable since a lone trailing comma could be missed:
[singleitem] = mylist

Rampant insanity, unpack the input to the identity lambda function:

# The only even semi-reasonable way to retrieve a single item and raise an exception on
# failure for too many, not just too few, elements as an expression, rather than a
# statement, without resorting to defining/importing functions elsewhere to do the work
singleitem = (lambda x: x)(*mylist)

All others silently ignore spec violation, producing first or last item:

Explicit use of iterator protocol:

singleitem = next(iter(mylist))

Destructive pop:

singleitem = mylist.pop()

Negative index:

singleitem = mylist[-1]

Set via single iteration for (because the loop variable remains available with its last value when a loop terminates):

for singleitem in mylist: break

There are many others (combining or varying bits of the above, or otherwise relying on implicit iteration), but you get the idea.

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I will add that the more_itertools library has a tool that returns one item from an iterable.

from more_itertools import one




iterable = ["foo"]
one(iterable)
# "foo"

In addition, more_itertools.one raises an error if the iterable is empty or has more than one item.

iterable = []
one(iterable)
# ValueError: not enough values to unpack (expected 1, got 0)


iterable = ["foo", "bar"]
one(iterable)
# ValueError: too many values to unpack (expected 1)

more_itertools is a third-party package > pip install more-itertools

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(This is an adjusted repost of my answer to a similar question related to sets.)

One way is to use reduce with lambda x: x.

from functools import reduce


> reduce(lambda x: x, [3]})
3


> reduce(lambda x: x, [1, 2, 3])
TypeError: <lambda>() takes 1 positional argument but 2 were given


> reduce(lambda x: x, [])
TypeError: reduce() of empty sequence with no initial value

Benefits:

  • Fails for multiple and zero values
  • Doesn't change the original list
  • Doesn't need a new variable and can be passed as an argument

Cons: "API misuse" (see comments).


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