从列表中获取第一个非None值-GettingthefirstnonNonevaluefromlist

73  

You can use next():

你可以使用next():

>>> a = [None, None, None, 1, 2, 3, 4, 5]
>>> next(item for item in a if item is not None)
1

If the list contains only Nones, it will throw StopIteration exception. If you want to have a default value in this case, do this:

如果列表仅包含Nones,则会抛出StopIteration异常。如果您想在这种情况下使用默认值,请执行以下操作:

>>> a = [None, None, None]
>>> next((item for item in a if item is not None), 'All are Nones')
All are Nones

4  

Adapt from the following (you could one-liner it if you wanted):

从以下内容进行调整(如果需要,可以单行调整):

values = (a, b, c, d)
not_NOne= (el for el in values if el is not None)
value = next(not_None, None)

This takes the first non None value, or returns None instead.

这将获取第一个非None值,或者返回None。

3  

first_true is an itertools recipe found in the Python 3 docs:

first_true是Python 3文档中的itertools配方:

def first_true(iterable, default=False, pred=None):
    """Returns the first true value in the iterable.

    If no true value is found, returns *default*

    If *pred* is not None, returns the first item
    for which pred(item) is true.

    """
    # first_true([a,b,c], x) --> a or b or c or x
    # first_true([a,b], x, f) --> a if f(a) else b if f(b) else x
    return next(filter(pred, iterable), default)

One may choose to implement the latter recipe or import more_itertools, a library that ships with itertools recipes and more:

可以选择实现后一个配方或导入more_itertools,一个带有itertools配方的库以及更多:

> pip install more_itertools

Use:

import more_itertools as mit

a = [None, None, None, 1, 2, 3, 4, 5]
mit.first_true(a, pred=lambda x: x is not None)
# 1

a = [None, None, None]
mit.first_true(a, default="All are None", pred=lambda x: x is not None)
# 'All are None'

Why use the predicate?

为什么要使用谓词?

"First non-None" item is not the same as "first True" item, e.g. [None, None, 0] where 0 is the first non-None, but it is not the first True item. The predicate allows first_true to be useable, ensuring any first seen, non-None, falsey item in the iterable is still returned (e.g. 0, False) instead of the default.

“First non-None”项目与“first True”项目不同,例如[None,None,0]其中0是第一个非None,但它不是第一个True项。谓词允许first_true可用,确保仍然返回迭代中的任何第一个看到的非None虚假项(例如0,False)而不是默认值。

a = [None, None, None, False]
mit.first_true(a, default="All are None", pred=lambda x: x is not None)
# 'False'