在列表中查找属性等于某个值(满足任何条件)的对象

Question

我有对象列表.我想在这个列表中找到一个(第一个或任何)对象,它具有等于的属性(或方法结果 - 无论如何)value.

找到它的最佳方法是什么？

这是测试用例:

  class Test:
      def __init__(self, value):
          self.value = value

  import random

  value = 5

  test_list = [Test(random.randint(0,100)) for x in range(1000)]

  # that I would do in Pascal, I don't believe isn't anywhere near 'Pythonic'
  for x in test_list:
      if x.value == value:
          print "i found it!"
          break

我认为使用生成器并reduce()没有任何区别,因为它仍然会遍历列表.

ps:方程式value只是一个例子.当然我们想要得到满足任何条件的元素.

Answer 1

next((x for x in test_list if x.value == value), None)

这将从列表中获取与条件匹配的第一个项目,None如果没有项目匹配则返回.这是我首选的单表达形式.

然而,

for x in test_list:
    if x.value == value:
        print "i found it!"
        break

天真的循环中断版本,完美的Pythonic - 它简洁,清晰,高效.为了使它符合单线的行为:

for x in test_list:
    if x.value == value:
        print "i found it!"
        break
else:
    x = None

如果您没有退出循环,这将分配None给x您break.

Answer 2

一个简单的例子：我们有以下数组

li = [{"id":1,"name":"ronaldo"},{"id":2,"name":"messi"}]

现在，我们要在数组中找到 id 等于 1 的对象

1. 使用next具有列表理解的方法

next(x for x in li if x["id"] == 1 )

2. 使用列表理解并返回第一项

[x for x in li if x["id"] == 1 ][0]

3. 自定义功能

def find(arr , id):
    for x in arr:
        if x["id"] == id:
            return x
find(li , 1)

输出所有上述方法是 {'id': 1, 'name': 'ronaldo'}

Answer 3

由于尚未提及完成.好过滤器过滤你的过滤元素.

功能编程ftw.

####### Set Up #######
class X:

    def __init__(self, val):
        self.val = val

elem = 5

my_unfiltered_list = [X(1), X(2), X(3), X(4), X(5), X(5), X(6)]

####### Set Up #######

### Filter one liner ### filter(lambda x: condition(x), some_list)
my_filter_iter = filter(lambda x: x.val == elem, my_unfiltered_list)
### Returns a flippin' iterator at least in Python 3.5 and that's what I'm on

print(next(my_filter_iter).val)
print(next(my_filter_iter).val)
print(next(my_filter_iter).val)

### [1, 2, 3, 4, 5, 5, 6] Will Return: ###
# 5
# 5
# Traceback (most recent call last):
#   File "C:\Users\mousavin\workspace\Scripts\test.py", line 22, in <module>
#     print(next(my_filter_iter).value)
# StopIteration


# You can do that None stuff or whatever at this point, if you don't like exceptions.

我知道通常在python列表中,首选理解是首选,或者至少这是我读过的内容,但我不认为问题是诚实的.当然,Python不是FP语言,但Map/Reduce/Filter是完全可读的,是函数式编程中最标准的标准用例.

你去吧 了解你的功能编程.

过滤条件清单

它不会比这更容易:

next(filter(lambda x: x.val == value,  my_unfiltered_list)) # Optionally: next(..., None) or some other default value to prevent Exceptions