Python：在列表中查找 [英] Python: Find in list
问题描述
item = someSortOfSelection（）
if myList中的item：
doMySpecialFunction（item）
但是有时它不适用于我所有的项目，就好像它们不是在列表中被识别（当它是一个字符串列表）。
是在列表中找到一个项目的最pythonic方法：<$ c $至于你的第一个问题：那个代码是完美无缺的，这个代码是完美的。如果 item 等于 myList 中的元素之一，那么应该工作正常。也许你试图找到一个不匹配其中一个项目的字符串，或者你正在使用一个不准确的浮点值。
至于你的第二个问题：实际上有几种可能的方法，如果在列表中找到的东西。
检查是否有内容
这是您描述的用例：检查列表中是否有内容。如您所知，您可以在运算符中使用：
3 in [1，2，3]＃=> True
过滤集合
即，找到满足一定条件的序列中的所有元素。您可以使用列表理解或生成器表达式：
matches = [x for x in x如果fulfills_some_condition（x）]
matches =（x for x in lst if x> 6）
后者将返回一个生成器，你可以将它想象成一种懒惰的列表，只有当你遍历它时才会生成。顺便说一句，第一个完全相同于 在这里，您可以看到工作中的高阶函数。在Python 3中，
$ p $ matches = filter（fulfills_some_condition，lst）
filter 不会返回一个列表，而是一个类似生成器的对象。
找到第一个如果你只想匹配一个条件的第一个东西（但你不知道它是什么），那么使用for循环（可能使用还有
else 子句，这并不是真正的众所周知的）。您也可以使用
下一个（x for lst if ...）
这将返回第一个匹配，或者如果找不到，则会引发 StopIteration 。或者，您可以使用
下一个（（x for lst中的x，如果...），[默认值]）
寻找物品的位置
，还有 index 方法，如果你想知道列表中某个元素的位置，有时可能有用：
$ b
[1,2,3] .index（2）＃=> 1
[1,2,3] .index（4）＃=> ValueError
但是，请注意，如果您有重复， .index 始终返回最低的索引：
$ $ $ $ $ $ $ $ $ 1,2,3,2] .index（2）＃= > 1
如果有重复项，并且您想要所有索引，则可以使用枚举（）代替：
3,2]）if x == 2]＃=> [1，3]
I have come across this delightful:
item = someSortOfSelection()
if item in myList:
doMySpecialFunction(item)
but sometimes it does not work with all my items, as if they weren't recognized in the list (when it's a list of string).
is it the most 'pythonic' way of finding an item in a list: if x in l:?
As for your first question: that code is perfectly fine and should work if item equals one of the elements inside myList. Maybe you try to find a string that does not exactly match one of the items or maybe you are using a float value which suffers from inaccuracy.
As for your second question: There's actually several possible ways if "finding" things in lists.
Checking if something is inside
This is the use case you describe: Checking whether something is inside a list or not. As you know, you can use the in operator for that:
3 in [1, 2, 3] # => True
Filtering a collection
That is, finding all elements in a sequence that meet a certain condition. You can use list comprehension or generator expressions for that:
matches = [x for x in lst if fulfills_some_condition(x)]
matches = (x for x in lst if x > 6)
The latter will return a generator which you can imagine as a sort of lazy list that will only be built as soon as you iterate through it. By the way, the first one is exactly equivalent to
matches = filter(fulfills_some_condition, lst)
in Python 2. Here you can see higher-order functions at work. In Python 3, filter doesn't return a list, but a generator-like object.
Finding the first occurrence
If you only want the first thing that matches a condition (but you don't know what it is yet), it's fine to use a for loop (possibly using the else clause as well, which is not really well-known). You can also use
next(x for x in lst if ...)
which will return the first match or raise a StopIteration if none is found. Alternatively, you can use
next((x for x in lst if ...), [default value])
Finding the location of an item
For lists, there's also the index method that can sometimes be useful if you want to know where a certain element is in the list:
[1,2,3].index(2) # => 1
[1,2,3].index(4) # => ValueError
However, note that if you have duplicates, .index always returns the lowest index:
[1,2,3,2].index(2) # => 1
If there are duplicates and you want all the indexes then you can use enumerate() instead:
[i for i,x in enumerate([1,2,3,2]) if x==2] # => [1, 3]
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