python列表的两个“包含”操作之间的区别

Question

I'm fairly new to python and have found that I need to query a list about whether it contains a certain item.
The majority of the postings I have seen on various websites (including this similar stackoverflow question ) have all suggested something along the lines of

for i in list
    if i == thingIAmLookingFor
        return True

However, I have also found from one lone forum that

if thingIAmLookingFor in list
    # do work

works.

I am wondering if the if thing in list method is shorthand for the for i in list method, or if it is implemented differently.

I would also like to which, if either, is more preferred.

Answer 1

In your simple example it is of course better to use in .

However ... in the question you link to , in doesn't work (at least not directly) because the OP does not want to find an object that is equal to something, but an object whose attribute n is equal to something.

One answer does mention using in on a list comprehension, though I'm not sure why a generator expression wasn't used instead:

if 5 in (data.n for data in myList):
    print "Found it"

But this is hardly much of an improvement over the other approaches, such as this one using any :

if any(data.n == 5 for data in myList):
    print "Found it"

Answer 2

the "if x in thing:" format is strongly preferred, not just because it takes less code, but it also works on other data types and is (to me) easier to read.

I'm not sure how it's implemented, but I'd expect it to be quite a lot more efficient on datatypes that are stored in a more searchable form. eg. sets or dictionary keys.

Answer 3

The if thing in somelist is the preferred and fastest way.

Under-the-hood that use of the in-operator translates to somelist.__contains__(thing) whose implementation is equivalent to: any((x is thing or x == thing) for x in somelist) .

Note the condition tests identity and then equality.

Answer 4

for i in list
    if i == thingIAmLookingFor
        return True

The above is a terrible way to test whether an item exists in a collection. It returns True from the function, so if you need the test as part of some code you'd need to move this into a separate utility function, or add thingWasFound = False before the loop and set it to True in the if statement (and then break), either of which is several lines of boilerplate for what could be a simple expression.

Plus, if you just use thingIAmLookingFor in list , this might execute more efficiently by doing fewer Python level operations (it'll need to do the same operations, but maybe in C, as list is a builtin type). But even more importantly, if list is actually bound to some other collection like a set or a dictionary thingIAmLookingFor in list will use the hash lookup mechanism such types support and be much more efficient, while using a for loop will force Python to go through every item in turn.

Obligatory post-script: list is a terrible name for a variable that contains a list as it shadows the list builtin, which can confuse you or anyone who reads your code. You're much better off naming it something that tells you something about what it means .