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Python: Why Lists do not have a find method?

Answer 1

+4 A:

I don't know why or maybe is buried in some PEP somewhere, but i do know 2 very basic "find" method for lists, and they are array.index() and the in operator. You can always make use of these 2 to find your items. (Also, re module, etc)

ghostdog74 2010-10-03 07:28:53

`index` method and `in` operator can look for single items in lists (Somebody please tell me if I can call them atomic). OP wants to know if he can find whether a list is a sub-list of another with items appearing in the same order.

dheerosaur 2010-10-03 07:35:04

But the basic principle behind is also iteration through the list, whether its nested or not, and finding them with these operators. OP is also free to develop own method/classes/generators etc for that.

ghostdog74 2010-10-03 07:37:35

The standard `find` method for arrays is to search for a single item. Nobody would be surprised to learn that an array implementation doesn't do substring searches.

Glenn Maynard 2010-10-03 07:37:49

@Glenn Maynard: With Python's otherwise very thorough treatment of lists, and its insistence on strings being treated similar to lists, I do find it surprising.

Tim Yates 2010-10-03 16:52:54

Answer 2

+1 A:

The "find" method for lists is index.

I do consider the inconsistency between string.find and list.index to be unfortunate, both in name and behavior: string.find returns -1 when no match is found, where list.index raises ValueError. This could have been designed more consistently. The only irreconcilable difference between these operations is that string.find searches for a string of items, where list.index searches for exactly one item (which, alone, doesn't justify using different names).

Glenn Maynard 2010-10-03 07:36:25

try `"asdfasdf".index('z')` next time your in an REPL.

aaronasterling 2010-10-03 07:50:50

@AaronMcSmooth: It highlights the absurdity of the oft-parroted "one obvious way to do something", when they can't even hold to it for core string methods.

Glenn Maynard 2010-10-03 08:18:44

but the point of my example is that they _do_ hold to it. `list` and `string` both have an `index` method that behaves exactly the same. it's just that `string` adds a `find` method on top of that. I'm not sure why they don't do it for `list` as well but there is no equivalence between `list.index` and `string.find`.

aaronasterling 2010-10-03 08:33:13

@AaronMcSmooth: The inconsistency is just shifted a little, with the underlying cause being the redundant search functions. (`getattr` handles this properly, allowing both a sentinel or an exception depending on what's needed.)

Glenn Maynard 2010-10-03 09:22:32

@AaronMcSmooth, although `string` and `list` both have an `.index()` method, it doesn't do quite the same thing. For strings it will find substring, but a sublist of a list -- which to me is another example of the inconsistency.

martineau 2010-10-03 12:38:33

@marineau, that's a great point. I suppose I'm wrong. not the first time by a mile

aaronasterling 2010-10-03 12:39:15

@AaronMcSmooth. How can you see and respond to my comment before I even hit the 'Add Comment' button? It really throws me off and now I can't fix the typos in it.

martineau 2010-10-03 12:43:58

Answer 3

+1 A:

Why would you need find if you have index? I think lists do not have find because technically find returns the first index of a substring. If you applied that notion to lists you would be able to say something like:

>[1,2,3,4,5].find([3,4])
2
># or
>["hello","world"].find("rld")
1

This is pretty counter intuitive for lists and doesn't seem very pythonic. There is list comprehension for accomplishing this sort of thing.

Gabriel 2010-10-03 07:37:31

why lists have both?

2010-10-03 08:01:32

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Python: Why Lists do not have a find method?

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