Question

Can someone explain to me why in python when we want to join a string we write:

'delim'.join(list)

and when we want to split a string we write:

str.split('delim')

coming from java it seems that one of these is backwards because in java we write:

//split:
str.split('delim');
//join
list.join('delim');

edit:
you are right. join takes a list. (though it doesnt change the question)

Can someone explain to me the rationale behind this API?

Answer 1

Join only makes sense when joining some sort of iterable. However, since iterables don’t necessarily contain all strings, putting join as a method on an iterable doesn’t make sense. (what would you expect the result of [1,"baz",my_custom_object,my_list].join("foo") to be?) The only other place to put it is as a string method with the understanding that everything in the iterable is going to be a string. Additionally, putting join as a string method allows it to be used with any iterable — tuples, lists, generators, custom objects which support iteration or even strings.

Also note that you are completely free to split a string in the same way that you join it:

list_of_strings='this, is , a, string, separated, by , commas.'.split(',')

Of course, the utility here isn’t quite as easy to see.