Why is it string.join(list) instead of list.join(string)?

It's because any iterable can be joined (e.g, list, tuple, dict, set), but its contents and the "joiner" must be strings.

For example:

'_'.join(['welcome', 'to', 'stack', 'overflow'])
'_'.join(('welcome', 'to', 'stack', 'overflow'))

'welcome_to_stack_overflow'

Using something other than strings will raise the following error:

TypeError: sequence item 0: expected str instance, int found

Because the join() method is in the string class, instead of the list class?

I agree it looks funny.

See http://www.faqs.org/docs/diveintopython/odbchelper_join.html:

Historical note. When I first learned Python, I expected join to be a method of a list, which would take the delimiter as an argument. Lots of people feel the same way, and there’s a story behind the join method. Prior to Python 1.6, strings didn’t have all these useful methods. There was a separate string module which contained all the string functions; each function took a string as its first argument. The functions were deemed important enough to put onto the strings themselves, which made sense for functions like lower, upper, and split. But many hard-core Python programmers objected to the new join method, arguing that it should be a method of the list instead, or that it shouldn’t move at all but simply stay a part of the old string module (which still has lots of useful stuff in it). I use the new join method exclusively, but you will see code written either way, and if it really bothers you, you can use the old string.join function instead.

--- Mark Pilgrim, Dive into Python

I agree that it's counterintuitive at first, but there's a good reason. Join can't be a method of a list because:

• it must work for different iterables too (tuples, generators, etc.)
• it must have different behavior between different types of strings.

There are actually two join methods (Python 3.0):

>>> b"".join
<built-in method join of bytes object at 0x00A46800>
>>> "".join
<built-in method join of str object at 0x00A28D40>

If join was a method of a list, then it would have to inspect its arguments to decide which one of them to call. And you can't join byte and str together, so the way they have it now makes sense.

Think of it as the natural orthogonal operation to split.

I understand why it is applicable to anything iterable and so can't easily be implemented just on list.

For readability, I'd like to see it in the language but I don't think that is actually feasible - if iterability were an interface then it could be added to the interface but it is just a convention and so there's no central way to add it to the set of things which are iterable.

Primarily because the result of a someString.join() is a string.

The sequence (list or tuple or whatever) doesn't appear in the result, just a string. Because the result is a string, it makes sense as a method of a string.

Both are not nice.

string.join(xs, delimit) means that the string module is aware of the existence of a list, which it has no business knowing about, since the string module only works with strings.

list.join(delimit) is a bit nicer because we're so used to strings being a fundamental type(and lingually speaking, they are). However this means that join needs to be dispatched dynamically because in the arbitrary context of a.split("\n") the python compiler might not know what a is, and will need to look it up(analogously to vtable lookup), which is expensive if you do it a lot of times.

if the python runtime compiler knows that list is a built in module, it can skip the dynamic lookup and encode the intent into the bytecode directly, whereas otherwise it needs to dynamically resolve "join" of "a", which may be up several layers of inheritence per call(since between calls, the meaning of join may have changed, because python is a dynamic language).

sadly, this is the ultimate flaw of abstraction; no matter what abstraction you choose, your abstraction will only make sense in the context of the problem you're trying to solve, and as such you can never have a consistent abstraction that doesn't become inconsistent with underlying ideologies as you start gluing them together without wrapping them in a view that is consistent with your ideology. Knowing this, python's approach is more flexible since it's cheaper, it's up to you to pay more to make it look "nicer", either by making your own wrapper, or your own preprocessor.

The variables my_list and "-" are both objects. Specifically, they're instances of the classes list and str, respectively. The join function belongs to the class str. Therefore, the syntax "-".join(my_list) is used because the object "-" is taking my_list as an input.

You can't only join lists and tuples. You can join almost any iterable. And iterables include generators, maps, filters etc

>>> '-'.join(chr(x) for x in range(48, 55))
'0-1-2-3-4-5-6'

>>> '-'.join(map(str, (1, 10, 100)))
'1-10-100'

And the beauty of using generators, maps, filters etc is that they cost little memory, and are created almost instantaneously.

Just another reason why it's conceptually:

str.join(<iterator>)

It's efficient only granting str this ability. Instead of granting join to all the iterators: list, tuple, set, dict, generator, map, filter all of which only have object as common parent.

Of course range(), and zip() are also iterators, but they will never return str so they cannot be used with str.join()

>>> '-'.join(range(48, 55))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sequence item 0: expected str instance, int found

I 100% agree with your issue. If we boil down all the answers and comments here, the explanation comes down to "historical reasons".

str.join isn't just confusing or not-nice looking, it's impractical in real-world code. It defeats readable function or method chaining because the separator is rarely (ever?) the result of some previous computation. In my experience, it's always a constant, hard-coded value like ", ".

I clean up my code — enabling reading it in one direction — using tools.functoolz:

>>> from toolz.functoolz import curry, pipe
>>> join = curry(str.join)
>>>
>>> a = ["one", "two", "three"]
>>> pipe(
...     a, 
...     join("; ")
>>> )
'one; two; three'

I'll have several other functions in the pipe as well. The result is that it reads easily in just one direction, from beginning to end as a chain of functions. Currying map helps a lot.