Question

I’m constantly wrapping my str.join() arguments in a list, e.g.

'.'.join([str_one, str_two])

The extra list wrapper always seems superfluous to me. I’d like to do…

'.'.join(str_one, str_two, str_three, ...)

… or if I have a list …

'.'.join(*list_of_strings)

Yes I’m a minimalist, yes I’m picky, but mostly I’m just curious about the history here, or whether I’m missing something. Maybe there was a time before splats?

Edit:

I’d just like to note that max() handles both versions:

max(iterable[, key])
max(arg1, arg2, *args[, key])

Answer 1

For short lists this won’t matter and it costs you exactly 2 characters to type. But the most common use-case (I think) for str.join() is following:

''.join(process(x) for x in some_input) 
# or
result = []
for x in some_input:
    result.append(process(x))
''.join(result)

where input_data can have thousand of entries and you just want to generate the output string efficiently.

If join accepted variable arguments instead of an iterable, this would have to be spelled as:

''.join(*(process(x) for x in some_input))
# or
''.join(*result)

which would create a (possibly long) tuple, just to pass it as *args.

So that’s 2 characters in a short case vs. being wasteful in large data case.

History note

(Second Edit: based on HISTORY file which contains missing release from all releases. Thanks Don.)

The *args in function definitions were added in Python long time ago:

==> Release 0.9.8 (9 Jan 1993) <==

Case (a) was needed to accommodate variable-length argument lists;
there is now an explicit “varargs” feature (precede the last argument
with a ‘*’). Case (b) was needed for compatibility with old class
definitions: up to release 0.9.4 a method with more than one argument
had to be declared as “def meth(self, (arg1, arg2, …)): …”.

A proper way to pass a list to such functions was using a built-in function apply(callable, sequence). (Note, this doesn’t mention **kwargs which can be first seen in docs for version 1.4).

The ability to call a function with * syntax is first mentioned in release notes for 1.6:

There’s now special syntax that you can use instead of the apply()
function. f(*args, **kwds) is equivalent to apply(f, args, kwds). You
can also use variations f(a1, a2, *args, **kwds) and you can leave one
or the other out: f(args), f(*kwds).

But it’s missing from grammar docs until version 2.2.

Before 2.0 str.join() did not even exists and you had to do from string import join.