[Python-ideas] Make map() better

[Nick Coghlan]

On 15 September 2017 at 03:38, Antoine Rozo <antoine.rozo at gmail.com> wrote:
>> Why is it ','.join(iterable), why isn't there join(',', iterable)
>
> Because join apply on a string, and strings are defined by the str class,
> not by a specific protocol (unlike iterables).

Join actually used to only be available as a function (string.join in
Python 2.x). However, nobody could ever remember whether the parameter
order was "join this series of strings with this separator" or "use
this separator to join this series of strings" - it didn't have the
same kind of natural ordering that map and filter did thanks to the
old "apply" builtin ("apply this function to these arguments" - while
the apply builtin itself is no longer around, the "callable(*args,
**kwds)" ordering that corresponds to the map() and filter() argument
ordering lives on).

This meant that when string.join became a builtin interface, it became
a string method since:

1. Strings are one of the few Python APIs that *aren't* protocol
centric - they're so heavily intertwined with the interpreter
implementation, that most folks don't even try to emulate or even
subclass them*.
2. As a string method, it's obvious what the right order has to be
(separator first, since it's the separator that has the method)

As a result, the non-obvious to learn, but easy to remember, method
spelling became the only spelling when string.join was dropped for
Python 3.0.

Cheers,
Nick.

* One case where it was actually fairly common for folks to define
their own str subclasses, rich filesystem path objects, finally gained
its own protocol in Python 3.6

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia