[Python-ideas] Add list.join() please

Terry Reedy tjreedy at udel.edu
Wed Jan 30 02:17:29 EST 2019

On 1/29/2019 7:12 PM, MRAB wrote:
> On 2019-01-29 23:30, Terry Reedy wrote:
>> On 1/28/2019 8:40 PM, Jamesie Pic wrote:
>>   > 0. os.path.join takes *args
>> Since at least 1 path is required, the signature is join(path, *paths).
>> I presume that this is the Python version of the Unix version of the
>> system call that it wraps..  The hidden argument is os.sep.  It is
>> equivalent to os.sep.join((path,)+paths) (though one would not write it
>> this way).
>>> 1. str.join takes a list argument,
>> This premise behind the (repeated) request is a false.  str.joins
>> arguments are a string (the joiner) and an *iterable of strings*, which
>> is an abstract subclass of the abstract concept 'iterable'.  And only a
>> small fraction of lists are lists of strings and therefore iterables of
>> strings.
> One the examples given was writing:
>  >>> '/'.join('some', 'path')
> To me, this suggests that what the OP _really_ wants is for str.join to 
> accept multiple arguments, much as os.path.join does.
> I thought that there would be a problem with that because currently the 
> single argument is an iterable, and you wouldn't want to iterate the 
> first argument of '/'.join('some', 'path').

> However, both min and max will accept either a single argument that's 
> iterated over or multiple arguments that are not, so there's a precedent 
> there.

I have done things like this in private code, but it makes for messy 
signatures.  The doc pretends that min has two signatures, given in the 

     min(iterable, *[, default=obj, key=func]) -> value
     min(arg1, arg2, *args, *[, key=func]) -> value

I believe that the actual signature is the uninformative min(*args, 
**kwargs).  The arg form, without key, is the original.  If min were 
being written today, I don't think it would be included.

Terry Jan Reedy

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