[Python-Dev] Re: A small proposed change to dictionaries' "get" method

[M.-A. Lemburg]

"Barry A. Warsaw" wrote:
> 
> >>>>> "GM" == Gareth McCaughan <Gareth.McCaughan@pobox.com> writes:
> 
>     GM> Consider the following piece of code, which takes a file
>     GM> and prepares a concordance saying on which lines each word
>     GM> in the file appears. (For real use it would need to be
>     GM> made more sophisticated.)
> 
>     |     line_number = 0
>     |     for line in open(filename).readlines():
>     |       line_number = line_number+1
>     |       for word in map(string.lower, string.split(line)):
>     |         existing_lines = word2lines.get(word, [])   |
>     |         existing_lines.append(line_number)          | ugh!
>     |         word2lines[word] = existing_lines           |
> 
> I've run into this same situation many times myself.  I agree it's
> annoying.  Annoying enough to warrant a change?  Maybe -- I'm not
> sure.
> 
>     GM> I suggest a minor change: another optional argument to
>     GM> "get" so that
> 
>     GM>     dict.get(item,default,flag)
> 
> Good idea, not so good solution.  Let's make it more explicit by
> adding a new method instead of a flag.  I'll use `put' here since this
> seems (in a sense) opposite of get() and my sleep addled brain can't
> think of anything more clever.  Let's not argue about the name of this
> method though -- if Guido likes the extension, he'll pick a good name
> and I go on record as agreeing with his name choice, just to avoid a
> protracted war.
> 
> A trivial patch to UserDict (see below) will let you play with this.
> 
> >>> d = UserDict()
> >>> word = 'hello'
> >>> d.get(word, [])
> []
> >>> d.put(word, []).append('world')
> >>> d.get(word)
> ['world']
> >>> d.put(word, []).append('gareth')
> >>> d.get(word)
> ['world', 'gareth']
> 
> Shouldn't be too hard to add equivalent C code to the dictionary
> object.

The following one-liner already does what you want:

	d[word] = d.get(word, []).append('world')

... and it's in no way more readable than your proposed
.put() line ;-)

-- 
Marc-Andre Lemburg
______________________________________________________________________
Business:                                      http://www.lemburg.com/
Python Pages:                           http://www.lemburg.com/python/