[Tutor] map & lambda to create dictionary from lists?

[Danny Yoo]

On Fri, 24 Aug 2001, Lance E Sloan wrote:

> The recent discussion of map and lambda coupled with a bit of ugly code
> I wrote recently inspired me to learn more about these things.
> 
> What I've got are two sequences.  The first is a sequence of strings
> that I want to use as the keys of a dictionary.  The second is a
> sequence of objects (strings, numbers, whatever) that I want to use as
> the values in the dictionary.  Of course, the two sequences have a
> one-to-one correspondence.
> 
> Here's what I've come up with using map and lambda so far and I want to
> know if it's Good:
> 
>     Python 2.0 (#1, Jan  8 2001, 10:18:58) 
>     [GCC egcs-2.91.66 19990314 (egcs-1.1.2 release)] on sunos5
>     Type "copyright", "credits" or "license" for more information.
>     >>> d = {}
>     >>> keys = ('name', 'age', 'food')
>     >>> values = ('Monty', 42, 'spam')
>     >>> junk = map(lambda k, v: d.update({k: v}), keys, values)
>     >>> junk
>     [None, None, None]
>     >>> d
>     {'name': 'Monty', 'age': 42, 'food': 'spam'}
> 
> Okay, so it works.  The list returned by map isn't all that useful,
> other than maybe for finding out how many pairs were processed.  I feel
> like I'm not using map the way it was intended.  Is this a Bad Thing?


> More importantly, is there a better way to do this?  I'd rather not
> reinvent the wheel with this.


I'm not sure if this is better; tell me what you think of this approach:

###
def itemsToDict(items):
    dict = {}
    for key, value in items:
        dict[key] = value

keys = ('name', 'age', 'foo')
values = ('Monty', 42, 'spam')
zipped_up_values = zip(keys, values)
dict = itemsToDict(zipped_up_values)
###

itemsToDict() seems to be a useful function; I use it all the time, but I
always have lingering doubts that I've reinvented the wheel somehow.


As a somewhat related note: map(), too, is special cased to do "zipping"
if we give it None in the function argument.

###
>>> map(None, [1, 2, 3], [4, 5, 6])
[(1, 4), (2, 5), (3, 6)]
###


Hope this helps!