combining lists in a dictionary

[Skaperen]

i have a dictionary in which the values of each element is a list. i am trying to come up with a not complicated comprehension that gives me a list that is all those values concatenated. the order of concatenation does not matter since the resulting list will be sorted. i tried sum() thinking that since it does + operations, it could be applied to lists to achieve concatenation, but i got a type error from that.

[ichabod801]

sum(dict.values(), [])

[Skaperen]

why was sum() designed to need the 2nd argument? why was sum() not designed to do its thing across any number of arguments like min() and max() were? my new mysum() function does.

def mysum(*args):
    if len(args)<1:
        return 0
    if isinstance(args[0],(list,tuple,set,frozenset,dict)):
        args=[x for x in args[0]]
    if args:
        s=args[0]
        for x in args[1:]:
            s+=x
        return s
    return 0

[ichabod801]

I don't know. Why do you have to complain about everything in Python?

[MckJohan]

just straightly pass the dict to sum function. 2nd arguments is just optional to add the result. hope this will help.

>>> dict = {'a':[1,2,3,4,5], 'b':[5,10,15,20]}
>>> sum(dict['a'])
15
>>> sum(dict['a'], 5)
20
>>> sum(dict['b'])
50
>>> sum(dict['b'], 50)
100
>>> 

[Skaperen]

i know how sum() works. i forgot about setting the 2nd argument to the initial value since i rarely use sum(). i use min(), max(), all(), and any() more often and was thinking that way.

i'm not complaining. i am more curious about the decisions because i have used a few other languages of nearly this model (not the syntax model, but the run-time model). and, i even designed one (but never implemented it). my curiosity also includes the syntax, like that question i recently had about the ambiguity if the ":" was omitted to start a new block. the creator side of my brain is always active (hence my nickname).

[perfringo]

There are many built-in tools for achieving desired result. One just need to find most suitable for use case. I would add to ichabod801 following possibilities:

Concat and sort in one go (using built-in itertools.chain):

sorted(i for i in itertools.chain(*d.values()))

As operation in question is not sum per se but concat (concatenation is what the + operator is written to do for Python's built-in lists). So why don't use built-in operator.concat? Combined with functools.reduce (used to be built-in function, now moved to functools) one can:

functools.reduce(operator.concat, d.values())

If interested how Python sum() evolved and what options were discussed one can look into mail.python.org archive. I provide one example thread named summing a bunch of numbers (or whatevers). But up until resignation simple explanation to 'why' is: 'this is the way Guido wanted (decided)'

If I try mysum function on dictionary d = {'a': [4], 'b': [2], 'c': [3]} with mysum(d) then 'abc' is returned. Is it expected behaviour?

[ichabod801]

I like the solution with chain, but isn't chain already a generator? Can't you just do:

sorted(itertools.chain(*d.values()))

[perfringo]

I like the solution with chain, but isn't chain already a generator? Can't you just do:

sorted(itertools.chain(*d.values()))

Mea culpa. You are absolutely right.

[DeaD_EyE]

You can get rid of the *:

sorted(itertools.chain.from_iterable(d.values()))

BTW: You will find inconsistencies in all existing languages (natural and logical). Humans make mistakes, humans build machines with bugs and humans are creating language definitions with "bugs". This is our nature. You can't be perfect and if you think you got the perfect solution, someone else is coming and will show all your mistakes you made, but even his improvement will never be perfect.