Transforming a dict of lists into a list of dicts

Question

I would like to transform a dict of one format into a dict of another format.

The raw_input dict will have the shape:

'key': ['list of strings']

I would like to reformat it into a list of dicts with the following shape:

[
    {'key': 'string'},
    {'key': 'string'},
    ...etc for each item in the list of strings
]

My implementation is functional but naive, using a doubly nested for loop:

raw_input = {
  'error': ['string 1', 'string2'],
  'error2': ['string 3']
}

def return_dict_as_list(raw_input):
    mylist = []
    for foo in raw_input: 
        for bar in raw_input[foo]: 
            mylist.append( { 'mykey': foo, 'myvalue': bar } )
    return mylist

The output is as expected but I can't help but think there's a better way.

I did try a couple of list comprehensions, but the output was not what was desired:

for example this:

elist = [[{'field': item, 'message': msg} for msg in raw[item]] for item in raw]

returns nested lists within a list, which I could unpack but doesn't seem very zen.

here's an online repl with the code: https://repl.it/repls/UnpleasantBiodegradableSystems

Answer 1

You can at least turn the inner for loop into a list/generator comprehension and use list.extend ("Flat is better than nested"):

def return_dict_as_list(raw_input):
    mylist = []
    for key, values in raw_input.items():
        my_list.extend({'mykey': key, 'myvalue': value} for value in values)
    return mylist

But you can even turn it into one list comprehension with two for loops in it (and no nested lists):

def return_dict_as_list(raw_input):
    return [{'mykey': key, 'myvalue': value}
            for key in raw_input
            for value in raw_input[key]]

This will be a bit faster, too, since list comprehensions loop at C speed.

Another approach is to make it a generator:

def to_list_generator(d):
    for key, values in d.items():
        for value in values:
            yield {'mykey': key, 'myvalue': value}

This makes it a lot easier to see what is happening (depending on your taste). It also means that you don't need to hold the new list in memory but can process one item at a time in some other function.