Merge two lists of lists - Python

Question

This is a great primer but doesn't answer what I need: http://stackoverflow.com/questions/464342/combining-two-sorted-lists-in-python

I have two Python lists, each is a list of datetime,value pairs:

list_a = [['1241000884000', 3], ['1241004212000', 4], ['1241006473000', 11]]

And:

list_x = [['1241000884000', 16], ['1241000992000', 16], ['1241001121000', 17], ['1241001545000', 19], ['1241004212000', 20], ['1241006473000', 22]]

1. There are actually numerous list_a lists with different key/values.
2. All list_a datetimes are in list_x.
3. I want to make a list, list_c, corresponding to each list_a which has each datetime from list_x and value_a/value_x.

Bonus:

In my real program, list_a is actually a list within a dictionary like so. Taking the answer to the dictionary level would be:

dict = {object_a: [['1241000884000', 3], ['1241004212000', 4], ['1241006473000', 11]], object_b: [['1241004212000', 2]]}

I can figure that part out though.

Answer 1

Here's some code that does what you asked for. You can turn your list of pairs into a dictionary straightforwardly. Then keys that are shared can be found by intersecting the sets of keys. Finally, constructing the result dictionary is easy given the set of shared keys.

dict_a = dict(list_a)
dict_x = dict(list_x)

shared_keys = set(dict_a).intersection(set(dict_x))

result = dict((k, (dict_a[k], dict_x[k])) for k in shared_keys)

Answer 2

"I want to make a list, list_c, corresponding to each list_a which has each datetime from list_x and value_a/value_x."

def merge_lists( list_a, list_x ):
    dict_x= dict(list_x)
    for k,v in list_a:
        if k in dict_x:
            yield k, (v, dict_x[k])

Something like that may work also.

merged= list( merge_lists( someDict['object_a'], someDict['object_b'] )

This may be slightly quicker because it only makes one dictionary for lookups, and leaves the other list alone.

Answer 3

Could try extend:

list_a.extend(list_b)

Answer 4

Nothing beats a nice functional one-liner:

reduce(lambda l1,l2: l1 + l2, list)

Answer 5

Both list concatenation and extension is available:

>>> mylist = [1, 2]
>>> mylist + [5, 6]         # concatenation
[1,2,5,6]

>>> mylist.extend([7, 8])   # extension
>>> print mylist
[1,2,7,8]

>>> mylist += [9, 10]       # extension
>>> print mylist
[1,2,7,8,9,10]