Combine two array's data using inner join

Organize your data differently (you can easily convert what you already have to two dicts):

d1 = { '2011-10-10': [1, 1],
       '2007-08-09': [5, 3]
     }
d2 = { '2011-10-10': [3, 4],
       '2007-09-05': [1, 1]
     }

Then:

d3 = { k : d1[k] + d2[k] for k in d1 if k in d2 }

It may be worth mentioning set data types. as their methods align to the type of problem. The set operators allow you to join sets easily and flexibly with full, inner, outer, left, right joins. As with dictionaries, sets do not retain order, but if you cast a set back into a list, you may then apply an order on the result join. Alternatively, you could use an ordered dictionary.

set1 = set(x[0] for x in arr1)
set2 = set(x[0] for x in arr2)
resultset = (set1 & set2)

This only gets you the union of dates in the original lists, in order to reconstruct arr3 you would need to append the [1:] data in arr1 and arr2 where the dates are in the result set. This reconstruction would not be as neat as using the dictionary solutions above, but using sets is worthy of consideration for similar problems.

You can convert the arrays to a dict, and back again.

d1 = dict((x[0],x[1:]) for x in arr1)
d2 = dict((x[0],x[1:]) for x in arr2)
keys = set(d1).union(d2)
n = []
result = dict((k, d1.get(k, n) + d2.get(k, n)) for k in keys)