searching a value of a dict (each value is a list)

[Zepo Len]

> I have a dictionary with million keys. Each value in the
> dictionary has a list with up to thousand integers.
> Follow is a simple example with 5 keys.
>
> dict = {1: [1, 2, 3, 4, 5],
>    2: [10, 11, 12],
>    900000: [100, 101, 102, 103, 104, 105],
>    900001: [20, 21, 22],
>    999999: [15, 16, 17, 18, 19]}
>
> I want to find out the key value which has a specific
> integer in the list of its value. For example, if I search
> 104 in the list, 900000 must be returned.
>
> How can I do this with Python? Ideas?

def find_key(dict, num):
   for k in dict:
     if num in dict[k]:
       return k