Finding matching keys in two large dictionaries and doing it fast

You could do this:

for key in myRDP:
    if key in myNames:
        print key, myNames[key]

Your first attempt was slow because you were comparing every key in myRDP with every key in myNames. In algorithmic jargon, if myRDP has n elements and myNames has m elements, then that algorithm would take O(n×m) operations. For 600k elements each, this is 360,000,000,000 comparisons!

But testing whether a particular element is a key of a dictionary is fast — in fact, this is one of the defining characteristics of dictionaries. In algorithmic terms, the key in dict test is O(1), or constant-time. So my algorithm will take O(n) time, which is one 600,000th of the time.

for key in myRDP:
    name = myNames.get(key, None)
    if name:
        print key, name

dict.get returns the default value you give it (in this case, None) if the key doesn’t exist.

Use the get method instead:

 for key in myRDP:
    value = myNames.get(key)
    if value != None:
      print key, "=", value

Copy both dictionaries into one dictionary/array. This makes sense as you have 1:1 related values. Then you need only one search, no comparison loop, and can access the related value directly.

Example Resulting Dictionary/Array:

[Name][Value1][Value2]

[Actinobacter][GATCGA...TCA][8924342]

[XYZbacter][BCABCA...ABC][43594344]

…

You could start by finding the common keys and then iterating over them. Set operations should be fast because they are implemented in C, at least in modern versions of Python.

common_keys = set(myRDP).intersection(myNames)
for key in common_keys:
    print key, myNames[key]

Here is my code for doing intersections, unions, differences, and other set operations on dictionaries:

class DictDiffer(object):
    """
    Calculate the difference between two dictionaries as:
    (1) items added
    (2) items removed
    (3) keys same in both but changed values
    (4) keys same in both and unchanged values
    """
    def __init__(self, current_dict, past_dict):
        self.current_dict, self.past_dict = current_dict, past_dict
        self.set_current, self.set_past = set(current_dict.keys()), set(past_dict.keys())
        self.intersect = self.set_current.intersection(self.set_past)
    def added(self):
        return self.set_current - self.intersect 
    def removed(self):
        return self.set_past - self.intersect 
    def changed(self):
        return set(o for o in self.intersect if self.past_dict[o] != self.current_dict[o])
    def unchanged(self):
        return set(o for o in self.intersect if self.past_dict[o] == self.current_dict[o])

if __name__ == '__main__':
    import unittest
    class TestDictDifferNoChanged(unittest.TestCase):
        def setUp(self):
            self.past = dict((k, 2*k) for k in range(5))
            self.current = dict((k, 2*k) for k in range(3,8))
            self.d = DictDiffer(self.current, self.past)
        def testAdded(self):
            self.assertEqual(self.d.added(), set((5,6,7)))
        def testRemoved(self):      
            self.assertEqual(self.d.removed(), set((0,1,2)))
        def testChanged(self):
            self.assertEqual(self.d.changed(), set())
        def testUnchanged(self):
            self.assertEqual(self.d.unchanged(), set((3,4)))
    class TestDictDifferNoCUnchanged(unittest.TestCase):
        def setUp(self):
            self.past = dict((k, 2*k) for k in range(5))
            self.current = dict((k, 2*k+1) for k in range(3,8))
            self.d = DictDiffer(self.current, self.past)
        def testAdded(self):
            self.assertEqual(self.d.added(), set((5,6,7)))
        def testRemoved(self):      
            self.assertEqual(self.d.removed(), set((0,1,2)))
        def testChanged(self):
            self.assertEqual(self.d.changed(), set((3,4)))
        def testUnchanged(self):
            self.assertEqual(self.d.unchanged(), set())
    unittest.main()

in python 3 you can just do

myNames.keys() & myRDP.keys()

Best and easiest way would be simply perform common set operations(Python 3).

a = {"a": 1, "b":2, "c":3, "d":4}
b = {"t1": 1, "b":2, "e":5, "c":3}
res = a.items() & b.items() # {('b', 2), ('c', 3)} For common Key and Value
res = {i[0]:i[1] for i in res}  # In dict format
common_keys = a.keys() & b.keys()  # {'b', 'c'}

Cheers!

def combine_two_json(json_request, json_request2):
intersect = {}

for item in json_request.keys():
    if item in json_request2.keys():
        intersect[item]=json_request2.get(item)
return intersect

You can simply write this code and it will save the common key in a list.
common = [i for i in myRDP.keys() if i in myNames.keys()]