> A = {'a': 1, 'b': 2}
> B = {'a': 8, 'c': 3}
> A.update(B)
> A
{'a': 8, 'b': 2, 'c': 3}

> A = {'a': 1, 'b': 2, 'c': 3}
> B = {'b': 4, 'c': 6, 'd': 8}
> for key,value in B.items():
...   if key in A:
...     A[key] += value
...   else:
...     A[key] = value
> dict(sorted(A.items(), key=lambda d:d[1]))
{'a': 1, 'b': 6, 'd': 8, 'c': 9}

> A = {'a': 1, 'b': 2, 'c': 3}
> B = {'b': 4, 'c': 6, 'd': 8}
> C = {}
> for key in list(set(A) | set(B)):
...   if A.get(key) and B.get(key):
...     C.update({key: A.get(key) + B.get(key)})
...   else:
...     C.update({key: A.get(key) or B.get(key)})
> C
{'c': 9, 'd': 8, 'a': 1, 'b': 6}

> A = {'a': 1, 'b': 2, 'c': 3}
> B = {'b': 4, 'c': 6, 'd': 8}
> def dict_union(d1, d2):
...   keys = d1.keys() | d2.keys()
...   temp = {}
...   for key in keys:
...     temp[key] = sum([d.get(key,0) for d in (d1, d2)])
...   return temp
> C = dict_union(A, B)
> C
{'d': 8, 'a': 1, 'b': 6, 'c': 9}

> A = {'a': 1, 'b': 2, 'c': 3}
> B = {'b': 4, 'c': 6, 'd': 8}
> C = {}
> for key1 in A:
...   for key2 in B:
...     if key1 in B:
...       C[key1] = A[key1] + B[key1]
...     else:
...       C[key1] = A[key1]
...       if key2 not in A:
...         C[key2] = B[key2]
> C
{'a': 1, 'd': 8, 'b': 6, 'c': 9}

> A = {'a': 1, 'b': 2, 'c': 3}
> B = {'b': 4, 'c': 6, 'd': 8}
> C = {}
> for key in A:
...   if B.get(key):
...     C[key] = A[key] + B[key]
...   else:
...     C[key] = A[key]
> for key in B:
...   if not A.get(key):
...     C[key] = B[key]
> C
{'a': 1, 'b': 6, 'c': 9, 'd': 8}