1. $update()$

dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}
dict1.update(dict2)
print(dict1)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

2. $dict(dict1, **dict2)$

dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}
dict3 = dict(dict1, **dict2)
print(dict3)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

3. $dict(dict1.items() | dict2.items())$

dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}
dict3 = dict(dict1.items() | dict2.items())
print(dict3)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

4. itertools

import itertools
dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}
dict3 = dict(itertools.chain(dict1.items(), dict2.items()))
print(dict3)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

5. ChainMap

from collections import ChainMap

dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}
dict3 = dict(ChainMap(dict1, dict2))
print(dict3)
# {'c': 3, 'd': 4, 'a': 1, 'b': 2}