Table of Contents
Merging Dictionaries in Python 2.7
Python 2.7 lacks the elegant dictionary merging capabilities of later versions. We primarily rely on the update() method or, less commonly, dictionary concatenation.
Using update()
The update() method modifies the dictionary it’s called upon, adding key-value pairs from another dictionary. Existing keys are overwritten by values from the second dictionary.
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
dict1.update(dict2)
print(dict1) # Output: {'a': 1, 'b': 3, 'c': 4}
Dictionary Concatenation
While possible, this approach is less efficient and requires creating a new dictionary. It’s generally less preferred than update().
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = dict(dict1.items() + dict2.items())
print(merged_dict) # Output: {'a': 1, 'b': 3, 'c': 4}
#Alternative using dict.copy():
merged_dict = dict1.copy()
merged_dict.update(dict2)
print(merged_dict) #Output: {'a': 1, 'b': 3, 'c': 4}
Python 3.5+ Dictionary Merging
Python 3.5 introduced the much cleaner dictionary unpacking using the ** operator. This creates a new dictionary, combining all key-value pairs. Duplicate keys are resolved by the value from the rightmost dictionary.
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = {**dict1, **dict2}
print(merged_dict) # Output: {'a': 1, 'b': 3, 'c': 4}
This is generally the preferred method for its readability and avoidance of in-place modification.
Comparison and Best Practices
The optimal method depends on your Python version and coding style. For Python 2.7, update() is recommended. For Python 3.5 and above, dictionary unpacking (**) offers superior readability and avoids potential side effects. In both cases, later dictionaries overwrite values of duplicate keys. For more complex key collision handling (e.g., combining values), you will need to implement custom logic.