5 Best Ways to Intersect Two Dictionaries Through Keys in Python

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πŸ’‘ Problem Formulation: Given two dictionaries in Python, how can one find the intersection based on their keys and possibly retain the matching key-value pairs? Suppose we have dict1 = {'apple': 2, 'banana': 3, 'cherry': 5} and dict2 = {'banana': 1, 'dragonfruit': 4, 'apple': 9}. We aim to find a dictionary which includes only the items with the keys that are present in both dict1 and dict2, leading to an output like {'apple': 2, 'banana': 3}.

Method 1: Using Dictionary Comprehension

Dictionary comprehension in Python is an elegant and efficient way to construct new dictionaries. This method involves creating a new dictionary by iterating over an iterable and generating key-value pairs. For intersecting two dictionaries, we can use dictionary comprehension to loop through the keys in the first dictionary and include them in the new dictionary only if they are also present in the second dictionary.

Here’s an example:

dict1 = {'apple': 2, 'banana': 3, 'cherry': 5}
dict2 = {'banana': 1, 'dragonfruit': 4, 'apple': 9}
intersection = {k: dict1[k] for k in dict1 if k in dict2}

print(intersection)

Output:

{'apple': 2, 'banana': 3}

This code snippet sets up two dictionaries and then uses dictionary comprehension to create a third dictionary named intersection. It iterates through the keys in dict1 and includes a key in intersection if and only if it also exists in dict2, using the corresponding value from dict1.

Method 2: Utilizing the & Operator with keys()

The & operator is used to perform a set intersection in Python. When applied to the keys() views of two dictionaries, it yields the common keys in both dictionaries. We can then create a new dictionary to hold these intersected keys with their values from one of the original dictionaries.

Here’s an example:

dict1 = {'apple': 2, 'banana': 3, 'cherry': 5}
dict2 = {'banana': 1, 'dragonfruit': 4, 'apple': 9}
intersection_keys = dict1.keys() & dict2.keys()
intersection = {k: dict1[k] for k in intersection_keys}

print(intersection)

Output:

{'apple': 2, 'banana': 3}

In this example, we use dict1.keys() & dict2.keys() to get a set of keys that exist in both dictionaries. We then use dictionary comprehension to build the new intersection dictionary that contains just these keys and their values from dict1.

Method 3: Using a Loop and Conditionals

A straightforward approach to finding intersected keys between two dictionaries is to iterate through one dictionary’s keys and use conditional logic to check for membership in the other’s keys. This is a manual way of producing the intersection without utilizing additional built-in methods or operations.

Here’s an example:

dict1 = {'apple': 2, 'banana': 3, 'cherry': 5}
dict2 = {'banana': 1, 'dragonfruit': 4, 'apple': 9}
intersection = {}

for key in dict1:
    if key in dict2:
    intersection[key] = dict1[key]

print(intersection)

Output:

{'apple': 2, 'banana': 3}

This snippet loops over each key in dict1 and checks whether the key also exists in dict2. If the key is present in both dictionaries, it is added to the intersection dictionary along with its value from dict1.

Method 4: The filter() Function and Lambda

The filter() function in Python is used to create an iterator from elements of an iterable for which a function returns true. By passing a lambda function that checks for key existence in both dictionaries, we can filter out the keys present in both. The resulting keys are then used to build the intersecting dictionary.

Here’s an example:

dict1 = {'apple': 2, 'banana': 3, 'cherry': 5}
dict2 = {'banana': 1, 'dragonfruit': 4, 'apple': 9}
intersection_keys = filter(lambda k: k in dict2, dict1)
intersection = {k: dict1[k] for k in intersection_keys}

print(intersection)

Output:

{'apple': 2, 'banana': 3}

Here, filter() with the accompanying lambda function sifts out the keys that are present in dict2. intersection dictionary then gets built through dictionary comprehension from these keys and their associated values in dict1.

Bonus One-Liner Method 5: Using Dictionary items() and Set Intersection

A more concise approach to intersecting two dictionaries is to leverage the items() method along with set intersection. This method is suited for a scenario where we seek to intersect both keys and values, thus ensuring that only exactly matching items are conserved.

Here’s an example:

dict1 = {'apple': 2, 'banana': 3, 'cherry': 5}
dict2 = {'banana': 3, 'dragonfruit': 4, 'apple': 2}
intersection = dict(dict1.items() & dict2.items())

print(intersection)

Output:

{'apple': 2, 'banana': 3}

The snippet uses the items() method to get the key-value pairs of both dictionaries as set-like objects and then applies the & operator to get the intersection. The result is converted back into a dictionary. This one-liner produces a dictionary with completely matching key-value pairs from both dictionaries.

Summary/Discussion

  • Method 1: Dictionary Comprehension. Strengths: Concise and clear. Weaknesses: May not be as efficient as other methods for very large dictionaries due to iterating over all keys.
  • Method 2: & Operator with keys(). Strengths: More efficient and concise, as it uses set operations. Weaknesses: Slightly less readable for beginners.
  • Method 3: Loop and Conditionals. Strengths: Clear logic, easy to understand. Weaknesses: More verbose and potentially less efficient than other methods.
  • Method 4: The filter() Function and Lambda. Strengths: Functional programming style, can be clear to those familiar with filter(). Weaknesses: May be confusing for those not used to lambda functions or filter().
  • Bonus Method 5: items() and Set Intersection. Strengths: Very concise, good for exact matches. Weaknesses: Limited to scenarios where both keys and values need to match exactly.