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How to Filter Lists of Dictionaries in Python

Filtering lists of dictionaries based on specific criteria is a common task in Python data manipulation.

This guide explores how to filter lists of dictionaries based on:

  • The values associated to keys
  • The presence of a specific key.
  • Unique value in a list.

Filtering by Value​

List comprehensions provide a concise way to filter dictionaries based on their values.

Checking for a Single Value​

To find all dictionaries where a key matches a single, specific value:

list_of_dictionaries = [
    {'id': 1, 'name': 'alice'},
    {'id': 2, 'name': 'anna'},
    {'id': 3, 'name': 'carl'},
    {'id': 4, 'name': 'anna'},
]

filtered_list_of_dicts = [
    dictionary for dictionary in list_of_dictionaries
    if dictionary['name'] == 'anna'
]

print(filtered_list_of_dicts)
# Output: [{'id': 2, 'name': 'anna'}, {'id': 4, 'name': 'anna'}]
  • The list comprehension filters the list_of_dictionaries to return dictionaries where the value for key 'name' is equal to "anna".

Checking for Multiple Values​

To filter based on multiple acceptable values, store them in a list and use the in operator:

list_of_dictionaries = [
    {'id': 1, 'name': 'alice'},
    {'id': 2, 'name': 'anna'},
    {'id': 3, 'name': 'carl'},
]

list_of_values = [1, 3]

filtered_list_of_dicts = [
    dictionary for dictionary in list_of_dictionaries
    if dictionary['id'] in list_of_values
]
print(filtered_list_of_dicts)
# Output: [{'id': 1, 'name': 'alice'}, {'id': 3, 'name': 'carl'}]
  • The in keyword is used to check if the id is one of the specified list_of_values.

Using a for Loop​

The same filtering can be achieved with a for loop, although this is usually more verbose:

list_of_dictionaries = [
    {'id': 1, 'name': 'alice'},
    {'id': 2, 'name': 'anna'},
    {'id': 3, 'name': 'carl'},
]

list_of_values = [1, 3]
filtered_list_of_dicts = []

for dictionary in list_of_dictionaries:
    if dictionary['id'] in list_of_values:
        filtered_list_of_dicts.append(dictionary)

print(filtered_list_of_dicts)
# Output: [{'id': 1, 'name': 'alice'}, {'id': 3, 'name': 'carl'}]
  • The for loop will add to the filtered_list_of_dicts all dictionaries where id key matches the list_of_values list.

Filtering for Unique Values​

To filter a list of dictionaries, keeping only the first dictionary encountered for each unique value of a specific key, use a dictionary comprehension:

list_of_dictionaries = [
    {'id': 1, 'name': 'alice'},
    {'id': 2, 'name': 'anna'},
    {'id': 1, 'name': 'carl'}, # Duplicate id
]

list_of_unique_dictionaries = list(
    {
        dictionary['id']: dictionary
        for dictionary in list_of_dictionaries
    }.values()
)

print(list_of_unique_dictionaries)
# Output: [{'id': 1, 'name': 'carl'}, {'id': 2, 'name': 'anna'}]
  • The dictionary comprehension {dictionary['id']: dictionary for dictionary in list_of_dictionaries} creates a dictionary using the 'id' as keys. Because dictionary keys must be unique, later dictionaries with the same 'id' overwrite earlier ones. This has the effect of keeping only the last dictionary with a given ID.
  • .values() extracts the dictionary values (the dictionaries themselves).
  • list() then transforms the result to a list.

Filtering Based on Key Existence​

To filter a list of dictionaries, keeping only those that have a specific key, use the dict.get() method within a list comprehension:

list_of_dictionaries = [
    {'id': 1, 'name': 'alice'},
    {'id': 2, 'name': 'anna'},
    {'id': 3, 'salary': 100},  # No 'name' key
]

filtered_list = [
    dictionary for dictionary in list_of_dictionaries
    if dictionary.get('name') is not None
]

print(filtered_list)
# Output: [{'id': 1, 'name': 'alice'}, {'id': 2, 'name': 'anna'}]
  • dictionary.get('name') attempts to retrieve the value associated with the key 'name'. If the key is not present, it returns None (by default) instead of raising a KeyError.
  • is not None ensures that only dictionaries with the 'name' key are included.

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