5 Best Ways to Create a List of Dictionaries in Python

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πŸ’‘ Problem Formulation: Often in Python, we need to organize data in a way that pairs each key with a corresponding value, making dictionaries the ideal structure. However, when one needs to store multiple such records, a list of dictionaries becomes essential. This article solves the problem of manipulating a collection of records such as [{‘name’: ‘Alice’, ‘age’: 25}, {‘name’: ‘Bob’, ‘age’: 30}, …] by showcasing different methods to create a list of dictionaries in Python.

Method 1: Using a Loop and Append

This method involves initializing an empty list and then using a for loop to create dictionaries, which are appended to the list. This is the most intuitive method and is most suitable when adding dictionaries procedurally based on some logic or conditions during iteration.

Here’s an example:

list_of_dicts = []
for i in range(5):
    list_of_dicts.append({'id': i, 'square': i**2})

print(list_of_dicts)

Output:

[{'id': 0, 'square': 0}, {'id': 1, 'square': 1}, {'id': 2, 'square': 4}, {'id': 3, 'square': 9}, {'id': 4, 'square': 16}]

This code snippet starts by initializing an empty list. The loop goes from 0 to 4, and in each iteration, it appends a new dictionary to the list, containing an ‘id’ and its ‘square’. It is easy to understand and control, especially for those new to Python.

Method 2: List Comprehension

List comprehension in Python allows you to write concise and readable code for generating lists. By incorporating dictionary creation into list comprehension, you can construct a list of dictionaries in a single line of code.

Here’s an example:

list_of_dicts = [{'id': i, 'square': i**2} for i in range(5)]
print(list_of_dicts)

Output:

[{'id': 0, 'square': 0}, {'id': 1, 'square': 1}, {'id': 2, 'square': 4}, {'id': 3, 'square': 9}, {'id': 4, 'square': 16}]

This code utilizes list comprehension to create the same list of dictionaries as Method 1 but in a more succinct manner. The expression inside the brackets defines the structure of each dictionary, which is generated for each value of ‘i’ in the specified range.

Method 3: Using the map() Function

The map() function can be used to apply a function to each item in an iterable. When combined with a lambda function that returns dictionaries, map() can efficiently create a list of dictionaries.

Here’s an example:

list_of_dicts = list(map(lambda i: {'id': i, 'square': i**2}, range(5)))
print(list_of_dicts)

Output:

[{'id': 0, 'square': 0}, {'id': 1, 'square': 1}, {'id': 2, 'square': 4}, {'id': 3, 'square': 9}, {'id': 4, 'square': 16}]

By mapping a lambda function over a range of numbers, dictionaries are created for each number and then converted into a list. This method is elegant for those who prefer functional programming.

Method 4: Using the dict() Function and a Zip

You can pair the dict() function with the zip() function to construct dictionaries from two parallel lists, one containing keys and the other containing values. This is useful when your data is already segregated into separate lists.

Here’s an example:

keys = ['id', 'square']
values = [[i, i**2] for i in range(5)]

list_of_dicts = [dict(zip(keys, v)) for v in values]
print(list_of_dicts)

Output:

[{'id': 0, 'square': 0}, {'id': 1, 'square': 1}, {'id': 2, 'square': 4}, {'id': 3, 'square': 9}, {'id': 4, 'square': 16}]

This snippet creates a list of keys and a list of values. It then zips the keys with each sublist of values, generating a dictionary, and proceeds to aggregate these into a list using list comprehension. This method is highly readable and useful for parallel data sequences.

Bonus One-Liner Method 5: Using a Generator

A generator expression can be utilized to generate a sequence of dictionaries. When typecast to a list, it becomes a compact and memory-efficient way to build a list of dictionaries, especially useful for large datasets.

Here’s an example:

list_of_dicts = list({'id': i, 'square': i**2} for i in range(5))
print(list_of_dicts)

Output:

[{'id': 0, 'square': 0}, {'id': 1, 'square': 1}, {'id': 2, 'square': 4}, {'id': 3, 'square': 9}, {'id': 4, 'square': 16}]

The generator expression within the list function is a more memory-efficient alternative to the list comprehension method. It avoids creating the entire list in memory at once, making it preferable when working with a huge number of dictionaries.

Summary/Discussion

  • Method 1: Using a Loop and Append. Intuitive and easy for beginners. Con: Can be verbose for simple cases.
  • Method 2: List Comprehension. Compact and Pythonic. Con: May be less readable for complex logic.
  • Method 3: Using the map() Function. Functional programming style. Con: Some find map() less intuitive than comprehensions.
  • Method 4: Using the dict() Function and a Zip. Clean and readable for parallel data. Con: Requires data to be well-organized in advance.
  • Method 5: Using a Generator. Memory-efficient for large datasets. Con: Slightly less straightforward than list comprehension for small datasets.