5 Best Ways to Convert a Python Dictionary to Two Lists

by
💡 Problem Formulation: When working with Python dictionaries, you may encounter a scenario where you need to separate the keys and values into two distinct lists. For instance, given a dictionary {'apple': 1, 'banana': 2, 'cherry': 3}, the desired output would be one list for the keys: ['apple', 'banana', 'cherry'], and another list for the values: [1, 2, 3]. This article explores five methods to achieve this conversion effectively.

Method 1: Using the keys() and values() Methods

The keys() and values() methods of a dictionary return its keys and values, respectively. These can be easily converted to lists using the list() function.

Here’s an example:

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}
keys_list = list(my_dict.keys())
values_list = list(my_dict.values())

Output:

keys_list: ['apple', 'banana', 'cherry']
values_list: [1, 2, 3]

This approach is straightforward and utilizes built-in dictionary methods to retrieve the keys and values before converting them into lists.

Method 2: Using List Comprehension

List comprehension provides a concise way to create lists. This approach uses list comprehension to iterate over dictionary items, creating separate lists for keys and values.

Here’s an example:

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}
keys_list = [key for key in my_dict]
values_list = [value for value in my_dict.values()]

Output:

keys_list: ['apple', 'banana', 'cherry']
values_list: [1, 2, 3]

List comprehension makes the code compact and elegant, but it might not be as immediately clear to beginners as the first method.

Method 3: Using the zip() Function

The zip() function is typically used to pair elements from two or more iterables. When used with the items() method of a dictionary, it can separate keys and values.

Here’s an example:

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}
keys_list, values_list = zip(*my_dict.items())

Output:

keys_list: ('apple', 'banana', 'cherry')
values_list: (1, 2, 3)

This approach results in tuples instead of lists. To convert them to lists, you can simply pass them through the list() function. This method is efficient and Pythonic, but it may be less intuitive to those unfamiliar with the zip() function.

Method 4: Using a For Loop

A traditional for loop can also be used to iterate through the dictionary’s items, appending keys and values to respective lists.

Here’s an example:

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}
keys_list = []
values_list = []
for key, value in my_dict.items():
    keys_list.append(key)
    values_list.append(value)

Output:

keys_list: ['apple', 'banana', 'cherry']
values_list: [1, 2, 3]

The for loop approach is perhaps the most explicit and understandable for beginners. However, it is more verbose and might not be as performant as other methods.

Bonus One-Liner Method 5: Using map() Function

The map() function applies a given function to each item of an iterable. This one-liner combines map() with items() to separate the keys and values.

Here’s an example:

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3}
keys_list, values_list = map(list, zip(*my_dict.items()))

Output:

keys_list: ['apple', 'banana', 'cherry']
values_list: [1, 2, 3]

This method is compact and efficient, but its functional programming style might be obscure for those unfamiliar with map() and zip().

Summary/Discussion

  • Method 1: keys() and values() with List Conversion. Straightforward and clear. Might not be the most elegant.
  • Method 2: List Comprehension. Compact and elegant. Learning curve for beginners.
  • Method 3: zip() Function. Pythonic and efficient. Tuples by default, extra step to convert to lists.
  • Method 4: For Loop. Explicit and easy to understand for new programmers. Less concise and possibly less performant.
  • Method 5: map() with zip(). One-liner and efficient. May be obscure to new programmers.