What is a Python List Comprehension?
In Python, there is a technique known as list comprehension. It offers a concise way to create a new list based on the values of an existing list.
The basic syntax of a list comprehension:
[expression for item in iterable if condition]
Here’s a breakdown of each part:
- expression: This is the operation you want to perform on each item in the iterable to create the elements of the new list.
- item: This represents each individual item in the iterable.
- iterable: This is the collection of items you want to iterate over, such as a list, tuple, or range.
- if condition (optional): This part allows you to filter the items based on a condition. Only items for which the condition evaluates to True will be included in the new list
For Loop vs List Comprehension
We’ll compare the traditional for loop with the shorter list comprehension method. Using examples, we’ll show how each one makes a list of numbers from 0 to 4.
For loop example:
numbers = []
for num in range(5):
numbers.append(num)
print(numbers)
# Output: [0, 1, 2, 3, 4]
List comprehension example:
numbers = [num for num in range(5)]
print(numbers)
# Output: [0, 1, 2, 3, 4]
Conditonals in List Comprehension
List comprehension can utilize conditional statement like if...else to filter existing lists.
If Conditional
In this example, list comprehension is used to filter even numbers from the range 1 to 9. The condition if num % 2 == 0 ensures that only even numbers are included in the new list.
even_numbers = [num for num in range(1, 10) if num % 2 == 0 ]
print(even_numbers)
# Output: [2, 4, 6, 8]
If-Else Conditional
even_odd_labels = ["even" if num % 2 == 0 else "odd" for num in range(1, 6)]
print(even_odd_labels)
# Output: ["odd", "even", "odd", "even", "odd"]
To summarize:
- When using only an if condition, it comes after the for loop expression because it serves as a filter to determine which items from the iterable should be included in the new list.
- When using if-else, it comes before the for loop expression because it involves transforming each item based on a condition.
Nested List
We will create a 3x3 nested list. We can achieve this using both nested for loop and list comprehension. The loop approach iterates over rows and columns to calculate each element. List comprehension provides a more concise alternative.
For loop
nested_list = []
# Iterate over rows
for i in range(3):
row = []
# Iterate over columns
for j in range(3):
# Calculate element and append to row
row.append(i * 3 + j + 1)
# Append row to nested list
nested_list.append(row)
print(nested_list)
# Output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
List Comprehension
nested_list = [
# creating sublist
[num for num in range(start, start + 3)]
# Outer loop
for start in range(1, 10, 3)
]
print(nested_list)
# Output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
In this code:
- The outer loop iterates over the starting numbers of each sublist (1, 4, and 7).
- Inside the list comprehension, another loop generates each sublist by iterating over the next 3 numbers starting from the current start value.
Flatten List
nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Flatten the nested list using list comprehension
flattened_list = [num for sublist in nested_list for num in sublist]
print(flattened_list)
In this example, the list comprehension iterates over each sublist in the nested list (for sublist in nested_list) and then iterates over each element in the sublist (for num in sublist). This results in a flattened list containing all elements from the nested list.
Resources
10 Python Comprehensions You SHOULD Be Using by Tech with Tim
How to use list comprehension in Python by 2MinutesPy
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