The World of Python Programming

One-Line Python Ifs: Ternary Operators & List Comprehensions

Basic Syntax & Control Flow
目次

1. Introduction

In Python programming, writing an if statement on a single line helps simplify code and is especially useful in situations where conditional branching occurs frequently. In this article, we explain in detail several techniques for writing Python “if” statements on a single line. Learn how to write readable, efficient code with practical examples such as the ternary operator and list comprehensions.

2. How to write one-line if statements using Python’s ternary operator

In Python, you can write an if statement on a single line using the ternary operator (conditional expression). It’s useful when returning a value based on a condition and allows you to keep your code concise.

Ternary operator syntax

The Python ternary operator is written like this:
variable = value1 if condition else value2
With this syntax, “value1” is returned if the condition is True, and “value2” is returned if it’s False.

Example: Checking even or odd

For example, you can check whether a number is even or odd and return different strings based on the result, like this:
number = 4
result = "even" if number % 2 == 0 else "odd"
print(result)  # Output: even
Because you can write the if statement on a single line like this, it’s very convenient for simple conditional branches.

Practical applications of the ternary operator

For example, you can use the ternary operator to display a message depending on whether a user is logged in.
is_logged_in = True
message = "Welcome" if is_logged_in else "Please log in"
print(message)  # Output: Welcome
The ternary operator is ideal for simple selections based on a condition. However, writing complex logic on one line can hurt readability, so it’s recommended to use it only for straightforward cases.

3. Combining List Comprehensions and if Statements

Python’s list comprehensions provide a powerful way to write concise code for creating lists. When combined with if statements, you can construct lists according to specified conditions.

Basic Syntax of List Comprehensions

When using an if statement in a list comprehension, you write it like this:
list = [expression for variable in iterable if condition]

Example: Creating a List of Even Numbers

To collect only the even numbers from 0 through 9 into a list, write it like this:
even_numbers = [x for x in range(10) if x % 2 == 0]
print(even_numbers)  # Output: [0, 2, 4, 6, 8]

Applied Examples of List Comprehensions

List comprehensions are also useful for filtering and transforming data. For example, they can help when extracting only words of a specific length from a list of strings.
words = ["apple", "banana", "cherry", "date"]
filtered_words = [word for word in words if len(word) > 5]
print(filtered_words)  # Output: ['banana', 'cherry']
By using list comprehensions, you can shorten your code while efficiently extracting or transforming data based on conditions.

4. One-line use of Python if and for statements

By combining for and if statements, you can write a single line of code that processes only elements that meet a given condition. This is useful for simplifying large list operations.

One-line syntax for if and for statements

The basic syntax is as follows.
list = [expression for variable in iterable if condition]

Example: Creating a list of squares of even numbers

To list the squares of the even numbers from 1 to 10, use the following.
squared_evens = [x**2 for x in range(1, 11) if x % 2 == 0]
print(squared_evens)  # Output: [4, 16, 36, 64, 100]

Applied example: Filtering data based on specific conditions

This code extracts only positive numbers from a list and collects them into a new list.
numbers = [-5, -1, 2, 8, -3, 7]
positive_numbers = [num for num in numbers if num > 0]
print(positive_numbers)  # Output: [2, 8, 7]
Writing if and for together on a single line like this reduces code redundancy while enabling efficient data manipulation.

5. One-line if with assignment expression (Walrus operator)

The assignment expression (walrus operator) :=, introduced in Python 3.8 and later, allows assignment to a variable within an if statement. Its key feature is that it enables you to evaluate a condition while also storing the result of the computation.

Syntax and Basic Usage of Assignment Expressions

Syntax and Basic Usage of Assignment Expressions

if (variable := expression) condition:
    process

Example: Using the length of a list as a condition

The length of the list is retrieved and evaluated as a condition, while also performing processing based on it.

some_list = [1, 2, 3, 4, 5]
if (length := len(some_list)) > 3:
    print(f"The length of the list is {length}, which is greater than 3")
# Output: The length of the list is 5, which is greater than 3

Practical Use Cases

By using the walrus operator, you can avoid redundant calculations and potentially improve performance. It is particularly effective in situations where the result of a calculation—such as list or string operations—needs to be reused.

6. Notes on Using One-Line if Statements

One-line if statements are powerful, but overusing them can reduce readability. Use them only for simple conditional checks, and switch to regular if statements when more complex logic is required.

7. Summary and Next Steps

In this article, we introduced various techniques for writing Python if statements on a single line. From simple conditional branches to list manipulation and using the walrus operator, we covered the benefits and caveats of one-line if statements. Try applying these techniques to make your everyday Python programming more efficient.
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