Python One Line Return if

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Problem: How to return from a Python function or method in single line?

Example: Consider the following “goal” statement:

def f(x):
    return None if x == 0

However, this leads to a Syntax error:

In this tutorial, you’ll learn how to write the return statement with an if expression in a single line of Python code. You can get an overview of the three methods in the interactive code shell:

Exercise: The code has no output. Print the results of all three function executions for a given x. Is it always the same?

Let’s dive into the three methods.

Method 1: As a Multi-Liner

The following method is the standard and most Pythonic way to accomplish this but using multiple lines:

def f(x):
    if x==0:
        return None

But how to write this as a one-liner?

Method 2: Direct One-Liner If

Nothing simpler than that—just write it into a single line!

def f(x):
    if x==0: return None

I should note that PEP 8 is actually fine with writing if block statements into a single line. Nevertheless, the default return value of a function is None so the code does really nothing.

Method 3: Ternary Operator

If you look for something more Pythonic, you can check out the ternary operator (also called “conditional expression”):

def f(x):
    return None if x==0 else 42

In this case, you also have to define a return value for the value 42. You should read the statement like this:

return (None if x == 0 else 42)

The statement inside the parentheses returns either None or 42—depending on the condition x == 0. If it is True, the value None is returned. If it is False, the value 42 is returned.

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