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
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.
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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