[Tut] Python One Line If Not None

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[Tut] Python One Line If Not None - xSicKxBot - 09-11-2020

Python One Line If Not None

To assign the result of a function

Code:
get_value()

to variable

Code:
x

if it is different from

Code:
None

, use the Walrus operator

Code:
if tmp := get_value(): x = tmp

within a single-line if block. The Walrus operator assigns the function’s return value to the variable

Code:
tmp

and returns it at the same time, so that you can check and assign it to variable

Code:
x

subsequently.

Problem: How to assign a value to a variable if it is not equal to

Code:
None

—using only a single line of Python code?

Example: Say, you want to assign the return value of a function get_value(), but only if it doesn’t return None. Otherwise, you want to leave the value as it is. Here’s a code example:

Code:
import random def get_value(): if random.random()>0.5: return None return 1 # Naive approach:
x = 42
tmp = get_value()
if tmp != None: x = tmp
print(tmp)

While this works, you need to execute the function

Code:
get_value()

twice which is not optimal. An alternative would be to assign the result of the

Code:
get_value()

function to a temporary variable to avoid repeated function execution:

Code:
x = 42
temp = get_value()
if temp != None: x = temp
print(x)

However, this seems clunky and ineffective. Is there a better way?

Let’s have an overview of the one-liners that conditionally assign a value to a given variable:

https://repl.it/@finxter/CrookedLivelyLocks?lite=true

Exercise: Run the code. Does it always generate the same result?

Method 1: Ternary Operator + Semicolon

The most basic ternary operator

Code:
x if c else y

consists of three operands

Code:
x

Code:
c

, and

Code:
y

. It is an expression with a return value. The ternary operator returns

Code:
x

if the Boolean expression

Code:
c

evaluates to

Code:
True

. Otherwise, if the expression

Code:
c

evaluates to

Code:
False

, the ternary operator returns the alternative

Code:
y

.

You can use the ternary operator to solve this problem in combination with the semicolon to write multiple lines of codeas a Python one-liner.

Code:
# Method 1
tmp = get_value(); x = tmp if tmp else x

You cannot run the

Code:
get_value()

function twice—to check whether it returns

Code:
True

and to assign the return value to the variable

Code:
x

. Why? Because it’s nondeterministic and may return different values for different executions.

Therefore, the following code would be a blunt mistake:

Code:
x = get_value() if get_value() else x

The variable

Code:
x

may still be

Code:
None

—even after the ternary operator has seemingly checked the condition.

Related articles:

Method 2: Walrus + One-Line-If

A beautiful extension of Python 3.8 is the Walrus operator. The Walrus operator

Code:
:=

is an assignment operator with return value. Thus, it allows you to check a condition and assign a value at the same time:

Code:
# Method 2
if tmp := get_value(): x = tmp

This is a very clean, readable, and Pythonic way. Also, you don’t have the redundant identity assignment in case the if condition is not fulfilled.

Related Article: The Walrus Operator in Python 3.8

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