Python Ternary Operation

Simple Ternary Operation

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Many may not know that Python does support ternary operation. Let’s see the function below.

def func(x):
    if x >= 0:
        return x
    else:
        return -x

We can simplify the function above with ternary operation, as below:

def func(x):
    return ( x if x >= 0 else -x )

Ternary Operator

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The ternary operator in Python is simply a shorter way of writing an if and if…else statement.

Here’s the syntax:

Syntax: [on_true] if [expression] else [on_false]

Ternary Usages

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Below are the examples to use Ternary Operator in Python if-else.

# Similar to abs() 
x = 1 
y = x if x >= 0 else -x

# Print whichever is smaller 
a, b = 10, 20
min = a if a < b else b
print(min)

# Advanced nested ternary operator
a, b = 10, 20
print ("Both a and b are equal" if a == b else "a is greater than b"
		if a > b else "b is greater than a")
# output: b is greater than a

Ternary with Tuples

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Explanation: In this example, we use a tuple for selecting an item. And if [x >= 0] is True, it will return 1. Otherwise it will return 0 as False. Basically, the condition [x>=0] has become the index for the tuple.

# Similar to abs() 
x = 1
y = (-x, x)[x >= 0]
# y is 1

# Print whichever is smaller
a, b = 10, 20
print((b, a)[a < b])
# output: 10

Some argue that this isn’t a proper ternary operator. See extra notes on why this method is less efficient and not recommended.

Ternary with Dictionary

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Similar to ternary with tuple().

# Get the smaller value
a, b = 10, 20
print({True: a, False: b} [a < b])
# output: 10

Ternary with Lambda

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Similar to ternary with tuple().

# Get the smaller value
a, b = 10, 20
print((lambda: b, lambda: a)[a < b]())
# output: 10

Ternary with Print-if

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# Get the greater value
a=5
b=7
# [statement_on_True] if [condition] else [statement_on_false]
print(a,"is greater") if (a>b) else print(b,"is Greater")
# output: 7 is Greater

Extra Notes

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OK. The extra notes here is about the argument on the usage of ternary with tuple() method.

Let’s dive into the situation below where we need to get the average (mean) value from a list of numbers.

At first, we have a function getAverage() as below.

def getAverage(nlist):
    return sum(nlist) / len(nlist)

But, what if there is an empty list been passed to the function? So, here is the enhanced version.

def getAverage(nlist):
    return sum(nlist) / len(nlist) if nlist else 0

Now, let’s try to apply ternary operator with tuple() like how we did above.

def getAverage(nlist):
    return (0, sum(nlist)/len(nlist))[len(nlist) >= 0]

Now, problem starts to arise. Because Python will create the tuple() without evaluate the condition (in this case [len(nlist)>=0]) first. This will cause ZeroDivisionError when an empty list is passed to the function.

From the efficiency perspective, this method is less efficient than if-else method. This method has to first create the tuple, including the sum(nlist)/len(nlist), then follow by evaluating the condition or subscribe. And this method relies on the conversion from True/False to index for a tuple().