Python's "Ternary Operator"

Welcome to another Python snippet post. This week we're going to take a brief look at conditional expressions in Python, sometimes referred to as Python's ternary operator.

Conditional expressions are a slightly obscure bit of syntax in Python, but they essentially allow us to assign values to variables based on some condition.

Let's take a look at a quick example:

x = 6
value = x if x < 10 else "Invalid value"  # 6

In this case we have some value bound to the variable x, and we check if the value of x is less than 10. If it is, we assign the number to value; otherwise, we assign the string, "Invalid value".

We can see a case where the value of x is not less than 10 below:

x = 10
value = x if x < 10 else "Invalid value"  # Invalid value

Let's break down the syntax.

First we start with the value to return if the condition is True. In our case, this is x. We then have the if keyword followed by some condition. In our case this is a comparison using the less than operator, but any expression which can evaluated to a Boolean value is fine. After the condition, we use the else keyword, followed by the value to return if the condition evaluates to False.

<value if condition True> if <condition> else <value if condition False>

One thing to keep in mind when it comes to conditional expressions is that we actually need all of the parts. We can't simply do away with the else clause if we don't care about it. We have to explicitly define a value for if the condition evaluates to False.

Failing to add an else clause results in a SyntaxError.

Conditional expressions can be chained by appending to the else clause, but the syntax is already confusing enough that I wouldn't ever recommend doing this:

x = 16
value = x if x < 10 else "Invalid value" if x < 15 else "Super invalid value" 

So, should you be using conditionally expressions all over your own code? Probably not.

Personally, I think the order of the conditions and the return values is pretty unintuitive, and it can be hard to follow the logic of these conditional expressions. Often it's much clearer to just use an if statement, even if it's a little longer:

x = 10

if x < 10:
	value = x
else:
	value = "Invalid value"

There are, however, plenty of examples of this structure being used in the wild, so it's important to be able to recognise it and understand it when it's used in other people's code.

Wrapping up

That's it for this snippet post. Hopefully you learnt something new, and I hope you find some use cases for conditional expressions in your own code. Sometimes they can be very useful.

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