Python Ternary Dictionary

Problem: Given a dictionary and a key. Can you use the ternary operator to return the key from the dictionary, but only if it exists to avoid the key error? If the key doesn’t exist, a “fall-back” function should be executed.

Example: Say, you want to do something like the following (pseudocode):

var = dict["key"] if dict.has_key("key") else "fallback"

Ternary Operator Short Recap

Ternary Operator: The most basic ternary operator x if c else y consists of three operands x, c, and y. It is an expression with a return value. The ternary operator returns x if the Boolean expression c evaluates to True. Otherwise, if the expression c evaluates to False, the ternary operator returns the alternative y.

Syntax: The three operands are written as x if c else y which reads as “return x if c else return y“. Let’s write this more intuitively as:

<OnTrue> if <Condition> else <OnFalse>

Related article: For a full tutorial on the ternary operator, check out our detailed blog article.

Method 1: Ternary Operator with Membership

Solution: You can use the ternary operator dict[key] if key in dict else "fallback" to accomplish this:

d = {'Alice': 17, 'Bob': 22}
key = 'Alice'

# Has key:
var = d[key] if key in d else -1
print(var)
# 17

# Doesn't have key:
key = 'Ann'
var = d[key] if key in d else -1
print(var)
# -1

The ternary operator returns the value associated to the given key—but only if the key exists. If it doesn’t exist, it returns the default value -1.

Method 2: dict.get(key, default)

However, a more Pythonic way to accomplish the same thing in a more readable and more concise way is to use the dictionary.get(key, default) function:

d = {'Alice': 17, 'Bob': 22}
key = 'Alice'

# Has key:
var = d.get(key, -1)
print(var)
# 17

# Doesn't have key:
key = 'Ann'
var = d.get(key, -1)
print(var)
# -1

The outer structure of the code is the same—but the get function with default value -1 semantically replaces the more complicated ternary operator.

Try it yourself:

Exercise: Create a more complicated default function that returns the key in reverse order and pass it into the get() function!

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