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Not using setdefault() to initialize a dictionary

When initializing a dictionary, it is common to see a code check for the existence of a key and then create the key if it does not exist. Although there is nothing wrong with this, the exact same idea can be accomplished more concisely by using the built-in dictionary method setdefault().


The code below checks if a key named list exists in a dictionary called dictionary. If it does not exist, then the code creates the key and then sets its value to an empty list. The code then proceeds to append a value to the list.

Although there is nothing wrong with this code, it is unnecessarily verbose. Later you will see how you can use setdefault() to accomplish the same idea more concisely.

dictionary = {}

if "list" not in dictionary:
    dictionary["list"] = []


Best practice

Use setdefault() to initialize a dictionary

The modified code below uses setdefault() to initialize the dictionary. When setdefault() is called, it will check if the key already exists. If it does exist, then setdefault() does nothing. If the key does not exist, then setdefault() creates it and sets it to the value specified in the second argument.

dictionary = {}

dictionary.setdefault("list", []).append("list_item")