Setting default dictionary values in Python

"The get method is the classic way to look up a value for a dictionary key without raising an exception for missing keys. Instead of this: Or this: We can do this: Here's what this would do for a key that's in the dictionary and one that isn't: The get method accepts two arguments: the key to look up and the default value to use if that key isn't in the dictionary. The second argument defaults to None:"

"We have a list of Item objects and we want to group these items by their color. We could use a for loop and the in operator to assign a dictionary key to a new list the first time we see that key: Or we could use the dictionary setdefault method: The setdefault method will assign a given key to a given value only if the key isn't already in the dictionary. So the setdefault method is doing that if check for us!"

Use dict.get(key, default) to return a value for a key without raising KeyError; the second argument defaults to None. dict.get does not modify the dictionary. Use dict.setdefault(key, default) to insert a default value only when the key is missing and return the value, enabling idioms like grouping by appending to the returned list. dict.setdefault returns the existing value if present. Use dict.fromkeys(iterable, value) to create a new dictionary with each given key initialized to the same default when all keys are known in advance. Choose the approach based on whether mutation or batch initialization is desired.

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