python dictionary keyerror

Handling the KeyError for python dictionary

python dictionary KeyError

The KeyError is quite commonly seen when dealing with the dictionary objects. when trying to access the dictionary while the key does not exists, then this error will be showing up. Usually to avoid this error, we will need to check if the key exists before accessing the value.

For instance, you can check if the key “country” exists in my_dict and then check if the values is “SGP” like the below. But the code does not look elegant.

my_dict = {"name" : "National University of Singapore", "address" : "21 Lower Kent Ridge Rd Singapore", "contact": "68741616"}
if my_dict.get("country") and my_dict["country"] == "SGP":
    print(f"country code is {my_dict['country']}")

You may also see someone uses the below way to make the code more concise. To pass in a default value if the key does not exists:

if my_dict.get("country", "") == "SGP":
    print(f"country code is {my_dict['country']}")

The Zen of Python tells us

Explicit is better than implicit.

So the above code actually does not follow this principal. If you go through the python documentation for dictionary, there is indeed a way to get the value of the key and meanwhile setting a default value if the key is new to the dictionary. Below code shows how it works:

if my_dict.setdefault("country", "") == "SGP":
    print(f"country code is {my_dict['country']}")

By doing the above, the key “country” will be added into the my_dict with a default value if the key does not exists previously, and then return the value of this key.

To extend the above setdefault method, if the value is a list of objects, you can also use this method to initialize it and then set the value.

my_dict.setdefault("faculty", []) # use list or set()
my_dict["faculty"].append("Computer Science")


As per always, welcome for any comments or questions.


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