What Is KeyError in Python?

by | Aug 7, 2020 | Technology Featured

While Python is a relatively simple programming language, a very large number of things can still go wrong with it. A computer program will only try to do what you tell it to, so you need to be careful to not make any typos and understand what, exactly, you are telling Python to do. Case in point, a keyerror isn’t necessarily a straightforward error to get; it occurs when Python tries to access a faulty key-value pair within a mapping and it can’t access a useable value. If you are looking to solve and avoid these kinds of errors, here are a few things to keep in mind.

What The Error Means

The first step to solving an error is knowing what the error means. A KeyError, essentially, means that a Python program tried to use a dictionary key that doesn’t exist to look up a value that didn’t exist. A Python dictionary is a specific way of organizing information and using one you can associate one kind of information with another. The best way to understand what this error means is to look at some documentation on the subject; you should be able to find a fair amount of information on the subject by searching Google for “keyerror python.” This kind of error is a kind of “LookupError,” so if you get a KeyError you have a pretty good idea of what to look for when debugging your code.

Python Dictionaries

After reviewing what a dictionary is, you will notice that there are a few specific qualities that a dictionary requires. Namely, the parenthesis or curly braces, the name of the dictionary, and the association between the key and the value as defined by a colon. A KeyError occurs when the key—the first half of the key-value pair—that the program tries to call doesn’t exist in the dict that you are trying to reference. The second half of that pairing, the value, isn’t technically at fault because the program never gets to the point where it would look up a value. With this in mind, if you are getting a KeyError then you should know exactly which part of the dictionary in question is at fault in your code, but there are a few circumstances where the issue might not be immediately apparent.


Fix the Code

If you still aren’t sure what the issue is with a KeyError, take into consideration how the dict is set up and what operations are performed on it prior to it being read by the code. Usually, a dict is set up statically. That is, the dict is defined for use by the program and it never changes. You can also have a dynamically allocated dict that can change depending on what information you want to store in it, and if you are having trouble with a dynamically allocated dict you should put some thought into what it should look like when Python returns an error, perhaps even by going through the code step-by-step if you have to.

There is no one-size-fits-all solution to most programming problems you will encounter in a professional programming career, so you need to know how the programming language you are working with operates. That being said, most programming languages are beholden to the same general logic and many of the skills you will learn with one will likely translate to another that you might learn later. In C++, for example, you can map values in a way that resembles Python mapping. While Python might be a simple, high-level programming language, it is still just as relevant as any other.

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