Python’s data types are one of its most important features. Most languages require 3rd party additions to handle even a portion of Python’s inherent flexibility with data types. Python’s data handling is a rare combination of power and ease of use that’s usually a joy to use. However, there’s really no such thing as a perfect programming language or library. And if you’re seeing a “keyerror 0” Python error you might start to second guess the language’s flexibility. But you’ll soon see why the error occurs, and some easy ways to fix it.
What Does the Keyerror Actually Mean?
The keyerror 0 Python error often seems confusing at first glance. Many users assume that the 0 refers to a built-in error type. But the error is actually referring to a conflict between a Python collection’s key and value combo. This most often occurs with a Python dictionary. In short, the error simply states that an attempt to access a key named 0 has failed. This usually means that there’s simply no value associated with a key of 0.
A Deeper Dive Into the Underlying Reasons for Keyerrors
A Python keyerror is generally fairly easy to fix when you understand why it’s happening. But that’s often easier said than done. The problem often comes about due to misunderstandings about the nature of Python’s dictionaries. And that fact can help us track down the underlying problem. Python’s dictionaries work through an association between a key and a value. This is distinctly different than Python’s list data type, which is positional. Consider the following Python code.
ourList = [‘zero’, ‘one’, ‘two’, ‘three’]
print(ourList[0])
This code makes use of a list, one of the most commonly used data types in Python programming. In this example, we have a list of string values that correspond to their position within the list. The starting position in Python’s numbering system is 0. As such the list begins with “zero”. We create the list, and then print out the value in the 0 position. The end result is that the word “zero” appears on the screen. But now consider something similar using a dictionary.
Ourdictionary = {}
ourDictionary = {1: ‘one’, 2: ‘two’, 3: ‘three’}
print(ourDictionary[2])
This example will run without an error message, but it might not show the result you’re expecting. It returns “two” even though that would appear to be in the 1 position when we start counting from 0. So what’s going on? We can get closer to an answer by making a small change in the previous code. Change ourDictionary[2] to ourDictionary[0] and run it again. We now see a keyerror 0. And the keyerror traceback points to our previously working print statement. The problem can be illustrated with the following example.
Ourdictionary = {}
ourDictionary = {‘a’: ‘one’, ‘b’: ‘two’, ‘c’: ‘three’}
print(ourDictionary[‘b’])
Note that this returns a “two” string just as the previous example did. And this is the core of what’s going on with the keyerror 0 problem. Python’s dictionaries work by a key value pairing rather than a numerical position. When we passed 2 as a parameter it wasn’t accessing the second position in the dictionary. In reality, 2 was just being used as a dictionary key, not a position.
It’s best to think of dictionaries as closer to the more complex data structures found in data science than to a standard list. You can conceptualize dictionaries as somewhat analogous to a column name and value in a multidimensional array.
How To Fix the Keyerror
There are a few ways to go about fixing the error. The first and most obvious fix comes from a situation where you’ve simply mistakenly used a dictionary like a list. In that case you just need to refer to the key rather than the position. If you needed to use an integer key system you could also just start at 0 and make sure every position was filled.
But what about if you need a dictionary where the keys are integers but where you can’t be certain of which values are filled? In short, what happens when you can’t be certain about a non-existent key in your dictionary? Thankfully Python’s developers foresaw this potential pitfall. Dictionaries have a number of method options available that provide some automatic exception handling. You can see it in effect within the following code.
Ourdictionary = {}
ourDictionary = {1: ‘one’, 2: ‘two’, 3: ‘three’}
print(ourDictionary.get(0) )
Note that the Python keyerror exception doesn’t appear when we pass a nonexistent key to “get” instead of directly referring to a key. Instead, we get a default value of “None”. This is the most common way of getting past the error. But it’s not the only option available. Consider this example of using a keyeror to our benefit rather than a detriment.
Ourdictionary = {}
ourDictionary = {1: ‘one’, 2: ‘two’, 3: ‘three’}
for x in range(0, 10):
try:
print(ourDictionary[x])
except KeyError:
print(str(x) + ” not found”)
We begin the code as usual by defining and populating a dictionary. But this time around we loop over x with a range of 0 to 10. For every loop, the code will print out the dictionary value associated with a key of x if the key exists. If a keyerror is raised the code won’t exit out. Instead, it catches the problem and simply prints out the key which wasn’t found. Then the code proceeds to the next iteration of the loop.
Keep in mind that these fixes are still fairly general-purpose by intent. The principles here can be easily adapted to your own code to fit your individual needs. Even fairly complex cases will typically still be fixed by these options.