Understanding the KeyError in json_normalize with Opened Files in Python

Discover why using `json_normalize` on a list in a script differs from using it on data from an opened file, and how to solve the `KeyError` issue in Python.
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Understanding the KeyError in json_normalize

When working with JSON data in Python, you may have encountered issues while trying to normalize nested data structures. One common source of confusion is the difference in behavior of the json_normalize function from the pandas library when applied to data stored in variables versus data read from files. This post addresses a specific question: Why does json_normalize work with a hardcoded list but throw a KeyError when using an opened file containing the same data?

The Problem: KeyError When Using json_normalize

To illustrate the issue, let’s look at a sample JSON structure and two different scripts attempting to normalize it:

Sample JSON Structure

The JSON data we are working with looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Script 1: Normalizing Data from a Variable

In this script, the data is stored in a variable called resp. The normalization works fine:

[[See Video to Reveal this Text or Code Snippet]]

Script 2: Attempting to Normalize from a File

This script reads data from a JSON file and attempts to normalize it:

[[See Video to Reveal this Text or Code Snippet]]

However, this results in:

[[See Video to Reveal this Text or Code Snippet]]

Solution: Debugging the KeyError

Step 1: Verify File Contents

The first thing to do is to ensure that the contents of the file are what you expect. Use the following code to print the data after loading:

[[See Video to Reveal this Text or Code Snippet]]

This will help confirm whether dft, the variable holding the loaded JSON data, is structured correctly to contain the key 'componentes_kit'.

Step 2: Check JSON Structure

If the structure is incorrect, it may be due to not correctly saving the JSON data in the file format. You may consider rechecking how the JSON was written into the file. If necessary, reproduce the writing of the JSON:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Closing the File

Make sure the file is closed after you’re done accessing it. You can use the with statement as it automatically handles this for you. Here’s an improved way to read and normalize:

[[See Video to Reveal this Text or Code Snippet]]

Step 4: Running the Normalization

After implementing the above changes, the normalization should work correctly, yielding the expected DataFrame without errors.

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

The difference in behavior of json_normalize when accessing JSON data from a variable versus an opened file often results from the file's contents or the way data is accessed. By following these debugging steps, you can effectively resolve the KeyError issue and successfully normalize your data.

If you continue experiencing difficulties, double-check the structure of the JSON and how you are referencing the keys in your pandas operations. Happy coding!