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How to Extract and Sort Data from Files in Python

JSON (JavaScript Object Notation) is the standard format for data exchange in web applications and APIs. A common task in Python data processing involves reading a JSON file, extracting specific lists, and sorting them based on a specific field (like a date or a name).

This guide demonstrates how to read a JSON file containing movie data and print a sorted list of titles ordered by their release date.

Understanding the Data Structure​

Before coding, we must understand the structure of our input file, movie.json. The JSON file typically contains a root object with keys, one of which holds a list of movie records.

Example movie.json:

{
  "movies": [
    {"name": "Inception", "published_at": "2010-07-16"},
    {"name": "Pulp Fiction", "published_at": "1994-10-14"},
    {"name": "The Dark Knight", "published_at": "2008-07-18"}
  ]
}

Step 1: Loading and Parsing JSON​

Python provides the built-in json module to handle this format. We use json.load() to parse a file directly into a Python dictionary.

import json

# ✅ Correct: Open the file and load it
with open("movie.json", "r") as file:
    data = json.load(file)

# Access the list using the dictionary key "movies"
movies_list = data["movies"]
note

json.load() vs json.loads(): Use json.load(file_object) to read from a file. Use json.loads(string) to read from a JSON string variable.

Step 2: Sorting Lists of Dictionaries​

We cannot simply call sorted() on a list of dictionaries because Python does not know which value inside the dictionary to compare. We must define a key using a lambda function.

Sorting Logic:

# ⛔️ Incorrect: Raises TypeError ( '<' not supported between instances of 'dict')
# sorted_movies = sorted(movies_list)

# ✅ Correct: Sort by the 'published_at' key
# Lambda x: represents an individual dictionary in the list
sorted_movies = sorted(movies_list, key=lambda x: x["published_at"])
tip

Since the dates are strings in "YYYY-MM-DD" format (ISO 8601), standard string comparison works perfectly for chronological sorting. You do not need to convert them to datetime objects for simple sorting.

Complete Code Solution​

Here is the complete movie.py script. It reads the file, sorts the data, and prints the formatted output.

import json

def extract_movie_info(file_path):
    """
    Reads a JSON file, extracts movie data, sorts by release date,
    and prints the details.
    """
    try:
        # 1. Read and Parse
        with open(file_path, "r") as file:
            data = json.load(file)
            
        # 2. Extract the list
        movies = data["movies"]
        
        # 3. Sort the list of dictionaries
        # key=lambda x: x["published_at"] tells sort to look at the date string
        sorted_movies = sorted(movies, key=lambda x: x["published_at"])

        # 4. Iterate and Print
        for movie in sorted_movies:
            name = movie["name"]
            published_at = movie["published_at"]
            print(f"movie: {name}, published: {published_at}")

    except FileNotFoundError:
        print(f"Error: The file '{file_path}' was not found.")
    except KeyError:
        print("Error: The JSON structure is missing required keys.")

if __name__ == "__main__":
    # Ensure movie.json exists in the same directory
    extract_movie_info("movie.json")

Execution Output:

movie: Pulp Fiction, published: 1994-10-14
movie: The Dark Knight, published: 2008-07-18
movie: Inception, published: 2010-07-16

Conclusion​

Processing JSON data in Python follows a standard pipeline:

  1. Context Manager: Use with open(...) to safely access the file.
  2. Parse: Use json.load() to convert JSON to Python lists and dictionaries.
  3. Sort: Use sorted(list, key=lambda ...) to organize complex data structures like dictionaries.

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