Whats the point of __init__.py on Vercel

The imports working perfectly fine locally, but failed after deploying on Vercel. The error I've experiencing is common when deploying Python applications with a specific file structure. My local environment likely handles imports differently than Vercel's serverless environment(because of the folder structure), which can cause import errors. The solution is to ensure that my project structure is recognized as a Python package

Why __init__.py is Essential 📦

In Python, a directory with a file named __init__.py is treated as a package [2]. Without this file, Python's import system will not recognize the directory as a source for modules, and you will get an ImportError. This is because the __init__.py file is what allows you to import files from within that directory as if they were part of a module [1].

When you run an app locally, your development environment's path settings often make it easy for Python to find your files, even if they're not in a formal package. However, in a production environment like Vercel, the file structure must adhere strictly to Python's package conventions.

The Problem on Vercel ⚙️

Vercel deploys your application's code to a serverless function environment. When an incoming request triggers your function, Vercel's environment looks for your code. If your project has a directory like my_module but lacks an __init__.py file, the import statement from my_module import my_file will fail because my_module is not a valid Python package [3].

The Solution: Adding the __init__.py File

To fix this, you simply need to add a (potentially empty) file named __init__.py to any directory that you want to import from.

Before (Failing)

my-vercel-app/
│
├── api/
│   └── my_api_function.py
│
└── utils/
    └── helper_functions.py

In this structure, my_api_function.py cannot successfully import helper_functions.py with from utils import helper_functions.

After (Working)

To make this work, add an __init__.py file to the utils directory.

my-vercel-app/
│
├── api/
│   └── my_api_function.py
│
└── utils/
    ├── __init__.py  # This makes 'utils' a package
    └── helper_functions.py

Now, when you deploy to Vercel, your import statement will work as expected.

# Inside my_api_function.py

from utils import helper_functions

# You can now call functions from helper_functions.py
result = helper_functions.my_helper_function()

The __init__.py file can be empty, but it can also contain code that runs when the package is imported. This is often used to initialize package-level variables or to perform setup tasks [4].

Summary

The __init__.py file is the key to creating a valid Python package. Without it, Python's import system cannot reliably locate your modules, leading to import errors in strict environments like Vercel. Adding this file to each of your subdirectories resolves this issue by declaring them as importable packages.

Sources

1. Python Documentation. "The import system." https://docs.python.org/3/reference/import.html
2. Real Python. "Python Modules and Packages: An Introduction." https://realpython.com/python-modules-packages/
3. Vercel Docs. "Serverless Functions." https://vercel.com/docs/functions/serverless-functions
4. GeeksforGeeks. "Python __init__.py file." https://www.geeksforgeeks.org/python-__init__-py-file/