How to Create and Update requirements.txt in Python (The Right Way)
Why do you need this file?
Imagine you send your Python code to a friend or upload it to a server. They try to run it, but it crashes immediately with an error: ModuleNotFoundError.
Why? Because your computer has libraries (like Django, Pandas, or NumPy) installed, but their computer does not.
A requirements.txt file is like a shopping list for your project. It tells other computers exactly which packagesтАФand which specific versionsтАФthey need to download to make your code work.
Here is how to manage it easily.
Step 1: Always Activate Your Virtual Environment
Before you do anything with requirements.txt, you must be inside your virtual environment. If you don't do this, you might accidentally include every single Python package installed on your entire laptop (even ones this project doesn't use!).
Windows: venv\Scripts\activate
Mac/Linux: source venv/bin/activate
(You know it works when you see (venv) in your terminal).
Step 2: Creating the File (The First Time)
Once your environment is active and you have installed your packages (like pip install django), you need to save that list to a file.
Run this simple command in your terminal:
pip freeze > requirements.txtWhat just happened?
pip freeze: asks Python "What is currently installed in this environment?">: takes that answer and writes it into a file instead of the screen.requirements.txt: the name of the file (you can name it anything, but this is the standard).
You will now see a new file in your folder containing lines like Django==4.2.7.
Step 3: Updating the File (When You Add New Packages)
This is where most people get confused.
If you install a new package, for example:
pip install requests
Your requirements.txt file does NOT update automatically. If you check the file, "requests" won't be there yet.
To update it, you must run the freeze command again:
pip freeze > requirements.txtThis will overwrite the old file with the new, updated list of packages.
Step 4: How to Use the File (Installing)
If you move your project to a new computer (or a friend wants to run it), you don't need to install packages one by one.
Just run this "magic" command:
pip install -r requirements.txtThis tells pip: "Read this file and install everything listed inside it."
Summary Checklist
- Activate your virtual environment.
- Install your packages (
pip install package_name). - Freeze them to the file (
pip freeze > requirements.txt). - Repeat step 3 every time you install something new.
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