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Editors and IDEs

This lesson is optional. You can complete the entire curriculum with a terminal and nothing else. Read this if you are curious, or if you want to start changing the code.

What is an editor? What is an IDE?

An editor is a program for writing text files. Code is text files. Word processors are not suitable, they add invisible formatting that breaks code.

An IDE (Integrated Development Environment) is an editor plus extra tools: running code, debugging, autocomplete, jumping to a function's definition.

The distinction is fuzzy and does not matter much. Both let you edit files.

Do I need one?

No. Moonfield is designed terminal-first, and every lesson works that way.

An editor helps when you want to:

• Read several source files while following a lesson
• Change a value and immediately see what breaks
• Search the whole project for a term
• Write your own scripts

The options

Terminal editors

Already installed, no setup, work over SSH.

• nano: the friendly one. Commands are listed at the bottom. Ctrl+O to

save, Ctrl+X to exit. If you have never used a terminal editor, start here.

• vim: powerful, and famously hard to exit. (It is :q!.) Worth learning

eventually, not today.

• micro: modern feel, familiar shortcuts. Needs installing.

nano src/moonfield/phase.py

Graphical editors

• VS Code: free, popular, excellent Python support. The usual first choice.
• VSCodium: VS Code without Microsoft's telemetry.
• PyCharm Community: free, Python-specific, heavier but very capable.
• Cursor, Zed, Sublime Text: all fine.

Whatever you already know is the right answer. Nothing here requires a particular one.

The one thing that actually matters: the interpreter

This is the concept that causes almost all IDE confusion, so it is worth a minute.

Your computer may have several Pythons: a system one, one from python.org, one from the app store, and one inside each project's virtual environment.

When you activate .venv in a terminal, you tell that terminal to use that Python. But your editor is a separate program. It does not know what your terminal did. Unless you tell it, it may use a completely different Python, one where Moonfield is not installed.

That produces the classic confusing symptom:

It runs fine in my terminal but my editor says ModuleNotFoundError: No module named 'moonfield'.

Nothing is broken. The editor is just looking in the wrong place.

Fixing it in VS Code

1. Open the moonfield folder (File → Open Folder)
2. Ctrl+Shift+P (Cmd+Shift+P on macOS)
3. Type "Python: Select Interpreter"
4. Choose the one with .venv in its path, usually marked "Recommended"

The chosen interpreter shows in the bottom status bar. If it does not say .venv, that is your problem.

Fixing it in PyCharm

Settings → Project → Python Interpreter → gear icon → Add → Existing Environment → point at moonfield/.venv/bin/python (or .venv\Scripts\python.exe on Windows).

Checking from anywhere

import sys
print(sys.executable)

If that path does not contain .venv, your editor is on the wrong interpreter.

A minimal setup

If you want VS Code configured for this project and nothing more:

1. Install VS Code
2. Install the Python extension by Microsoft
3. Open the moonfield folder
4. Select the .venv interpreter as above
5. Open a terminal inside VS Code (Ctrl+ ` ``): it usually activates the

venv automatically

That is all. Skip the rest of the extension marketplace for now.

Checkpoint

• I know what an editor and an IDE are
• I know I do not need either for this curriculum
• I understand why the Python interpreter setting matters
• If I use an editor, I have pointed it at my .venv
• I know how to check which Python is running

Try it yourself

1. Open src/moonfield/phase.py in any editor and find REFERENCE_NEW_MOON
2. Change it by one day, run moonfield phase, see what happens
3. Change it back
4. Print sys.executable from both your terminal and your editor and compare

Getting stuck?

Getting Unstuck

Next: Module 01, Time and place.