The Python REPL

The REPL is an interactive Python prompt where you can type one line of code, press Enter, and see the result right away. No files, no saving — just experiment.

REPL stands for Read–Eval–Print–Loop:

  1. Read your input
  2. Evaluate it
  3. Print the result
  4. Loop back for more

Think of it like a calculator for Python. It's the fastest way to test a function, check what a variable looks like, or sanity-check an import.

Starting the REPL

Open a terminal and type:

python

(or python3 on some systems). You'll see something like:

Python 3.12.1 (main, ...)
Type "help", "copyright", "credits" or "license" for more information.
>>>

The >>> is the prompt — it means "Python is waiting for you to type something."

To exit the REPL, type exit() and press Enter, or press Ctrl+D (macOS/Linux) or Ctrl+Z then Enter (Windows).

Your First REPL Session

Try typing each of these lines and pressing Enter after each:

>>> 2 + 2
4
>>> name = "Ari"
>>> name
'Ari'
>>> print("Hello,", name)
Hello, Ari
>>> len("hello")
5

Notice:

  • You don't need print() to see a value — just type the expression and Python will show its result.
  • Variables you set (name = "Ari") stay around for the rest of the session.
  • Strings display with quotes ('Ari') when printed automatically, but without quotes when you use print().

Importing Your Own Code

This is the most useful thing for project work. If you have a file recommend.py with a function recommend_clps, you can test it without running the whole app:

>>> from recommend import recommend_clps
>>> from scraper import get_clp_events
>>> events = get_clp_events()
>>> recommend_clps(events, ["music"])

Important: start the REPL from the same folder as your .py files (use cd to get there first). Otherwise Python won't find them.

cd path/to/your/project
python

Multi-line Code

If you type something that needs more lines (like a function or an if statement), the prompt changes to ... to show Python is waiting for the rest:

>>> def greet(name):
...     return "Hello, " + name
...
>>> greet("Ari")
'Hello, Ari'

Press Enter on an empty ... line to finish the block.

Handy REPL Tricks

Use the up arrow to recall previous commands — great for re-running a test after editing code.

Check what type something is:

>>> type(events)
<class 'list'>
>>> type(events[0])
<class 'dict'>

Peek inside a list or dict:

>>> events[0]
{'title': 'Music Night', 'interests': ['music', 'community']}
>>> events[0].keys()
dict_keys(['title', 'interests'])

Get help on anything:

>>> help(str.upper)
>>> help(recommend_clps)

(Press q to exit the help screen.)

If You Change Your Code

The REPL loads your module once. If you edit recommend.py while the REPL is running, the old version is still in memory. The simplest fix: exit the REPL (exit()) and start a fresh one.

When to Use a REPL vs. a Test File

  • REPL: quick, one-off checks. "Does this function return what I expect?"
  • Test file (e.g. test_recommend.py): checks you want to re-run later, or a sequence of calls that's annoying to retype.

A test file is just a normal Python script — run it with python test_recommend.py.

VS Code Shortcut

In VS Code you can open an integrated Python REPL with:

  • Open a .py file
  • Press Shift+Enter on a line → it sends that line to a Python terminal
  • Or run the Python: Start REPL command from the command palette (Cmd+Shift+P / Ctrl+Shift+P)

Tip: the REPL is your friend. Any time you're not sure what a piece of code does, paste it in and see.