The REPL or a file: two ways to run Python
When to type into the interactive shell, when to save a file, and what Python 3.14 added to the shell.
There are two ways to run Python, and beginners usually only know one. You can run it one line at a time and get an answer after each line, or you can save your code in a file and run the whole thing.
The REPL: ask, get an answer
Type python in a terminal and press enter. You get the REPL, the interactive shell, with a >>> prompt:
> python
>>> 40 * 0.2
8.0
>>> price = 40
>>> price + price * 0.2
48.0Each line runs the moment you press enter. Ask it forty times zero point two, and it answers 8.0 straight away. Store a price, use it on the next line, and you get 48.0. No file, and no run button.
Since Python 3.14 the shell also colours your code as you type, the way an editor does.
The catch: close the window and everything is gone. The REPL remembers price only for as long as it is open.
A file: keep it, run it again
Put the same three lines in a file called total.py:
price = 40
tax = price * 0.2
print("total:", price + tax)Run it with:
python total.pytotal: 48.0Same answer, and this time you keep it. You can run it again tomorrow, change one number, or send it to someone.
Notice the print. The REPL shows the value of every line for you. A file shows only what you print.
Which one to use
- Use the REPL to experiment: check what a function returns, try an expression, test an idea in ten seconds.
- Use a file for anything you want to keep, run again, or build on.
REPL to experiment, file to keep.
Both are covered step by step in Your first Python program.
This article goes with the video Run Python one line at a time. Watch it on YouTube.