The REPL vs Script

Python can run code in two different ways: interactively (REPL) or as a script. Understanding the difference prevents confusion.

REPL (Interactive Mode)

REPL stands for "Read-Eval-Print Loop." It's what happens when you type python (or python3) in your terminal and see the >>> prompt.

Characteristics:

• Runs one line at a time
• Shows results immediately after each line
• Great for testing and experimenting
• Variables persist between lines (until you exit)

>>> print("Hello")
Hello
>>> print("World")
World
>>> name = "Alice"
>>> print(name)
Alice

When to use: Testing code, trying things out, learning, debugging

Script (File Mode)

A script is a .py file that Python runs from top to bottom.

Characteristics:

• Runs all lines in order, once
• Only shows output from print() statements
• Great for saving and reusing code
• Variables exist for the entire program run

# saved as example.py
print("First line")
print("Second line")
print("Third line")

When you run python example.py, Python executes all lines in order, from top to bottom.

When to use: Writing programs, saving your work, creating reusable code

Key differences

Feature	REPL (Interactive)	Script (File)
Execution	One line at a time	All lines, top to bottom
Output	Shows result of every expression	Only shows print() output
Persistence	Until you exit	For the program run
Best for	Testing, learning	Writing programs

Common confusion

"Why did it work in the REPL but not in my script?"

This usually happens because:

1. In REPL: You might have typed some code earlier that created variables or set things up
2. In Script: The script starts completely fresh—nothing from previous runs exists

# In REPL, you might have done:
>>> name = "Alice"
>>> # Then later...
>>> print(name)  # Works! name exists from earlier

# But in a script:
print(name)  # Error! name was never created in this script
name = "Alice"

Solution: Make sure everything your script needs is created in the script itself, in the right order (top to bottom).

Using VS Code

If you're using VS Code (or another code editor):

• Interactive mode: Use the integrated terminal and type python to start a REPL
• Script mode: Write code in a .py file and run it with the "Run" button or python filename.py

Both modes are useful:

• Use interactive mode to test ideas quickly
• Use script mode to write and save your programs

Summary

Python can run code in two different ways:

• REPL (Interactive Mode): Runs one line at a time, shows results immediately, great for testing and learning. Start it by typing python in your terminal.
• Script (File Mode): Runs all lines in a .py file from top to bottom, only shows print() output, great for saving and reusing code. Run it with python filename.py.

Key differences:

• REPL runs one line at a time; scripts run all lines in order
• REPL shows results immediately; scripts only show print() output
• REPL is great for testing; scripts are great for writing programs

Common pitfall: Code that works in the REPL might not work in a script if you forgot to include setup code. Always make sure your script has everything it needs, in the right order (top to bottom).

Understanding when to use each mode will make you a more effective Python programmer!