Python Interpreter and Read–Eval–Print Loop¶

Python programs run inside a program called the Python interpreter.

The interpreter reads Python code, executes it, and produces results. Python also provides an interactive execution environment known as the REPL.

REPL stands for:

• Read
• Evaluate
• Print
• Loop

flowchart LR
    A[Read]
    B[Evaluate]
    C[Print]
    D[Loop]

    A --> B --> C --> D --> A

This cycle allows Python to execute commands interactively.

1. Starting the Interpreter¶

The Python interpreter can be started from the terminal or command prompt.

Typical commands:

python

or

python3

If Python starts successfully, a prompt appears:

>>>

This prompt indicates that Python is ready to receive commands.

Example:

>>> 2 + 3
5

The interpreter immediately evaluates the expression and prints the result.

2. Basic REPL Interaction¶

The REPL allows users to enter Python statements and expressions interactively.

Example:

>>> x = 10
>>> x * 2
20

Each command is executed as soon as it is entered.

This interactive execution makes it easy to test ideas and explore the language.

3. Using the REPL for Exploration¶

The REPL is especially useful for:

• experimenting with code
• testing expressions
• learning language features
• exploring modules

Example:

>>> import math
>>> math.sqrt(25)
5.0

This quick feedback makes the REPL a powerful learning and debugging tool.

4. Exiting the Interpreter¶

The interpreter session can be exited using the exit() function:

exit()

Keyboard shortcuts may also be used:

After exiting, control returns to the operating system shell.

5. REPL vs Script Execution¶

Python programs can be executed in two primary ways.

flowchart TD
    A[Python Code]
    A --> B[Interactive REPL]
    A --> C[Script Execution]

The REPL is useful for quick testing, while scripts are used for larger programs.

6. Example Script¶

A Python program can be saved as a file and executed by the interpreter.

Example file: square.py

x = int(input("Number: "))
print(x * x)

Run the script from the terminal:

python square.py

Example interaction:

Number: 5
25

Scripts allow programs to be reused and shared.

7. Summary¶

Key ideas from this section:

• the Python interpreter executes Python code
• the REPL provides an interactive programming environment
• expressions entered in the REPL are evaluated immediately
• Python programs can also be saved and executed as scripts
• both REPL interaction and script execution are important development tools

The interpreter is the core component that allows Python programs to run and produce results.