~/Building a Simple Python REPL

Mar 15, 2020

A REPL is a language shell that reads user input, evaluates it, prints the result, and loops back. In Python, you can create a basic REPL with a simple loop and the eval function.

Here is an example:

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while True:
    try:
        command = input('>>> ')
        if command in ('exit', 'quit'):
            break
        result = eval(command)
        print(result)
    except Exception as e:
        print('Error:', e)

This script will:

• Continuously read input from the user.
• Evaluate the input with eval.
• Print the result.
• Quit if user enters exit or quit.

For more complex statements, use exec instead of eval. For example:

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while True:
    try:
        command = input('>>> ')
        if command in ('exit', 'quit'):
            break
        exec(command)
    except Exception as e:
        print('Error:', e)

Be cautious: using eval or exec can be dangerous if you run untrusted code. See the Python docs on eval for security guidelines.

This is the simplest way to build a Python REPL.