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The Python interpreter is your gateway to running Python code. This guide covers how to start the interpreter, pass arguments, and understand its interactive mode.

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Invoking the Interpreter

The Python interpreter is usually installed as /usr/local/bin/python3.15 on Unix systems. You can start it by typing: 
python3.15
On Windows, if you’ve installed Python from the Microsoft Store, use: 
python
Or with the py launcher: 
py

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Exiting the Interpreter

To exit the interpreter:
  • Unix/Linux: Press Ctrl-D
  • Windows: Press Ctrl-Z then Enter
  • All platforms: Type quit()

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Command Line Editing

The interpreter supports:
  • Interactive line editing
  • History substitution
  • Code completion on most systems
Test if command line editing works by typing a word and pressing the Left arrow key. If the cursor moves, editing is enabled.

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Running Python Scripts

You can run Python code in several ways:

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Execute a File

python script.py

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Execute a Command

python -c "print('Hello, World!')"
Since Python statements often contain spaces or special characters, quote the entire command.

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Execute a Module

python -m http.server

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Interactive Mode After Script

Run a script and then enter interactive mode: 
python -i script.py

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Argument Passing

When you run a Python script, command-line arguments are available in sys.argv: 
import sys

print("Script name:", sys.argv[0])
print("Arguments:", sys.argv[1:])
Special cases:
  • No script: sys.argv[0] is an empty string
  • With -c: sys.argv[0] is '-c'
  • With -m: sys.argv[0] is the full module name

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Interactive Mode

When you start the interpreter without a script, it enters interactive mode: 
$ python3.15
Python 3.15 (default, May 7 2025, 15:46:04)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

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Prompts

  • Primary prompt (>>>): Ready for a new statement
  • Secondary prompt (...): Continuation of a multi-line statement
>>> the_world_is_flat = True
>>> if the_world_is_flat:
...     print("Be careful not to fall off!")
...
Be careful not to fall off!

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Source Code Encoding

By default, Python source files are treated as UTF-8 encoded. This allows using characters from most languages: 
# UTF-8 is the default
name = "François"
greeting = "你好"

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Declaring a Different Encoding

To use a different encoding, add a special comment as the first line: 
# -*- coding: encoding -*-
Example for Windows-1252: 
# -*- coding: cp1252 -*-

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With a Shebang Line

If your script starts with a shebang, the encoding goes on the second line: 
#!/usr/bin/env python3
# -*- coding: cp1252 -*-
For portable code, stick to UTF-8 encoding and ASCII identifiers even if you can use non-ASCII characters in strings and comments.

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Next Steps

Now that you know how to invoke the interpreter, let’s explore Python basics in the Introduction to Python.

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