Concepts / Debugging Python Scripts

Debugging Python Scripts

A Python script is a text file containing Python code, saved with the .py extension, that the interpreter executes from top to bottom.

  • Programming

Two Ways to Run Python

When learning Python, you can enter an instruction directly into the interactive interpreter or place a collection of instructions in a saved script. The interpreter is useful when you want immediate feedback from a small experiment. A script is more suitable when you are building a complete program that must be organized, saved, reused, or run again.

What do you think happens?

You need to repeat a multi-step data analysis tomorrow. Which approach is more suitable: entering each instruction again in the interactive interpreter or saving the instructions in a script?

  • Enter every instruction again in the interactive interpreter

  • Save the instructions in a Python script

Reveal answer
Answer:

Save the instructions in a Python script

A script is persistent and reusable. The interactive interpreter is designed for one-off commands and experimentation, and its entered code disappears when the interpreter is closed.

The Interactive Interpreter

The interactive Python interpreter acts as a sandbox for experimentation. You type a command, press Enter, and immediately see the result. This makes it useful for learning, testing a small piece of code, checking how a function behaves, or exploring Python's built-in features. Each command is evaluated immediately, and the interpreter presents a prompt for the next command.

Interactive interpreterSaved script
Commands are entered one at a timePython code is stored together in a text file
Results are shown immediatelyThe interpreter runs the saved code as a program
Useful for experimentation and small testsUseful for complete, reusable programs
Entered code disappears when the interpreter closesThe file can be saved and run again
A prompt waits for the next commandThere is no interactive prompt during execution

The two execution environments support different kinds of work.

Interactiveinterpretercommand by commandPromptnext commandSaved script.py fileScript executionno promptevaluates immediatelyruns saved code
How does entering code interactively differ from running all the code in a saved script?

Anatomy of a Python Script

Definition

A Python script is a text file containing Python code, saved with the .py extension, that the interpreter executes from top to bottom.

The script is saved as plain text. You can create it with a simple text editor, a code-focused editor, or an integrated development environment. The important requirement is that the editor saves plain text rather than a formatted document. The .py extension identifies the file as containing Python code for the interpreter to execute.

Python codewritten in an editorPlain-text file.py extensionInterpreteropens the fileExecutionbeginning to endsave asrunreads and executes
What happens to Python code as it moves from an editor to a saved .py file and then to the interpreter?

Execution from Beginning to End

When you run a script, the interpreter opens the file and reads the code from the beginning to the end. It then executes each instruction in order, as if you had entered the instructions one by one in the interactive interpreter. In script mode, there is no prompt and no interactive feedback during execution. The script runs silently unless you explicitly use print() to display output.

Worked example

Tracing a Script with an Error

A saved script contains three ordered instructions. The interpreter completes the first instruction, encounters an error during the second instruction, and has a third instruction after it. What happens?

  1. 1
    Start at the beginning

    The interpreter opens the script and begins reading its contents from the start.

  2. 2
    Execute the first instruction

    The first instruction is executed in its position in the script.

  3. 3
    Reach the error

    When the interpreter encounters an error, it stops and reports the problem.

  4. 4
    Do not continue

    The instruction after the error is not reached because execution has already stopped.

Result

A script normally proceeds from top to bottom, but an error interrupts execution at the point where the problem occurs.

Script startbeginningInstruction 1executedInstruction 2error occursErrorexecution stopsInstruction 3not reachedread nextcontinueproblemno further execution
In what order does the interpreter execute a script, and where does an error stop execution?

Why Complex Work Needs Scripts

In the real world

Consider a program that analyzes data from a hundred files, performs calculations, and generates a report. Entering every step into the interactive interpreter would be tedious, error-prone, and difficult to reproduce. If you made a mistake or wanted to run the program again later, you would need to retype the instructions. A script keeps the instructions available so the complete process can be run again.

Scripts become increasingly important as a program grows. They provide a persistent place to keep related instructions, allow code to be organized logically, and can include comments explaining what sections do. These qualities make a script more reliable to reuse than a sequence of commands entered from memory.

  • Use the interactive interpreter for learning, quick experiments, small tests, and exploring how a function or module works.
  • Use a script when solving a real problem or building a complete program.
  • Use a script when you want to save code and run it again later.
  • Use a script when related instructions need logical organization and comments.

Common Execution Mistakes

  • Common mistake

    Treating the interactive interpreter as permanent storage

    What it looks like

    Entering a useful sequence of commands interactively and then closing the interpreter without saving it in a script.

    Why it fails

    The code entered in the interactive interpreter disappears when the interpreter is closed.

    Do this instead

    Move code that must be reused into a plain-text Python script saved with the .py extension.

  • Common mistake

    Expecting a running script to behave like the interactive interpreter

    What it looks like

    Running a script and expecting a prompt or automatic display of every result.

    Why it fails

    Script mode has no interactive prompt and runs silently unless output is explicitly displayed with print().

    Do this instead

    Use the interactive interpreter for immediate feedback, or explicitly use print() in a script when output should be displayed.

  • Common mistake

    Assuming an error allows later instructions to continue

    What it looks like

    Expecting the interpreter to execute instructions after the point where the script reports an error.

    Why it fails

    The interpreter stops and reports the problem when an error occurs.

    Do this instead

    Treat the error location as the point where execution stopped, then inspect the preceding and current instructions.

  • Common mistake

    Saving Python code as a formatted document

    What it looks like

    Using a word-processing document as the script file.

    Why it fails

    The source requires a plain-text file rather than a formatted document.

    Do this instead

    Use a text editor, code-focused editor, or integrated development environment that saves plain text, and use the .py extension.

  • Common mistake

    Retyping a growing program one command at a time

    What it looks like

    Re-entering all instructions for a multi-step analysis whenever the task must be repeated.

    Why it fails

    This approach is tedious, error-prone, and difficult to reproduce reliably.

    Do this instead

    Organize the instructions in a reusable script.

Practice the Workflow

EASY

Describe the complete workflow for turning Python instructions into a reusable program. Include the type of editor or file format required, the file extension, what the interpreter does when the file runs, and what happens if an error occurs.

Hints
  • Begin with writing Python code in an editor.
  • Mention that the file must be plain text and use the .py extension.
  • Explain that the interpreter reads and executes the script from top to bottom.
  • State that an error stops execution and the interpreter reports the problem.
EASY

You want to explore how one Python function behaves before adding it to a larger program. Decide whether the interactive interpreter or a saved script is the better starting point, and explain why.

Hints
  • Look for the option that gives immediate feedback.
  • The source recommends one environment specifically for experimentation and testing a small piece of code.

Key Takeaways

  1. 1
    A Python script is a plain-text file containing Python code and normally uses the .py extension.
  2. 2
    The interactive interpreter evaluates commands immediately and is best suited to experimentation and small tests.
  3. 3
    A script is read and executed from top to bottom, without an interactive prompt during execution.
  4. 4
    An error stops script execution and causes the interpreter to report the problem.
  5. 5
    Scripts are persistent, reusable, and easier to organize for complex or repeatable work.

At a glance

  • Use the interactive interpreter for immediate feedback during experimentation.
  • Use a saved .py script for complete programs and work that must be reused.
  • Create a script in a plain-text editor, save it with the .py extension, and run it with the interpreter.
  • The interpreter executes a script from top to bottom and stops when an error occurs.
  • Scripts make complex tasks manageable by preserving, organizing, and reusing related instructions.

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