Writing and Running Scripts

Module 4: IDEs & Tools

Notebooks and scripts

A Python script is a text file with the extension .py that contains Python code. Compared with a notebook:

	Notebook (.ipynb)	Script (.py)
Structure	Cells that you run one by one	One program that runs top to bottom
Output	Shown below each cell	Shown in the terminal
Best for	Exploring, learning, reports	Larger projects, automation, reusable code
Reusable from other files	Difficult	Yes, with import

Notebooks are great for exploring. When your code grows, or you want to reuse it, move it to a script.

Your first script

1. In VS Code, open your course folder (File > Open Folder…).
2. Create a new file named hello.py.
3. A script usually has three parts, in this order: imports, functions, main code.

import random


def greet(name):
    """Return a greeting for the given name."""
    return f"Hello, {name}!"


name = "BAM"
print(greet(name))
print("Lucky number:", random.randint(1, 9))

4. Run it with the Run Python File button (the triangle at the top right), or in a terminal:

python hello.py

Note

The terminal must be in the same folder as the file. In VS Code, the integrated terminal opens in the folder you have open.

Execution order

Python runs a script from the first line to the last. Functions are only defined when Python reads the def; their code runs only when you call them.

print("1. start")

def say_hello():
    print("3. inside the function")

print("2. after the definition")
say_hello()
print("4. end")

The numbers show the order in which the messages appear.

The main function

When we put a script in a file, we usually write the code that does the work inside a function called main, and we call it at the end:

def compute_square(x):
    return x ** 2


def main():
    print(compute_square(7))


if __name__ == "__main__":
    main()

What does if __name__ == "__main__": mean?

• Python gives every file a special variable called __name__.
• If you run the file directly (python script.py), __name__ is equal to "__main__", so the code inside the if runs.
• If another file imports this one, __name__ is the name of the file, so the code inside the if is skipped.

This lets the same file work as a program and as a collection of functions that other files can import.

The main guard decides what happens when a file is run or imported.

Getting input from the user

The input() function pauses the script and waits for the user to type text in the terminal. It always returns a string:

text = input("Enter some numbers separated by spaces: ")
numbers = [float(x) for x in text.split()]
print(numbers)

Typing 2 4 4 4 gives [2.0, 4.0, 4.0, 4.0]. Here split() cuts the string at the spaces and float() converts each piece to a number.

Importing functions from another script

You can reuse functions from one script in another. Suppose we have two files in the same folder:

# script_sum.py
def sum_fun(a, b):
    return a + b


def main():
    print(sum_fun(2, 3))


if __name__ == "__main__":
    main()

# script_calc.py
from script_sum import sum_fun


def diff_fun(a, b):
    return a - b


def main():
    print(sum_fun(10, 4))
    print(diff_fun(10, 4))


if __name__ == "__main__":
    main()

To import several functions from the same file:

from script_sum import sum_fun, another_function

WarningImportant

• The script you import from must be in the folder you are working in (or in a folder Python can find).
• Do not include the .py in the import: write from script_sum import sum_fun, not from script_sum.py import sum_fun.
• Because of if __name__ == "__main__":, importing script_sum does not run its main().

Practice

When something fails, read the error message first. If you need a reminder of debugging techniques, try the Debugging Questions.

NoteExercise: Guessing Game

Do the Guessing Game as a script guessing_game.py. Import the random library to generate the number. The game must end when the user guesses the number or types exit, and then print the number of guesses.

TipSolution

Save this as guessing_game.py and run it with python guessing_game.py:

import random


def guessing_game():
    number = random.randint(1, 9)   # both ends are included
    guesses = 0

    while True:
        guess = input("Guess a number between 1 and 9, or type 'exit': ")

        if guess.lower() == "exit":
            break

        guess = int(guess)
        guesses += 1

        if guess < number:
            print("Too low!")
        elif guess > number:
            print("Too high!")
        else:
            print("Exactly right!")
            break

    print(f"Game over. You took {guesses} guesses.")


def main():
    guessing_game()


if __name__ == "__main__":
    main()

random.randint(a, b) includes both limits, so random.randint(1, 9) can return 9. Compare with np.random.randint(1, 10), where the upper limit is excluded. If the user types something that is not a number or exit, int(guess) raises a ValueError: you can handle it with try / except ValueError.

NoteExercise: Standard Deviation

Write a script std.py with two functions:

• compute_std calculates the standard deviation of a list of numbers.
• main asks the user to type some numbers separated by spaces, then computes and prints their standard deviation.

Use input() to read from the terminal.

TipSolution

import math


def compute_std(numbers):
    """Return the (population) standard deviation of a list of numbers."""
    n = len(numbers)
    mean = sum(numbers) / n
    variance = sum((x - mean) ** 2 for x in numbers) / n
    return math.sqrt(variance)


def main():
    text = input("Enter some numbers separated by spaces: ")
    numbers = [float(x) for x in text.split()]
    print(f"Standard deviation: {compute_std(numbers)}")


if __name__ == "__main__":
    main()

For the input 2 4 4 4 5 5 7 9 the result is 2.0.

NoteExercise: Mean and Standard Deviation

Write a script mean_std.py with two functions:

• compute_mean calculates the mean of a list of numbers.
• main asks the user for numbers and prints their mean and standard deviation.

Do not copy compute_std: import it from std.py.

TipSolution

from std import compute_std


def compute_mean(numbers):
    """Return the mean of a list of numbers."""
    return sum(numbers) / len(numbers)


def main():
    text = input("Enter some numbers separated by spaces: ")
    numbers = [float(x) for x in text.split()]
    print(f"Mean: {compute_mean(numbers)}")
    print(f"Standard deviation: {compute_std(numbers)}")


if __name__ == "__main__":
    main()

Both files must be in the same folder. Thanks to the main guard, importing std does not ask for numbers twice.

NoteExercise: Centipede Game

The centipede game starts with a pot of money and is played by two players. On their turn, a player can either take the money, or pass the turn to the other player, in which case the pot doubles. The game ends when one player takes the money.

Write a script centipede.py to play against the computer:

• The pot starts at 1 EUR and the first player is chosen at random.
• When it is the computer’s turn, it passes with a probability of \(100 \cdot (1/\text{pot})^{1/3}\ \%\) and takes the money otherwise.
• Play several times. How much money can you get?

TipSolution

import random


def computer_passes(amount):
    """The computer passes with probability (1 / amount) ** (1 / 3)."""
    probability = (1 / amount) ** (1 / 3)
    return random.random() < probability


def play():
    amount = 1
    player_turn = random.choice([True, False])
    while True:
        if player_turn:
            choice = input(f"Your turn. Pot: {amount} EUR. Take or pass? ").strip().lower()
            if choice == "take":
                print(f"You take {amount} EUR!")
                return amount
            amount *= 2
        else:
            if computer_passes(amount):
                print(f"The computer passes. Pot: {amount * 2} EUR")
                amount *= 2
            else:
                print(f"The computer takes {amount} EUR. You get nothing.")
                return 0
        player_turn = not player_turn


if __name__ == "__main__":
    play()

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Need Help?

If you have any questions, feel free to reach out via email at dprecioso@faculty.ie.edu

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