2. はじめての Python プログラム

# hello.py
print("Hello, World!")

Hello, World!

# greetings.py
print("Welcome to Python programming!")
print("This is your first real program.")
print("Python makes programming fun and accessible.")

Welcome to Python programming!
This is your first real program.
Python makes programming fun and accessible.

# multiple_items.py
print("Python", "is", "awesome!")
print("I", "am", "learning", "to", "code")

Python is awesome!
I am learning to code

# mixed_output.py
print("The answer is", 42)
print("Python version", 3.12, "is powerful")

The answer is 42
Python version 3.12 is powerful

# quotes.py
print("This uses double quotes")
print('This uses single quotes')
print("Both work exactly the same way")

This uses double quotes
This uses single quotes
Both work exactly the same way

# apostrophes.py
print("It's a beautiful day!")
print("Python's syntax is clean")

It's a beautiful day!
Python's syntax is clean

# This causes an error:
print('It's a beautiful day!')  # Error! Python sees three separate quotes

# quotations.py
print('She said, "Hello!"')
print('The book is called "Python Basics"')

She said, "Hello!"
The book is called "Python Basics"

# spacing.py
print("First section of output")
print("More information here")
print()  # Empty line
print("Second section of output")
print("This is separated from the first section")

First section of output
More information here
 
Second section of output
This is separated from the first section

# newlines.py
print("First line\nSecond line\nThird line")

First line
Second line
Third line

# tabs.py
print("Name:\tJohn")
print("Age:\t25")
print("City:\tNew York")

Name:	John
Age:	25
City:	New York

# backslash.py
print("The file path is C:\\Users\\Documents")

The file path is C:\Users\Documents

# comment_example.py
# This is a comment. Python ignores this line completely.
print("This line runs")  # This comment explains the print statement
# print("This line does NOT run because it's commented out")
print("This line also runs")

This line runs
This line also runs

# display_info.py
# Display a welcome message to the user
print("Welcome to the program!")
 
print("Processing data...")  # Inform the user of progress

# Bad comment - just repeats what the code obviously does
print("Hello")  # Print "Hello"

# Good comment - explains the purpose or context
# Greet the user before requesting their information
print("Hello")

# temperature_converter.py
# This program converts temperatures from Fahrenheit to Celsius
# Formula: C = (F - 32) × 5/9
 
# Display program title
print("Temperature Converter")
print("Fahrenheit to Celsius")
print()
 
# We'll learn how to get user input in section 2.4
# For now, we'll use a fixed temperature value
fahrenheit = 75  # Temperature in Fahrenheit to convert
 
# Perform the conversion using the standard formula
# Subtract 32 first (order matters due to the formula)
celsius = (fahrenheit - 32) * 5 / 9
 
# Display the result
print("Temperature:", fahrenheit, "°F =", celsius, "°C")

Temperature Converter
Fahrenheit to Celsius
 
Temperature: 75 °F = 23.88888888888889 °C

# multi_line_comments.py
# This program demonstrates a longer explanation
# that spans multiple lines. Each line starts
# with a # symbol.
print("Program starts here")

# docstring_preview.py
"""
This is a multi-line string that can serve as a comment.
It's not technically a comment—it's a string that Python
creates but doesn't use. However, it's often used for
longer explanations at the beginning of files.
"""
print("Program starts here")

# testing.py
print("This always runs")
# print("This is disabled for testing")
print("This also always runs")

variable_name = value

# variables_basic.py
message = "Hello, Python!"
print(message)

Hello, Python!

# variable_types.py
greeting = "Welcome!"  # A string (text)
age = 25  # A number (integer)
price = 19.99  # A number with a decimal point (float)
is_student = True  # A boolean (True or False)
 
print(greeting)
print(age)
print(price)
print(is_student)

Welcome!
25
19.99
True

# no_variables.py
print("Welcome, John!")
print("John, your account balance is $1000")
print("Thank you for banking with us, John!")

# with_variables.py
name = "John"
balance = 1000
 
print("Welcome,", name + "!")
print(name, "your account balance is $", balance)
print("Thank you for banking with us,", name + "!")

# reassignment.py
score = 0
print("Initial score:", score)
 
score = 10
print("After first update:", score)
 
score = 25
print("After second update:", score)

Initial score: 0
After first update: 10
After second update: 25

# updating_values.py
count = 5
print("Starting count:", count)
 
count = count + 1  # Add 1 to count
print("After adding 1:", count)
 
count = count + 10  # Add 10 to count
print("After adding 10:", count)

Starting count: 5
After adding 1: 6
After adding 10: 16

# valid_names.py
user_name = "Alice"
age2 = 30
_private_value = 42
totalPrice = 99.99

# These cause errors:
# 2age = 30  # Cannot start with a number
# user-name = "Alice"  # Cannot contain hyphens
# total price = 99.99  # Cannot contain spaces
# print = "Hello"  # Cannot use Python keywords

# dynamic_greeting.py
name = "Alice"
age = 28
city = "Seattle"
 
print("Personal Information")
print("====================")
print("Name:", name)
print("Age:", age)
print("City:", city)
print()
print("Summary:", name, "is", age, "years old and lives in", city)

Personal Information
====================
Name: Alice
Age: 28
City: Seattle
 
Summary: Alice is 28 years old and lives in Seattle

# basic_input.py
name = input("What is your name? ")
print("Hello,", name + "!")

What is your name? Alice
Hello, Alice!

# input_as_string.py
age = input("How old are you? ")
print("You entered:", age)
print("The type is:", type(age))

How old are you? 25
You entered: 25
The type is: <class 'str'>

# This causes an error:
age = input("How old are you? ")
next_year_age = age + 1  # Error! Can't add a number to a string

# input_conversion.py
age_string = input("How old are you? ")
age = int(age_string)  # Convert string to integer
 
next_year = age + 1
print("Next year, you will be", next_year, "years old")

How old are you? 25
Next year, you will be 26 years old

# input_conversion_compact.py
age = int(input("How old are you? "))
next_year = age + 1
print("Next year, you will be", next_year, "years old")

# float_input.py
price = float(input("Enter the price: $"))
tax_rate = 0.08
tax = price * tax_rate
total = price + tax
 
print("Price: $", price)
print("Tax: $", tax)
print("Total: $", total)

Enter the price: $19.99
Price: $ 19.99
Tax: $ 1.5992
Total: $ 21.5892

# multiple_inputs.py
print("Please enter your information:")
print()
 
first_name = input("First name: ")
last_name = input("Last name: ")
age = int(input("Age: "))
city = input("City: ")
 
print()
print("Summary")
print("=======")
print("Name:", first_name, last_name)
print("Age:", age)
print("Location:", city)

Please enter your information:
 
First name: John
Last name: Smith
Age: 30
City: Boston
 
Summary
=======
Name: John Smith
Age: 30
Location: Boston

# Good prompts
name = input("Enter your full name: ")
temperature = float(input("Enter temperature in Fahrenheit: "))
quantity = int(input("How many items do you want to purchase? "))

# format_prompts.py
date = input("Enter date (MM/DD/YYYY): ")
phone = input("Enter phone number (XXX-XXX-XXXX): ")
price = float(input("Enter price in dollars (without $): "))

# friendly_prompts.py
print("Welcome to the Temperature Converter!")
print()
temp = float(input("Please enter a temperature in Fahrenheit: "))

# temperature_converter.py
# This program converts a temperature from Fahrenheit to Celsius
# Formula: C = (F - 32) × 5/9
 
print("Temperature Converter")
print("Fahrenheit to Celsius")
print("=" * 30)  # Print a line of equal signs
print()
 
# INPUT: Get temperature from user
fahrenheit = float(input("Enter temperature in Fahrenheit: "))
 
# PROCESS: Convert using the formula
celsius = (fahrenheit - 32) * 5 / 9
 
# OUTPUT: Display the result
print()
print("Result:", fahrenheit, "°F =", celsius, "°C")

Temperature Converter
Fahrenheit to Celsius
==============================
 
Enter temperature in Fahrenheit: 75
 
Result: 75.0 °F = 23.88888888888889 °C

# rectangle_area.py
# Calculate the area of a rectangle
# Formula: Area = length × width
 
print("Rectangle Area Calculator")
print("=" * 30)
print()
 
# INPUT: Get dimensions from user
length = float(input("Enter the length: "))
width = float(input("Enter the width: "))
 
# PROCESS: Calculate area
area = length * width
 
# OUTPUT: Display the result
print()
print("A rectangle with length", length, "and width", width)
print("has an area of", area, "square units")

Rectangle Area Calculator
==============================
 
Enter the length: 5.5
Enter the width: 3.2
 
A rectangle with length 5.5 and width 3.2
has an area of 17.6 square units

# shopping_total.py
# Calculate total cost including tax
 
print("Shopping Total Calculator")
print("=" * 30)
print()
 
# INPUT: Get price and tax rate
price = float(input("Enter item price: $"))
tax_rate = float(input("Enter tax rate (as decimal, e.g., 0.08 for 8%): "))
 
# PROCESS: Calculate tax and total
tax_amount = price * tax_rate
total = price + tax_amount
 
# OUTPUT: Display itemized breakdown
# Note: Comma in print() adds a space after $
print()
print("Purchase Summary")
print("-" * 30)
print("Item price:    $", price)
print("Tax rate:      ", tax_rate * 100, "%")
print("Tax amount:    $", tax_amount)
print("-" * 30)
print("Total:         $", total)

Shopping Total Calculator
==============================
 
Enter item price: $49.99
Enter tax rate (as decimal, e.g., 0.08 for 8%): 0.08
 
Purchase Summary
------------------------------
Item price:    $ 49.99
Tax rate:       8.0 %
Tax amount:    $ 3.9992
------------------------------
Total:         $ 53.9892

# error_example.py
print("Starting the program")
print("This line works fine"
print("This line might not be reached")

  File "error_example.py", line 2
    print("This line works fine"
                                ^
SyntaxError: '(' was never closed

# missing_colon.py
name = input("What is your name? ")
if name == "Alice"  # Missing colon
    print("Hello, Alice!")

  File "missing_colon.py", line 2
    if name == "Alice"
                     ^
SyntaxError: expected ':'

# mismatched_quotes.py
message = "Hello, world!'
print(message)

  File "mismatched_quotes.py", line 1
    message = "Hello, world!'
              ^
SyntaxError: unterminated string literal (detected at line 1)

# name_error.py
print("Starting calculation")
result = total + 10  # 'total' was never created
print(result)

Traceback (most recent call last):
  File "name_error.py", line 2, in <module>
    result = total + 10
             ^^^^^
NameError: name 'total' is not defined

# name_error_fixed.py
print("Starting calculation")
total = 0  # Define the variable first
result = total + 10
print(result)

# type_error.py
age = input("How old are you? ")
next_year = age + 1  # Can't add a number to a string
print("Next year:", next_year)

Traceback (most recent call last):
  File "type_error.py", line 2, in <module>
    next_year = age + 1
                ~~~~^~~
TypeError: can only concatenate str (not "int") to str

# type_error_fixed.py
age = int(input("How old are you? "))  # Convert to integer
next_year = age + 1
print("Next year:", next_year)

# value_error.py
age = int(input("How old are you? "))
print("You are", age, "years old")

How old are you? twenty
Traceback (most recent call last):
  File "value_error.py", line 1, in <module>
    age = int(input("How old are you? "))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'twenty'

# simple_traceback.py
print("Starting program")
name = input("Enter your name: ")
age = int(input("Enter your age: "))
print("Hello,", name)
print("Next year you'll be", age + 1)

Starting program
Enter your name: Alice
Enter your age: abc
Traceback (most recent call last):
  File "simple_traceback.py", line 3, in <module>
    age = int(input("Enter your age: "))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'abc'

# debugging_with_print.py
price = input("Enter price: ")
print("DEBUG: price =", price, "type =", type(price))  # Debugging line
 
tax = price * 0.08  # This will cause an error
print("Tax:", tax)

# isolating_error.py
name = input("Enter name: ")
# age = int(input("Enter age: "))  # Commented out for testing
# city = input("Enter city: ")  # Commented out for testing
print("Name:", name)

# simplified_version.py
# Start with just the basic structure
print("Program starting")
# Add one feature at a time and test

# incremental_development.py
# Step 1: Get input and test
name = input("Enter name: ")
print("DEBUG: Got name:", name)
 
# Step 2: Add more input and test
# age = int(input("Enter age: "))
# print("DEBUG: Got age:", age)
 
# Step 3: Add processing and test
# ... and so on

# Clear variable names make errors less likely
user_age = int(input("Enter your age: "))
next_year_age = user_age + 1

# temperature_with_comments.py
# Get temperature in Fahrenheit from user
fahrenheit = float(input("Enter temperature in Fahrenheit: "))
 
# Convert to Celsius using the standard formula: C = (F - 32) × 5/9
celsius = (fahrenheit - 32) * 5 / 9
 
# Display the result
print(fahrenheit, "°F =", celsius, "°C")