Python Hello World

print("Hello, World!")

# Navigate to the folder containing hello.py
cd ~/projects/my-first-python

# Run the file
python3 hello.py     # macOS / Linux
python hello.py      # Windows (if Python 3 is the default)
py hello.py          # Windows py launcher (most reliable)

# Run a one-liner without a file (-c flag)
python3 -c "print('Hello from command line')"

# In VS Code: open the file and press F5 (or Ctrl+F5 to run without debug)

# Basic print
print("Hello, World!")        # Hello, World!
print(42)                     # 42
print(3.14)                   # 3.14
print(True)                   # True

# Multiple arguments - separated by space by default
print("Name:", "Alice", "Age:", 30)   # Name: Alice Age: 30

# sep - change the separator
print("2024", "06", "15", sep="-")    # 2024-06-15
print("a", "b", "c", sep=", ")       # a, b, c

# end - change what is appended after (default is "\n")
print("Loading", end="")
print("...", end="")
print(" done")
# Loading... done   (all on one line)

# Print empty line
print()

# f-string (the modern way to format output)
name = "Alice"
age  = 30
print(f"Hello, {name}! You are {age} years old.")
# Hello, Alice! You are 30 years old.

# print to stderr (useful for error messages)
import sys
print("Error: file not found", file=sys.stderr)

# Single-line comment - starts with #
# Everything after # on the same line is ignored

x = 5  # inline comment - after code on the same line

# Multi-line comments - use consecutive # lines
# This is line 1 of a longer explanation
# This is line 2
# This is line 3

# Docstrings - triple-quoted strings used to document functions/classes
def greet(name):
    """Return a greeting string for the given name."""
    return f"Hello, {name}!"

# Docstrings are accessible at runtime via __doc__
print(greet.__doc__)  # Return a greeting string for the given name.

# Triple-quoted strings can span multiple lines (used as multi-line docstrings)
def calculate_area(width, height):
    """
    Calculate the area of a rectangle.

    Args:
        width: The width of the rectangle.
        height: The height of the rectangle.

    Returns:
        The area as a float.
    """
    return width * height

# input() always returns a string
name = input("What is your name? ")
print(f"Hello, {name}!")

# Convert to int or float for numeric input
age_str = input("How old are you? ")
age     = int(age_str)   # convert string to integer
print(f"In 10 years you will be {age + 10}.")

# Shorthand - convert in one line
height = float(input("Enter your height in meters: "))
print(f"Height: {height}m")

# Always validate user input in real programs
try:
    count = int(input("Enter a number: "))
    print(f"You entered: {count}")
except ValueError:
    print("That was not a valid number.")

$ python3
>>> # Expressions are evaluated and printed automatically
>>> 2 + 2
4
>>> "hello".upper()
'HELLO'

>>> # Assign variables - they persist in the session
>>> x = 10
>>> x * 3
30

>>> # _ holds the last result
>>> 5 * 5
25
>>> _ + 1
26

>>> # Multi-line code - use ... continuation
>>> for i in range(3):
...     print(i)
...
0
1
2

>>> # Exit
>>> exit()    # or Ctrl+D

# 1. Imports (standard library first, then third-party, then local)
import os
import sys

# 2. Constants (UPPER_SNAKE_CASE by convention)
MAX_RETRIES = 3
DEFAULT_NAME = "World"

# 3. Functions and classes
def greet(name=DEFAULT_NAME):
    """Print a greeting."""
    print(f"Hello, {name}!")

def main():
    """Entry point of the program."""
    if len(sys.argv) > 1:
        name = sys.argv[1]  # command-line argument
    else:
        name = input("Enter your name: ")
    greet(name)

# 4. Entry point guard - runs only when executed directly, not when imported
if __name__ == "__main__":
    main()