1. Comments

• Single-line comment: # comment
• Multi-line comment: """comment""" or '''comment'''

2. Variables

• Purpose: A variable is a name that references the memory address where data is stored.
• Defining a variable:
  • Format: variable_name = value
  • Identifiers: composed of letters, digits, and underscores; cannot start with a digit; cannot use keywords; case-sensitive.
  • Naming conventions:
    • Meaningful names
    • PascalCase: first letter of each word capitalized
    • camelCase: first word lowercase, subsequent words capitalized
    • snake_case: words separated by underscores (e.g., my_name)
• Data types:
  • int, float, bool (True/False), str, list, tuple, set, dict

number1 = 1          # int
decimal1 = 1.1       # float
greeting = 'hello world'  # str
flag = True          # bool
list_data = [10, 20, 30]   # list
tuple_data = (10, 20, 30)  # tuple
set_data = {10, 20, 30}    # set
dict_data = {'name': 'Amy', 'age': 18}  # dict

3. Output

• Formatted output:
  • %s – string, %d – signed decimal integer, %f – float, %c – charcater

name = 'Amy'
age = 18
gender = 'female'
weight = 55.5
print('This year I am %d years old' % age)
print('My name is %s' % name)
print('My gender is %s' % gender)
print('This year I am %d years old, but next year I will be %d' % (age, age+1))
print('I am %s, this year I weigh %f kg, but I plan to lose 5 kg, so next year I will be %f kg' % (name, weight, weight-5))

• Formatting digits:
  • %06d – pad integer with zeros to at least 6 digits
  • %.2f – show 2 decimal places
• f-strings: f'{expression}'

print(f'My name is {name}, next year I will be {age+1} years old')

• Escape characters:
  • \n – newline
  • \t – tab

print('hello\nworld')
print('hello\tworld')

Output:

hello
world
hello   world

• End character: By default, print() ends with \n. You can change it using end=:

print('hello', end="\n")
print('world', end="\t")
print('hello', end="...")
print('Python')  # default end="\n"

Output:

hello
world   hello...Python

4. Input

• Syntax: input('prompt: ')
• Characteristics:
  • Execution pauses until user provides input.
  • Input is usually stored in a variable.
  • input() always returns a string.

password = input('Please enter your password: ')
print(f'Your password is {password}')
print(type(password))

5. Type Conversion

• Syntax: new_type(variable)

number = input('Enter a number: ')  # user enters 1
print(type(number))  # <class 'str'>
print(type(int(number)))  # <class 'int'>

value1 = 1
value2 = '10'
print(float(value1))  # 1.0
print(float(value2))  # 10.0

• Type conversion works only if the string contains a valid representation of the target type.