1.1 Literals

A literal is a fixed value written directly in the source code. It represents a constant value that doesn't change during program execution.

Common literal types in Python:

100
3.14159
"Software Engineer"

print(100)
print(3.14159)
print("Software Engineer")

1.2 Comments

Comments are annotations ignored by the Python interpreter. They help developers understand code.

Single-line comments use the # symbol. Everything after # on that line is treated as a comment. A space after # improves readability. Single-line comments typically explain specific lines or small code blocks.

Multi-line comments use triple quotes ''' or """. All content within these quotes is ignored and can span multiple lines. Multi-line comments are commonly used to document modules, classes, and functions.

# This is a single-line comment

"""This is a multi-line comment
that spans several lines
for comprehensive documentation"""

1.3 Variables

Variables store values that can be modified during program execution.

Definition format:

variable_name = value

Key characteristic: The value stored in a variable can change throughout the program's lifecycle.

Outputting multiple values: Use commas within the print() function to display multiple items.

# Store account balance in a variable
balance = 1000
print("Current balance:", balance)

# Withdraw 250 dollars
balance = balance - 250
print("After withdrawal:", balance, "dollars")

user_name = "Alice"
user_age = 28

1.4 Data Types

Checking data types: Use the type() function to determine a value's data type.

About variables: Variables themselves don't have types. A variable marked as a string means it contains a string value, not that the variable itself is classified as a string type.

# Method 1: Print type information directly
print(type("Hello World"))
print(type(999))
print(type(2.71828))

# Method 2: Store type() results in variables
string_type = type("Hello World")
int_type = type(999)
float_type = type(2.71828)
print(string_type)
print(int_type)
print(float_type)

# Method 3: Use type() to check the type of data stored in a variable
country = "United States"
country_type = type(country)
print(country_type)

1.5 Type Conversion

Conversion funcsions: str(), int(), float()

Important rules:

• Any type can be converted to a string
• Converting a string to a number requires the string to contain only numeric characters
• Converting a float to an integer truncates the decimal portion, losing precision

# Convert numbers to strings
num_str = str(42)
print(type(num_str), num_str)
float_str = str(3.14159)
print(type(float_str), float_str)

# Convert strings to numbers
num = int("42")
print(type(num), num)
num2 = float("3.14159")
print(type(num2), num2)

# Conversion error example - string must contain only digits
# num3 = int("twenty")
# print(type(num3), num3)

# Integer to float
float_num = float(42)
print(type(float_num), float_num)

# Float to integer - decimal is truncated
int_num = int(3.14159)
print(type(int_num), int_num)

1.6 Identifiers

Identifiers are names created by the developer to label variables, classes, functions, and other code elements.

Naming rules:

• Character restrictions: Only Chinese characters, English letters, digits, and underscores are allowed. Digits cannot start the name.
• Case sensitivity: Variable and variable are treated as different identifiers
• Reserved keywords: Cannot use Python's built-in keywords

Naming conventions for variables:

• Names should be descriptive and self-explanatory
• Use snake_case (words separated by underscores)
• Keep all letters lowercase

# Rule 1: Only letters, digits, underscores; cannot start with a digit
# Invalid: 2nd_place = "runner"
# Invalid: total! = 100
first_name = "Bob"
_last = "Smith"
item2 = "widget"

# Rule 2: Case-sensitive
PythonCourse = "Advanced"
python_course = "Beginner"
print(PythonCourse)
print(python_course)

# Rule 3: Cannot use reserved keywords
# Invalid: def = 1
# Invalid: class = "Container"
Class = 1

1.7 Operators

Arithmetic operators:

• Addition: +
• Subtraction: -
• Multiplication: *
• Division: /
• Floer division: //
• Modulus: %
• Exponentiation: **

Assignment operators:

• Simple assignment: =
• Compound assignments: +=, -=, *=, /=, //=, %=, **=

1.8 String Definition Methods

Python supports three ways to define strings:

1. Single quotes: 'text'
2. Double quotes: "text"
3. Triple quotes: '''text''' or """text""" - allows multi-line strings

Quote nesting:

• Use backslash \ for escaping quotes
• Single quotes can contain double quotes, and vice versa

1.9 String Concatenation

Concatenation method: Use the + operator to join string literals or string variables.

Limitation: Cannot directly concatenate strings with non-string types.

# Concatenating string literals
print("Learn " + "Python " + "Today")

# Concatenating literal with variable
title = "Engineer"
company = "TechCorp"
phone = 5551234
print("Position: " + title + " at " + company)
# This would error: print("Phone: " + phone)

1.10 String Formatting with Placeholders

Syntax: "%placeholder" % variable

Common placeholders:

• %s - strings
• %d - integers
• %f - floats

# Using placeholders for string concatenation
developer = "Backend Specialist"
message = "Career path: %s" % developer
print(message)

# Combining numbers and strings
cohort = 23
salary = 95000
message = "Web Development track, cohort %d, average salary: $%s" % (cohort, salary)
print(message)

company = "InnovateTech"
founded = 2015
stock_value = 45.67
message = "%s founded in %d, stock price: %f" % (company, founded, stock_value)
print(message)

1.11 Format Specifier Precision Control

Syntax: Use m.n format like %5d, %7.2f, %.2f

• m controls minimum width
• .n controls decimal precision
• Both can be omitted

Behavior:

• If m is less than the number's width, m has no effect
• .n rounds the decimal portion

num1 = 42
num2 = 3.14159
print("Width 5:", "%5d" % num1)
print("Width 1:", "%1d" % num1)
print("Width 8, precision 2:", "%8.2f" % num2)
print("Auto width, precision 2:", "%.2f" % num2)

1.12 f-String Formatting

Syntax: f"{variable} {variable}"

Advantages:

• Ignores type specifications
• No precision control
• Cleaner and more readable for quick formatting

1.13 Formatting Expressions

Expression definition: Code that evaluates to a specific result, such as 1 + 1, len("text"), or 5 * 8.

During variable assignment like total = 25 + 30, the right side is an expression that produces a result assigned to the variable.

Formatting methods:

• f"{expression}"
• "%s\%d\%f" % (expr1, expr2, expr3)

1.14 User Input with input()

The input() function reads data from the keyboard.

Usage: input("prompt message") displays a prompt before waiting for user input.

Critical note: All input values are returned as strings, regardless of what the user types.