Python 3 introduced several breaking changes from Python 2:

• Print function: print "hello" vs print("hello")
• Unicode strings: Default string type is Unicode in Python 3
• Integer division: 5/2 returns 2.5 in Python 3, 2 in Python 2
• Range function: range() returns an iterator in Python 3
• Input function: raw_input() was renamed to input()

Python 2 reached end-of-life in 2020, so Python 3 is the current standard.

PEP 8 is the official style guide for Python code. It provides conventions for:

• Indentation: Use 4 spaces per indentation level
• Line length: Maximum 79 characters
• Naming conventions: snake_case for variables, UPPER_CASE for constants
• Import organization: Standard library, third-party, local imports
• Whitespace: Proper spacing around operators and after commas

Python uses indentation to define code blocks instead of braces {}. This enforces consistent code structure and improves readability.

if x > 0:
    print("Positive")  # 4 spaces indentation
    if x > 10:
        print("Greater than 10")  # 8 spaces indentation
else:
    print("Non-positive")

Incorrect indentation results in IndentationError.

Keywords are reserved words with special meaning: if, else, for, while, def, class, import, etc.

Identifiers are names for variables, functions, classes, etc. Rules:

• Start with letter or underscore
• Contain letters, digits, underscores
• Case-sensitive
• Cannot be keywords

# Valid identifiers
variable_name = 10
_private_var = 20
ClassName = "example"

# Invalid identifiers
# 123variable = 10  # Cannot start with digit
# class = "test"    # Cannot use keyword

Python has several built-in data types:

Numeric: int, float, complex
Sequence: str, list, tuple
Set: set, frozenset
Mapping: dict
Boolean: bool
None: NoneType

integer = 42
floating = 3.14
string = "Hello"
boolean = True
none_value = None

# List - mutable
my_list = [1, 2, 3]
my_list[0] = 10  # Works

# Tuple - immutable
my_tuple = (1, 2, 3)
# my_tuple[0] = 10  # TypeError

Dictionary is a mutable, unordered collection of key-value pairs. Keys must be immutable and unique.

student = {
    "name": "John",
    "age": 25,
    "grades": [85, 90, 78]
}

# Accessing values
print(student["name"])  # John
print(student.get("age"))  # 25

# Adding/updating
student["email"] = "john@email.com"