Python Data Types

Python Data Types: Built-in Taxonomy & Mutability

Everything in Python is an object. Python's built-in data types are categorized by whether their internal state can be altered in place (mutable) or requires creating a new object upon modification (immutable).

Built-in Type Hierarchy & Mutability

flowchart TD
    A["Python Data Types"] --> B["Immutable Types"]
    A --> C["Mutable Types"]
    B --> D["Numeric: int, float, complex"]
    B --> E["Sequence: str, tuple, bytes"]
    B --> F["Other: bool, frozenSet"]
    C --> G["Sequence: list, bytearray"]
    C --> H["Mapping: dict"]
    C --> I["Set: set"]

Type Mutability Overview

Practical Code Example

from typing import Any, Dict, List, Tuple

def inspect_data_types() -> None:
    # Explicit type assignments
    user_id: int = 1001
    user_score: float = 98.5
    user_name: str = "Alice Developer"
    active_status: bool = True
    tags: List[str] = ["python", "backend", "fastapi"]
    location_coords: Tuple[float, float] = (37.7749, -122.4194)
    profile: Dict[str, Any] = {"role": "Engineer", "level": 3}

    print(f"user_id: {type(user_id).__name__} = {user_id}")
    print(f"user_score: {type(user_score).__name__} = {user_score}")
    print(f"tags: {type(tags).__name__} = {tags}")
    print(f"coords: {type(location_coords).__name__} = {location_coords}")

    # Inspecting Mutability via id()
    print("
--- Mutability Check ---")
    original_str = "hello"
    str_id_before = id(original_str)
    original_str += " world"
    print(f"String modification changes ID: {str_id_before != id(original_str)}")

    original_list = [1, 2]
    list_id_before = id(original_list)
    original_list.append(3)
    print(f"List modification retains ID: {list_id_before == id(original_list)}")

if __name__ == "__main__":
    inspect_data_types()

Best Practices & Gotchas

• Never Use Mutable Defaults in Functions: Avoid def func(data=[]) because default mutable arguments persist across function calls; use data: list | None = None instead.
• Use isinstance() for Type Checks: Always check types using isinstance(obj, TargetType) rather than comparing type(obj) == TargetType.
• Use tuple for Heterogeneous Fixed Records: Tuples consume less memory than lists and guarantee immutability.

Self-Check Challenge

Write a function analyze_container(container: Any) -> None that prints its data type, length (if applicable), and tests if mutating an element changes its memory id().