Introduction to Python Type Annotations

Brief History

• 1990: Guido van Rossum created Python (dynamically typed language)
• 2000: Initial mention of type hinting
• 2014: PEP 484 introduced type annotations
• Today: Type annotations in Python 3.6+
  • Python remains dynamically typed

Update on Current State

• Type annotations don’t change Python’s behavior
• Serve as a form of structured documentation

Examples and Syntax

• Basic Type Annotations: V1: int = hello gives an error with type checker (variable type mismatch)
• Class Annotations: class Foo: var1: int Foo.var1 doesn’t influence runtime, stored in __annotations__
• Function Annotations:

 def bar(values: List[int]) -> int:
  return sum(values)

• Inheritance with Annotations:

 class Animal:
   pass

 class Dog(Animal):
   pass

Type Syntax Notes

• Union: var: Union[int, float, str] or var: int | float | str
• Custom Types:

 MyNumber = Union[int, float]
 def func(value: MyNumber) -> MyNumber:
   return value

• Generics: Lists and Dictionaries

 from typing import List, Dict
 List[int]
 Dict[str, int]

• Any: value: Any
• Tuples:

 Tuple[int, str]
 Tuple[int, ...]  # Infinite length
 Tuple[Union[int, str], ...]

Protocols and TypeVars

• Protocols: Interfaces specifying methods without implementation

from typing import Protocol
class EmailSender(Protocol):
    def send_email(self, to: str, subject: str, body: str) -> None:
        ...

• TypeVars: For generic types with constraints

 from typing import TypeVar
 T = TypeVar('T', str, int)

Tools and Best Practices

• Type Checkers: mypy (most mature), pytype, Pylance, pyright
• Install and Run mypy:

 pip install mypy
 mypy <your_code>

• Configuration:
  • Use mypy.ini, mypy.conf, or setup.cfg for project settings
• Stub Files: For third-party packages without annotations (use .pyi files)

Benefits of Type Annotations

• Improves code documentation and readability
• Enhances IDE features and auto-completion
• Helps catch bugs early and ensures code reliability

Conclusion

• Recommendation: Start using a type checker in your codebase
• Contact: [Contact details provided]

Example Additional Terms

• Type Hinting: Often used interchangeably with type annotations
• Type Comments: For backwards compatibility with older Python versions