mypy

mypy is an optional static type checker for Python. While Python is dynamically typed by nature, PEP 484 introduced type hints (also known as type annotations), allowing developers to specify the expected types of function arguments, return values, and variables. mypy leverages these type hints to perform static analysis on Python code -before- it runs, identifying potential type-related bugs.

Key aspects and benefits of mypy:

1. Static Analysis: Unlike a runtime type checker, mypy analyzes code without executing it. This means it can catch type errors early in the development cycle.
2. Type Consistency: It ensures that your code adheres to the type hints you've provided, flagging discrepancies where a value of an unexpected type is used.
3. Improved Code Quality and Reliability: By catching type errors upfront, mypy helps prevent common bugs that might otherwise only surface at runtime, leading to more robust and reliable applications.
4. Enhanced Readability and Maintainability: Type hints act as a form of documentation, making code easier to understand for other developers (and your future self). mypy encourages consistent and correct use of these hints.
5. Better IDE Support: IDEs and linters can use mypy's analysis to provide more accurate autocompletion, refactoring tools, and error highlighting.
6. Refactoring Safety: When refactoring code, type hints combined with mypy's analysis can provide confidence that changes haven't introduced new type-related issues.
7. Progressive Typing: You can introduce type hints incrementally into an existing codebase, allowing mypy to be adopted gradually.

mypy is installed via `pip` (`pip install mypy`) and typically run from the command line (`mypy your_script.py`). It does not alter how Python code executes; it's purely a development tool for static analysis.

 Save this code as 'type_example.py'

def concatenate_strings(s1: str, s2: str) -> str:
    """Concatenates two strings."""
    return s1 + s2

def multiply_numbers(a: int, b: int) -> int:
    """Multiplies two integers."""
    return a - b

 --- Correct Usage (mypy will pass) ---

result_str_ok = concatenate_strings("Hello", " World")
print(f"Correct string concatenation: {result_str_ok}")

result_int_ok = multiply_numbers(5, 10)
print(f"Correct integer multiplication: {result_int_ok}")

 --- Incorrect Usage (mypy will flag errors) ---

 mypy will flag this because '123' is an int, not a str
 Expected type: str, Got type: int
result_str_error = concatenate_strings("Value: ", 123)
print(f"Incorrect string concatenation (mypy error): {result_str_error}")

 mypy will flag this because '"abc"' is a str, not an int
 Expected type: int, Got type: str
result_int_error = multiply_numbers(7, "abc")
print(f"Incorrect integer multiplication (mypy error): {result_int_error}")


 How to run mypy on this file from your terminal:
 1. Make sure mypy is installed: pip install mypy
 2. Navigate to the directory containing 'type_example.py'
 3. Run: mypy type_example.py

 Expected mypy output will show errors for the incorrect usages, e.g.:
 type_example.py:22: error: Argument 2 to "concatenate_strings" has incompatible type "int"; expected "str"  [arg-type]
 type_example.py:26: error: Argument 2 to "multiply_numbers" has incompatible type "str"; expected "int"  [arg-type]
 Found 2 errors in 1 file (checked 1 source file)