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Chapter 9: Decorators and Closures — Notes

1. When do decorators execute?

Decorators execute at import time (when the module is loaded). This is why registration decorators (like @app.route in Flask or @singledispatch) work instantly when the script starts. The decorated function, however, only runs when explicitly called at runtime.

2. Variable Scope and UnboundLocalError

When Python compiles a function, it decides variable scope:

  1. If a variable is assigned a value inside the function, it is deemed local.
  2. If it is only read, it is deemed a global or free variable.

If you try to read a local variable before it is assigned, you get an UnboundLocalError.

b = 6
def f(a):
    print(a)
    print(b)  # UnboundLocalError!
    b = 9     # Because of this assignment, Python classifies `b` as local.

3. Closures

A closure is a function with an extended scope that encompasses free variables (variables that are neither global nor local, but bound in an outer enclosing function).

  • If you only read a free variable in a closure, you don't need special syntax.
  • If you need to re-assign a free variable, you must use the nonlocal keyword to prevent Python from shadowing it as a local variable.

4. functools.wraps

When you write a decorator that returns an inner wrapper function, the wrapper takes on the name and docstring of the inner function (e.g., __name__ becomes 'wrapper'). Always use @functools.wraps(func) on the inner function to copy __name__, __doc__, and signature metadata from the original function.

5. Standard Library Highlights

  • @functools.lru_cache(maxsize=128): Memoizes the results of a function. Requires arguments to be hashable.
  • @functools.singledispatch: Transforms a normal function into a single-dispatch generic function, allowing you to overload the function based on the type of its first argument.

6. Parameterized Decorators

To pass arguments to a decorator (e.g., @retry(max_attempts=3)), you need a decorator factory. This requires 3 layers of nested functions:

  1. The Factory (takes the parameters, returns the Decorator)
  2. The Decorator (takes the function, returns the Wrapper)
  3. The Wrapper (takes the args/kwargs, executes the function)