Decorators Basics

Basic Decorator

  1. # simple decorator
  2. def my_decorator(func):
  3.     def wrapper():
  4.         print("Before")
  5.         func()
  6.         print("After")
  7.     return wrapper
  8. @my_decorator
  9. def say_hello():
  10.     print("Hello!")
  11. say_hello()
  12. #ans: Before
  13. #ans: Hello!
  14. #ans: After

Decorator with Arguments

  1. # decorator for functions with args
  2. def my_decorator(func):
  3.     def wrapper(*args, **kwargs):
  4.         print("Before")
  5.         result = func(*args, **kwargs)
  6.         print("After")
  7.         return result
  8.     return wrapper
  9. @my_decorator
  10. def add(a, b):
  11.     return a + b
  12. add(5, 3)
  13. #ans: Before, After, returns 8

Manual Decoration

  1. # without @ syntax
  2. def greet():
  3.     print("Hello")
  4. greet = my_decorator(greet)
  5. #ans: equivalent to @my_decorator

Preserving Metadata

  1. from functools import wraps
  2. # preserve function info
  3. def my_decorator(func):
  4.     @wraps(func)
  5.     def wrapper(*args, **kwargs):
  6.         return func(*args, **kwargs)
  7.     return wrapper
  8. @my_decorator
  9. def my_func():
  10.     """Docstring"""
  11.     pass
  12. my_func.__name__
  13. #ans: "my_func" (not "wrapper")

Exercises - Part 1

  1. # what is returned?
  2. def decorator(func):
  3.     def wrapper():
  4.         return func()
  5.     return wrapper
  6. @decorator
  7. def func():
  8.     return 5
  9. func()
  10. #ans: 5

Exercises - Part 2

  1. # decorator execution?
  2. def decorator(func):
  3.     print("Decorating")
  4.     return func
  5. @decorator
  6. def func():
  7.     pass
  8. #ans: "Decorating" (at definition time)

Exercises - Part 3

  1. # nested decorators?
  2. def dec1(func):
  3.     def wrapper():
  4.         print("1")
  5.         return func()
  6.     return wrapper
  7. def dec2(func):
  8.     def wrapper():
  9.         print("2")
  10.         return func()
  11.     return wrapper
  12. @dec1
  13. @dec2
  14. def func():
  15.     pass
  16. func()
  17. #ans: 1, 2 (bottom to top)

Exercises - Part 4

  1. # decorator without wrapper?
  2. def decorator(func):
  3.     return func
  4. @decorator
  5. def func():
  6.     pass
  7. #ans: valid (identity decorator)

Exercises - Part 5

  1. # accessing original?
  2. def decorator(func):
  3.     def wrapper():
  4.         return func()
  5.     wrapper.original = func
  6.     return wrapper
  7. @decorator
  8. def func():
  9.     pass
  10. func.original

Exercises - Part 6

  1. # decorator modifying return?
  2. def double(func):
  3.     def wrapper(*args):
  4.         return func(*args) * 2
  5.     return wrapper
  6. @double
  7. def add(a, b):
  8.     return a + b
  9. add(2, 3)
  10. #ans: 10

Exercises - Part 7

  1. # decorator with closure?
  2. def make_bold(func):
  3.     def wrapper():
  4.         return f"<b>{func()}</b>"
  5.     return wrapper
  6. @make_bold
  7. def get_text():
  8.     return "Hello"
  9. get_text()
  10. #ans: "<b>Hello</b>"

Exercises - Part 8

  1. # class as decorator?
  2. class Decorator:
  3.     def __init__(self, func):
  4.         self.func = func
  5.     def __call__(self, *args):
  6.         return self.func(*args)
  7. @Decorator
  8. def func():
  9.     pass

Exercises - Part 9

  1. # without @wraps?
  2. def decorator(func):
  3.     def wrapper():
  4.         return func()
  5.     return wrapper
  6. @decorator
  7. def func():
  8.     pass
  9. func.__name__
  10. #ans: "wrapper"

Exercises - Part 10

  1. # decorator return None?
  2. def decorator(func):
  3.     def wrapper():
  4.         func()
  5.     return wrapper
  6. @decorator
  7. def func():
  8.     return 5
  9. func()
  10. #ans: None (wrapper doesn't return)

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