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Introduction to Decorators
Power Up Your Python Code
Geir Arne Hjelle -- `geirarne@gmail.com`
Pycon US May 12, 2021

Exercise 1

Write a decorator that prints BEFORE before calling the decorated function and AFTER afterwards.

>>> @before_and_after
... def greet(name):
...     print(f"Hi {name}!")
...
>>> greet("PyCon")
BEFORE
Hi PyCon!
AFTER

Post your code to https://forms.gle/bW87NFstp24G6gSaA

Exercise 2

Write a decorator that runs the decorated function twice and returns a 2-tuple with both return values.

>>> import random
>>> @do_twice
... def roll_dice():
...     return random.randint(1, 6)
...
>>> roll_dice()
(3, 1)

Exercise 3

Write a decorator that stores references to decorated functions in a dictionary.

>>> FUNCTIONS = {}
>>> @register
... def roll_dice():
...     return random.randint(1, 6)
...
>>> FUNCTIONS
{'roll_dice': <function __main__.roll_dice()>}
>>> FUNCTIONS["roll_dice"]()
2

Exercise 4

Write a decorator that repeatedly calls the decorated function as long as it raises an exception.

>>> @retry
... def only_roll_sixes():
...     number = random.randint(1, 6)
...     if number != 6:
...         raise ValueError(number)
...     return number
...
>>> only_roll_sixes()
6

Exercise 5 (hard)

Rewrite @retry so that it only retries on specific, user-defined exceptions.

>>> @retry(ValueError)
... def calculation():
...     number = random.randint(-5, 5)
...     if abs(1 / number) > 0.2:
...         raise ValueError(number)
...     return number
...
>>> calculation()
# -5, 5, or ZeroDivisionError

Exercise 6

Adapt @retry so that it only tries a maximum of max_retries times.

>>> @retry(max_retries=3)
... def only_roll_sixes():
...     number = random.randint(1, 6)
...     if number != 6:
...         raise ValueError(number)
...     return number
...
>>> only_roll_sixes()
# 6 or ValueError

. . .

Challenge: Can you make @retry count all retries across several function calls?

Exercise 7

Write a class-based @Retry decorator that keeps track of the number of retries across all function calls.

>>> @Retry
... def only_roll_sixes():
...     # Same as before
...
>>> only_roll_sixes()
Retry attempt 1
Retry attempt 2
6
>>> only_roll_sixes()
Retry attempt 3
6

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