A short presentation by Slater Victoroff on a couple of ways to explore the secret frayed edges of the python language.

1. USING (AND ABUSING)
PYTHON'S MAGIC METHODS
TO REDUCE GOO CODE

2. WHO AM I?
• Work at indico (small local
company)
• Previously at Olin College,
Pearson, edX, fetchnotes,
and freelance work
• Semi-retired SO & quora user
• Love playing with the dirty
little secrets of python

3. PART 1: USING MAGIC
METHODS

4. YOU CAN’T CONTROL
ACCESS TO ATTRIBUTES IN
PYTHON

5. class MutableObject(object):
def __init__(self, a):
self.a = a
>>> test = MutableObject("one")
>>> test.a = "two"
NORMAL PYTHON ATTRIBUTES

6. class MutableObject(object):
def __init__(self, a):
self._a = a
>>> test = MutableObject("one")
>>> test._a = "two" # Feels Wrong
TELLING OTHER USERS NOT TO TOUCH YOUR
ATTRIBUTES

7. class StubbornObject(object):
def __setattr__(self, key, value):
if hasattr(self, key):
raise ValueError(“Already set in my ways")
else:
object.__setattr__(self, key, value)
>>> test = StubbornObject()
>>> test.a = "one"
>>> test.a = “two” # Now actually errors
ACTUALLY STOPPING PEOPLE FROM TOUCHING
YOUR ATTRIBUTES

8. PART 2: (AB)USING
MAGIC METHODS

9. results = my_object.attribute
if not isinstance(results, list):
results = list(results)
# Do Something
EVER WRITTEN CODE LIKE THIS?

10. class ChangelingObject(object):
list_fields = {"a", "b", "c"}
def __setattr__(self, key, value):
if key in self.list_fields:
value = [value]
object.__setattr__(self, key, value)
>>> test = ChangelingObject()
>>> test.a = 1
>>> print test.a
[1]
NEVER AGAIN WITH SPECIAL PYTHON MAGIC

11. PART 3: ABUSING MAGIC
METHODS

12. def add_money(user_id, amount):
session = Session()
user_object =
session.query(User).filter_by(id=user_id).first()
current_value = user_object.balance
try:
user_object.balance = current_value + amount
session.commit()
except ORMException:
session.rollback

13. class User(object):
# Standard ORM stuff
def __iadd__(self, other):
current = user_object.balance
try:
user_object.balance = current + other
session.commit()
except ORMException:
session.rollback()

14. def add_money(user_id, amount):
session = Session()
user_object =
session.query(User).filter_by(id=user_id).first()
user_object += amount
SHORT CODE, BUT AT WHAT COST?

15. PART 4: HERE THERE BE
DRAGONS

16. class Food(object):
recipes = {
frozenset({"flour", "water"}): "dough",
frozenset({"dough", "yeast"}): "bread",
frozenset({"avocado", "onion"}): "guac",
frozenset({"guac", "bread"}): "tasty snack"
}
def __init__(self, ingredient):
self.name = ingredient
def _mix(self, second):
current_pantry = frozenset({self.name, second.name})
try:
return self.recipes[current_pantry]
except KeyError:
raise ValueError("%s and %s don't mix, trust me" % (self.name,
second.name))
def __add__(self, other):
return Food(self._mix(other))
def __iadd__(self, other):
self.name = self._mix(other)
def __repr__(self):
return self.name

17. >>> step_one = Food("flour") + Food("water") +
Food("yeast")
>>> step_two = Food("avocado") + Food("onion")
>>> print step_one + step_two
tasty snack
PLEASE NEVER ACTUALLY DO THIS

18. MAGIC METHODS ARE
EXTREMELY POWERFUL, BUT
REMEMBER:

19. ALWAYS CODE AS IF THE
PERSON WHO ENDS UP
MAINTAINING YOUR CODE IS
A VIOLENT PSYCHOPATH WHO
KNOWS WHERE YOU LIVE.
— Jeff Atwood