Using and Abusing Magic methods in Python

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

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

YOU CAN’T CONTROL
ACCESS TO ATTRIBUTES IN
PYTHON

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

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

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

PART 2: (AB)USING
MAGIC METHODS

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

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

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

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()

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?

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

>>> 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

MAGIC METHODS ARE
EXTREMELY POWERFUL, BUT
REMEMBER:

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

Using and Abusing Magic methods in Python

More Related Content

What's hot

Similar to Using and Abusing Magic methods in Python

Recently uploaded

Using and Abusing Magic methods in Python