The document discusses Python magic methods, which are special methods that start and end with double underscores. Magic methods are not called directly but are invoked when built-in functions or operators are used. Some common magic methods include __init__, __str__, __len__, __add__, and __getitem__. The document provides examples of implementing magic methods like __init__, __str__, and operator overloading methods like __add__ to make custom classes behave like built-in types. It also discusses differences in magic methods between Python 2 and 3 and advises only implementing magic methods when needed and with documentation.

1. Magic Methods
Python meetup
Ines Jelovac
Zagreb, 21 February 2017

2. Why magic methods?

3. What are magic methods?
● Methods with double underscores at the beginning and the
end
○ __init__
○ __str__
● Where is magic?
○ Magic methods are not invoked directly!

4. How magic methods are invoked?
● Calling Python’s built in functions
len([1, 2, 3])
● Using operators
1 + 1
5 > 3
● Using for loop
for x in [1, 2, 3]:
print(x)

5. Which methods are invoked?
● Calling Python’s built in functions
len([1, 2, 3]) # __len__()
● Using operators
1 + 1 # __add__()
5 > 3 # __gt__()
● Using for loop
for x in [1, 2, 3]: # __iter__()
print(x) # __next__()

6. Example: __len__
x = [1, 2, 3, 4, 5]
len(x)
>> 5
Magic method:
x.__len__()

7. Example: __str__
from datetime import datetime
x = datetime.now()
print(x)
>> 2017-02-21 18:15:02.161633
Magic method:
x.__str__()

8. Example: __getitem__
x = {
‘A’: 1,
‘B’: 2
}
x[‘A’]
>> 1
Magic method:
x.__getitem__(key) # key=’A’

9. Example: __setitem__
x = {1: ‘a’, 2: ‘3’}
x[3] = ‘c’ # x.__setitem__(self, name, value)
Another magic method:
3.__hash__()
● Must be implemented if instance is
used in hashable collections
● Returns integer, immutable
for instance
● Works with __cmp__ or __eq__

10. When to implement?
● to control string representation of an object
○ __repr__
○ __str__
● to initialize an instance
○ __init__
● when writing libraries, frameworks
○ to make objects behave like built-in types
○ Django example:
■ model_instance_a == model_instance_b

11. Implementing: __init__
● “Constructor”
○ __new__
○ __init__
● Example
class ComplexNumber:
def __init__(self, x, y):
self.x = x # x = Re
self.y = y # y = Im
z = ComplexNumber(3, 5)

12. Implementing: __str__
● string representation of an object
○ str(x)
○ print(x)
● Example
class ComplexNumber:
...
def __str__(self):
return ‘{} + {}i’.format(self.x, self.y)

13. Operator overloading: +
complex_number_a = ComplexNumber(3, 5)
complex_number_b = ComplexNumber(2, 1)
complex_number_a + complex_number_b
>> TypeError
● Magic method
○ __add__
○ complex_number_a.__add__(complex_number_b)

14. Operator overloading: +
● Magic method
○ __add__
● Example
class ComplexNumber:
...
def __add__(self, other):
real = self.x + other.x
imag = self.y + other.y
return ComplexNumber(real, imag)

15. Operator overloading: +
complex_number_a = ComplexNumber(3, 5)
complex_number_b = ComplexNumber(2, 1)
complex_number_a + complex_number_b
>> 5 + 6i
complex_number_a + 4
>> AttributeError
'int' object has no attribute 'x'

16. Operator overloading: +
● Magic method
○ __add__
● Example
class ComplexNumber:
…
def __add__(self, other):
if isinstance(other, ComplexNumber):
real = self.x + other.x
imag = self.y + other.y
return ComplexNumber(real, imag)
else:
return ComplexNumber(self.x + other, self.y)

17. Operator overloading: +
complex_number_a = ComplexNumber(3, 5)
complex_number_a + 4
>> 7 + 5i
4 + complex_number_a
>> TypeError
unsupported operand type(s) for +: 'int' and 'ComplexNumber'

18. Operator overloading: +
● Magic method
○ __radd__
● Example
class ComplexNumber:
...
def __radd__(self, other):
return self + other

19. Operator overloading: +
complex_number_a = ComplexNumber(3, 5)
4 + complex_number_a
>> 7 + 5i
complex_number_a += 4
>> 7 + 5i

20. Implementing: __all__
● example : django/db/__init__.py
__all__ = ['backend', 'connection', 'connections', ...]
● controls * imports
from django.db import *

21. When not to implement?
class ComplexNumber():
...
def __del__(self):
self.connection.close() # :(
...
del counter # ?
Magic method:
__del__ is called by garbage collector!

22. Python 2 and Python 3
● String representation
○ Python 2: __str__ and __unicode__
○ Python 3: __str__
● Boolean
○ Python 2: __nonzero__
○ Python 3: __bool__
● Comparison
○ Python 2: __cmp__
○ Python 3: uses other magic methods

23. Conclusion
● Be aware when magic methods are invoked
○ Some actions invoke more than just one magic method
● Implement them with caution
○ Make sure to understand purpose of
magic method
○ Write documentation