Magic MethodsPython meetup

Ines JelovacZagreb, 21 February 2017

Why magic methods?

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!

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)

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__()

Example: __len__

x = [1, 2, 3, 4, 5]

len(x)

>> 5

Magic method:

x.__len__()

Example: __str__

from datetime import datetime

x = datetime.now()

print(x)

>> 2017-02-21 18:15:02.161633

Magic method:

x.__str__()

Example: __getitem__

x = {‘A’: 1,‘B’: 2

}

x[‘A’]

>> 1

Magic method:

x.__getitem__(key) # key=’A’

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__

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

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)

Implementing: __str__

● string representation of an object○ str(x)○ print(x)

● Example

class ComplexNumber:...

def __str__(self):

return ‘{} + {}i’.format(self.x, self.y)

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)

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)

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'

Operator overloading: +

● Magic method○ __add__

● Example

class ComplexNumber:…

def __add__(self, other):

if isinstance(other, ComplexNumber):real = self.x + other.ximag = self.y + other.yreturn ComplexNumber(real, imag)

else:return ComplexNumber(self.x + other, self.y)

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'

Operator overloading: +

● Magic method○ __radd__

● Example

class ComplexNumber:...

def __radd__(self, other):

return self + other

Operator overloading: +

complex_number_a = ComplexNumber(3, 5)

4 + complex_number_a

>> 7 + 5i

complex_number_a += 4

>> 7 + 5i

Implementing: __all__

● example : django/db/__init__.py

__all__ = ['backend', 'connection', 'connections', ...]

● controls * imports

from django.db import *

When not to implement?

class ComplexNumber():

...

def __del__(self):

self.connection.close() # :(

...

del counter # ?

Magic method:

__del__ is called by garbage collector!

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

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