June Bangpypers meetup - Magic Methods - Python

comparison.py

# comparison

from functools import total_ordering


@total_ordering
class Person(object):

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __eq__(self, other):
        return self.name == other.name and self.age == other.age

    def __gt__(self, other):
        return self.age > other.age

    # def __lt__(self, other):
    #     return

init.py

# __init__


class Person(object):
    def __init__(self, name):
        self.name = name

new.py

# __new__


class Person(object):

    def __new__(cls, *args, **kwargs):
        print "args:", args, "kwargs:", kwargs
        inst = object.__new__(cls, *args, **kwargs)
        print "instance:", inst, "class:", cls
        return inst

    def __init__(self, name):
        print "in init"
        self.name = name

representations.py

# representations


class Person(object):

    def __init__(self, name):
        self.name = name

    def __str__(self):
        print "str"

        return 'Person: ' + self.name

    def __repr__(self):
        print "repr"
        return 'Person("' + self.name + '")'

    def __unicode__(self):
        return u'Person utf: ' + self.name

sequences.py

# sequences

from collections import defaultdict


class ContactBook(object):
    version = '0.1'

    def __init__(self):
        self.objects = []

    def add(self, contact):
        self.objects.append(contact)

    def all(self):
        return self.objects

    def index(self):
        d = defaultdict(list)
        for contact in self.all():
            d[contact[0].lower()].append(contact)
        return d

    def __iter__(self):
        for item in self.all():
            yield item

    def __getitem__(self, key):
        if isinstance(key, int):
            return self.all()[key]
        else:
            return self.index()[key]

    def __setitem__(self, key, value):
        pass

    def __len__(self):
        return len(self.all())

    def __enter__(self):
        return self

    def __exit__(self, )

slides.md

Magic Methods - Python

Vignesh Sarma

http://github/vigneshsarma

---

What are magic methods?

Reference: http://www.rafekettler.com/magicmethods.html

Add magic to your classes

Surrounded by __

---

Object Initialization

__init__

!python
class Person(object):
    def __init__(self, name):
        self.name = name

• Only initializes
• does not create
• similar to constructor in other languages
• but who created the object?

---

Object creation

__new__

!python
class Person(object):

    def __new__(cls, *args, **kwargs):
        print "args:", args, "kwargs:", kwargs
        inst = object.__new__(cls, *args, **kwargs)
        print "instance:", inst, "class:", cls
        return inst

    def __init__(self, name):
        print "in init"
        self.name = name

In [0]: Person(name="hi")

---

__new__

• static method
• has to return new object
• need to call the super objects new method

---

Representations

!python
class Person(object):

    def __init__(self, name):
        self.name = name

    def __str__(self):
        return 'Person: ' + self.name

    def __repr__(self):
        return 'Person("' + self.name + '")'

• str for humans.
• repr for programs.

---

Operators

!python
>>> 1 + 2
>>> 1 == 1
>>> 1 > 2
>>> # wont work
>>> custom_object  == another_object

---

Comparison

• __cmp__(self, other) for all comparisons.
• __eq__(self, other) equality operator, ==.
• __ne__(self, other) inequality operator, !=.
• __lt__(self, other) less-than operator, <.
• __gt__(self, other) greater-than operator, >.
• __le__(self, other) less-than-or-equal-to operator, <=.
• __ge__(self, other) greater-than-or-equal-to operator, >=.

---

Arithmetic Operators

• override operators like +, - etc for your object.

---

Sequences

!python
class ContactBook(object):
    version = '0.1'

    def __init__(self):
        self.objects = []

    def add(self, contact):
        self.objects.append(contact)

    def all(self):
        return self.objects

    def index(self):
        d = defaultdict(list)
        for contact in self.all():
            d[contact[0].lower()].append(contact)
        return d

---

Context Managers

!python
with open('foo.txt') as bar:
    print bar.read()

Equvalent to

!python
bar = open(foo.txt)
try:
    print bar.read()
finally:
    bar.close()

• enter(self)
• exit(self, exception_type, exception_value, traceback)

---

END