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Classes and Objects

Table of Contents

List Attributes

>>> dir(list)  # check all attr of list
['__add__', '__class__', ...]

Get Instance Type

>>> ex = 10
>>> isinstance(ex, int)
True

Declare a Class

>>> def fib(self, n):
...     if n <= 2:
...         return 1
...     return fib(self, n-1) + fib(self, n-2)
...
>>> Fib = type('Fib', (object,), {'val': 10,
...                               'fib': fib})
>>> f = Fib()
>>> f.val
10
>>> f.fib(f.val)
55

Equals to

>>> class Fib(object):
...     val = 10
...     def fib(self, n):
...         if n <=2:
...             return 1
...         return self.fib(n-1)+self.fib(n-2)
...
>>> f = Fib()
>>> f.val
10
>>> f.fib(f.val)
55

Has / Get / Set Attributes

>>> class Example(object):
...   def __init__(self):
...     self.name = "ex"
...   def printex(self):
...     print("This is an example")
...
>>> ex = Example()
>>> hasattr(ex,"name")
True
>>> hasattr(ex,"printex")
True
>>> hasattr(ex,"print")
False
>>> getattr(ex,'name')
'ex'
>>> setattr(ex,'name','example')
>>> ex.name
'example'

Check Inheritance

>>> class Example(object):
...   def __init__(self):
...     self.name = "ex"
...   def printex(self):
...     print("This is an Example")
...
>>> issubclass(Example, object)
True

Get Class Name

>>> class ExampleClass(object):
...   pass
...
>>> ex = ExampleClass()
>>> ex.__class__.__name__
'ExampleClass'

New and Init

__init__ will be invoked

>>> class ClassA(object):
...     def __new__(cls, arg):
...         print('__new__ ' + arg)
...         return object.__new__(cls, arg)
...     def __init__(self, arg):
...         print('__init__ ' + arg)
...
>>> o = ClassA("Hello")
__new__ Hello
__init__ Hello

__init__ won't be invoked

>>> class ClassB(object):
...     def __new__(cls, arg):
...         print('__new__ ' + arg)
...         return object
...     def __init__(self, arg):
...         print('__init__ ' + arg)
...
>>> o = ClassB("Hello")
__new__ Hello

The Diamond Problem

The problem of multiple inheritance in searching a method

>>> def foo_a(self):
...     print("This is ClsA")
...
>>> def foo_b(self):
...     print("This is ClsB")
...
>>> def foo_c(self):
...     print("This is ClsC")
...
>>> class Type(type):
...     def __repr__(cls):
...         return cls.__name__
...
>>> ClsA = Type("ClsA", (object,), {'foo': foo_a})
>>> ClsB = Type("ClsB", (ClsA,), {'foo': foo_b})
>>> ClsC = Type("ClsC", (ClsA,), {'foo': foo_c})
>>> ClsD = Type("ClsD", (ClsB, ClsC), {})
>>> ClsD.mro()
[ClsD, ClsB, ClsC, ClsA, <type 'object'>]
>>> ClsD().foo()
This is ClsB

Representation of a Class

>>> class Example(object):
...    def __str__(self):
...       return "Example __str__"
...    def __repr__(self):
...       return "Example __repr__"
...
>>> print(str(Example()))
Example __str__
>>> Example()
Example __repr__

Callable Object

>>> class CallableObject(object):
...   def example(self, *args, **kwargs):
...     print("I am callable!")
...   def __call__(self, *args, **kwargs):
...     self.example(*args, **kwargs)
...
>>> ex = CallableObject()
>>> ex()
I am callable!

Context Manager

# replace try: ... finally: ...
# see: PEP343
# common use in open and close

import socket

class Socket(object):
    def __init__(self, host, port):
        self.host = host
        self.port = port

    def __enter__(self):
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.bind((self.host,self.port))
        sock.listen(5)
        self.sock = sock
        return self.sock

    def __exit__(self,*exc_info):
        if exc_info[0] is not None:
            import traceback
            traceback.print_exception(*exc_info)
        self.sock.close()

if __name__=="__main__":
    host = 'localhost'
    port = 5566
    with Socket(host, port) as s:
        while True:
            conn, addr = s.accept()
            msg = conn.recv(1024)
            print(msg)
            conn.send(msg)
            conn.close()

Using contextlib

from contextlib import contextmanager

@contextmanager
def opening(filename, mode='r'):
   f = open(filename, mode)
   try:
      yield f
   finally:
      f.close()

with opening('example.txt') as fd:
   fd.read()

Property

>>> class Example(object):
...     def __init__(self, value):
...        self._val = value
...     @property
...     def val(self):
...         return self._val
...     @val.setter
...     def val(self, value):
...         if not isinstance(value, int):
...             raise TypeError("Expected int")
...         self._val = value
...     @val.deleter
...     def val(self):
...         del self._val
...
>>> ex = Example(123)
>>> ex.val = "str"
Traceback (most recent call last):
  File "", line 1, in
  File "test.py", line 12, in val
    raise TypeError("Expected int")
TypeError: Expected int

Equals to

>>> class Example(object):
...     def __init__(self, value):
...        self._val = value
...
...     def _val_getter(self):
...         return self._val
...
...     def _val_setter(self, value):
...         if not isinstance(value, int):
...             raise TypeError("Expected int")
...         self._val = value
...
...     def _val_deleter(self):
...         del self._val
...
...     val = property(fget=_val_getter, fset=_val_setter, fdel=_val_deleter, doc=None)
...

Computed Attributes

@property computes a value of a attribute only when we need. Not store in memory previously.

>>> class Example(object):
...   @property
...   def square3(self):
...     return 2**3
...
>>> ex = Example()
>>> ex.square3
8

Descriptor

>>> class Integer(object):
...   def __init__(self, name):
...     self._name = name
...   def __get__(self, inst, cls):
...     if inst is None:
...       return self
...     else:
...       return inst.__dict__[self._name]
...   def __set__(self, inst, value):
...     if not isinstance(value, int):
...       raise TypeError("Expected int")
...     inst.__dict__[self._name] = value
...   def __delete__(self,inst):
...     del inst.__dict__[self._name]
...
>>> class Example(object):
...   x = Integer('x')
...   def __init__(self, val):
...     self.x = val
...
>>> ex1 = Example(1)
>>> ex1.x
1
>>> ex2 = Example("str")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
  File "<stdin>", line 11, in __set__
TypeError: Expected an int
>>> ex3 = Example(3)
>>> hasattr(ex3, 'x')
True
>>> del ex3.x
>>> hasattr(ex3, 'x')
False

Singleton Decorator

Singleton is a design pattern that restricts the creation of instances of a class so that it only creates one instance of the class that implements it.

#!/usr/bin/env python3
"""Singleton decorator class."""

class Singleton(object):

    def __init__(self, cls):
        self.__cls = cls
        self.__obj = None

    def __call__(self, *args, **kwargs):
        if not self.__obj:
            self.__obj = self.__cls(*args, **kwargs)
        return self.__obj

if __name__ == "__main__":
    # Testing ...

    @Singleton
    class Test(object):

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

    a = Test("Hello")
    b = Test("World")

    print("id(a):", id(a), "id(b):", id(b), "Diff:", id(a)-id(b))

Static and Class Methond

@classmethod is bound to a class. @staticmethod is similar to a python function but define in a class.

>>> class example(object):
...   @classmethod
...   def clsmethod(cls):
...     print("I am classmethod")
...   @staticmethod
...   def stmethod():
...     print("I am staticmethod")
...   def instmethod(self):
...     print("I am instancemethod")
...
>>> ex = example()
>>> ex.clsmethod()
I am classmethod
>>> ex.stmethod()
I am staticmethod
>>> ex.instmethod()
I am instancemethod
>>> example.clsmethod()
I am classmethod
>>> example.stmethod()
I am staticmethod
>>> example.instmethod()
Traceback (most recent call last):
  File "", line 1, in
TypeError: unbound method instmethod() ...

Abstract Method

abc is used to define methods but not implement

>>> from abc import ABCMeta, abstractmethod
>>> class base(object):
...   __metaclass__ = ABCMeta
...   @abstractmethod
...   def absmethod(self):
...     """ Abstract method """
...
>>> class example(base):
...   def absmethod(self):
...     print("abstract")
...
>>> ex = example()
>>> ex.absmethod()
abstract

Another common way is to raise NotImplementedError

>>> class base(object):
...   def absmethod(self):
...     raise NotImplementedError
...
>>> class example(base):
...   def absmethod(self):
...     print("abstract")
...
>>> ex = example()
>>> ex.absmethod()
abstract

Using slot to Save Memory

#!/usr/bin/env python3

import resource
import platform
import functools


def profile_mem(func):
    @functools.wraps(func)
    def wrapper(*a, **k):
        s = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
        ret = func(*a, **k)
        e = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss

        uname = platform.system()
        if uname == "Linux":
            print(f"mem usage: {e - s} kByte")
        elif uname == "Darwin":
            print(f"mem usage: {e - s} Byte")
        else:
            raise Exception("not support")
        return ret
    return wrapper


class S(object):
    __slots__ = ['attr1', 'attr2', 'attr3']

    def __init__(self):
        self.attr1 = "Foo"
        self.attr2 = "Bar"
        self.attr3 = "Baz"


class D(object):

    def __init__(self):
        self.attr1 = "Foo"
        self.attr2 = "Bar"
        self.attr3 = "Baz"


@profile_mem
def alloc(cls):
    _ = [cls() for _ in range(1000000)]


alloc(S)
alloc(D)

output:

$ python3.6 s.py
mem usage: 70922240 Byte
mem usage: 100659200 Byte

Common Magic

# see python document: data model
# For command class
__main__
__name__
__file__
__module__
__all__
__dict__
__class__
__doc__
__init__(self, [...)
__str__(self)
__repr__(self)
__del__(self)

# For Descriptor
__get__(self, instance, owner)
__set__(self, instance, value)
__delete__(self, instance)

# For Context Manager
__enter__(self)
__exit__(self, exc_ty, exc_val, tb)

# Emulating container types
__len__(self)
__getitem__(self, key)
__setitem__(self, key, value)
__delitem__(self, key)
__iter__(self)
__contains__(self, value)

# Controlling Attribute Access
__getattr__(self, name)
__setattr__(self, name, value)
__delattr__(self, name)
__getattribute__(self, name)

# Callable object
__call__(self, [args...])

# Compare related
__cmp__(self, other)
__eq__(self, other)
__ne__(self, other)
__lt__(self, other)
__gt__(self, other)
__le__(self, other)
__ge__(self, other)

# arithmetical operation related
__add__(self, other)
__sub__(self, other)
__mul__(self, other)
__div__(self, other)
__mod__(self, other)
__and__(self, other)
__or__(self, other)
__xor__(self, other)