第二章、深入类和对象

2.1.鸭子类型和多态

“当看到一只鸟走起来像鸭子、游泳起来像鸭子、叫起来也像鸭子，那么这只鸟就可以被称为鸭子。”

 我们并不关心对象是什么类型，到底是不是鸭子，只关心行为。

实例一：

# 鸭子类型和多态简单实例

class Dog(object):
    def say(self):
        print('a dog')


class Cat(object):
    def say(self):
        print('a cat')

class Duck(object):
    def say(self):
        print('a duck')

animal_list = [Dog,Cat,Duck]

for animal in animal_list:
    animal().say()


#运行结果

a dog
a cat
a duck

 

 实例二：

类只要实现了__getitem__方法，它就是可迭代的,并不关心对象的本身，只关心行为，然后就可以当做extend的参数。

class Company(object):
    def __init__(self, employee_list):
        self.employee = employee_list

    def __getitem__(self, item):
        return self.employee[item]


company = Company(["11", "22", "33"])

a = ['derek1','derek2']

name_set = set()

name_set.add('tom1')
name_set.add(('tom2'))

#extend里面的参数介绍
#def extend(self, iterable):  # real signature unknown; restored from __doc__
    #""" L.extend(iterable) -> None -- extend list by appending elements from the iterable """


#extend里面可以添加任何可迭代的参数，给类添加一个魔法函数__getitem__,类就变成可迭代的，所以可以extend进去
a.extend(company)
print(a)       #['derek1', 'derek2', '11', '22', '33']

a.extend(name_set)
print(a)       #['derek1', 'derek2', '11', '22', '33', 'tom2', 'tom1']

2.2.抽象基类（abc模块）

抽象基类的作用类似于JAVA中的接口。在接口中定义各种方法，然后继承接口的各种类进行具体方法的实现。抽象基类就是定义各种方法而不做具体实现的类，任何继承自抽象基类的类必须实现这些方法，否则无法实例化

（1）判断类时候有某种属性

#判断类是否有某种属性

class Company(object):
    def __init__(self, employee_list):
        self.employee = employee_list

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

com = Company(["11", "22", "33"])

#hasattr判断类有没有某种属性，方法也是类的属性
print(hasattr(com,"__len__"))   #True

#虽然用hasattr可以判断，但正确的方式是定义一个抽象基类

#我们在某些情况下希望判定某个对象的类型，可以用抽象基类
from collections.abc import Sized
print(isinstance(com,Sized))    #True
# print(len(com))

class Sized(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __len__(self):
        return 0

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Sized:
            return _check_methods(C, "__len__")
        return NotImplemented

Sized源码

（2）abc模块

简单抽象基类实例

#模拟一个抽象基类

#写一个抽象基类，它的子类必须要重写抽象基类里面的方法
import abc
#定义一个抽象基类
class CacheBase(metaclass=abc.ABCMeta):
    @abc.abstractclassmethod
    def get(self,key):
        pass

    @abc.abstractclassmethod
    def set(self,key,value):
        pass

#子类,必须有抽象基类里面的方法，get和set
#假入不写set方法会报错

class RedisCache(CacheBase):
    def get(self,key):
        pass
    
    # def set(self,key,value):
    #     pass

redis_cache = RedisCache()

#模拟一个抽象基类

#写一个抽象基类，它的子类必须要重写抽象基类里面的方法
import abc
#定义一个抽象基类
class CacheBase(metaclass=abc.ABCMeta):
    @abc.abstractclassmethod
    def get(self,key):
        pass

    @abc.abstractclassmethod
    def set(self,key,value):
        pass

#子类,必须有抽象基类里面的方法，get和set
#假入不写set方法会报错

class RedisCache(CacheBase):
    def get(self,key):
        pass

    def set(self,key,value):
        pass

redis_cache = RedisCache()

正确

 

 

 

 （3）abc里面所有的抽象基类

from collections.abc import *

所有的抽象基类

# Copyright 2007 Google, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.

"""Abstract Base Classes (ABCs) for collections, according to PEP 3119.

Unit tests are in test_collections.
"""

from abc import ABCMeta, abstractmethod
import sys

__all__ = ["Awaitable", "Coroutine",
           "AsyncIterable", "AsyncIterator", "AsyncGenerator",
           "Hashable", "Iterable", "Iterator", "Generator", "Reversible",
           "Sized", "Container", "Callable", "Collection",
           "Set", "MutableSet",
           "Mapping", "MutableMapping",
           "MappingView", "KeysView", "ItemsView", "ValuesView",
           "Sequence", "MutableSequence",
           "ByteString",
           ]

# This module has been renamed from collections.abc to _collections_abc to
# speed up interpreter startup. Some of the types such as MutableMapping are
# required early but collections module imports a lot of other modules.
# See issue #19218
__name__ = "collections.abc"

# Private list of types that we want to register with the various ABCs
# so that they will pass tests like:
#       it = iter(somebytearray)
#       assert isinstance(it, Iterable)
# Note:  in other implementations, these types might not be distinct
# and they may have their own implementation specific types that
# are not included on this list.
bytes_iterator = type(iter(b''))
bytearray_iterator = type(iter(bytearray()))
#callable_iterator = ???
dict_keyiterator = type(iter({}.keys()))
dict_valueiterator = type(iter({}.values()))
dict_itemiterator = type(iter({}.items()))
list_iterator = type(iter([]))
list_reverseiterator = type(iter(reversed([])))
range_iterator = type(iter(range(0)))
longrange_iterator = type(iter(range(1 << 1000)))
set_iterator = type(iter(set()))
str_iterator = type(iter(""))
tuple_iterator = type(iter(()))
zip_iterator = type(iter(zip()))
## views ##
dict_keys = type({}.keys())
dict_values = type({}.values())
dict_items = type({}.items())
## misc ##
mappingproxy = type(type.__dict__)
generator = type((lambda: (yield))())
## coroutine ##
async def _coro(): pass
_coro = _coro()
coroutine = type(_coro)
_coro.close()  # Prevent ResourceWarning
del _coro
## asynchronous generator ##
async def _ag(): yield
_ag = _ag()
async_generator = type(_ag)
del _ag


### ONE-TRICK PONIES ###

def _check_methods(C, *methods):
    mro = C.__mro__
    for method in methods:
        for B in mro:
            if method in B.__dict__:
                if B.__dict__[method] is None:
                    return NotImplemented
                break
        else:
            return NotImplemented
    return True

class Hashable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __hash__(self):
        return 0

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Hashable:
            return _check_methods(C, "__hash__")
        return NotImplemented


class Awaitable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __await__(self):
        yield

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Awaitable:
            return _check_methods(C, "__await__")
        return NotImplemented


class Coroutine(Awaitable):

    __slots__ = ()

    @abstractmethod
    def send(self, value):
        """Send a value into the coroutine.
        Return next yielded value or raise StopIteration.
        """
        raise StopIteration

    @abstractmethod
    def throw(self, typ, val=None, tb=None):
        """Raise an exception in the coroutine.
        Return next yielded value or raise StopIteration.
        """
        if val is None:
            if tb is None:
                raise typ
            val = typ()
        if tb is not None:
            val = val.with_traceback(tb)
        raise val

    def close(self):
        """Raise GeneratorExit inside coroutine.
        """
        try:
            self.throw(GeneratorExit)
        except (GeneratorExit, StopIteration):
            pass
        else:
            raise RuntimeError("coroutine ignored GeneratorExit")

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Coroutine:
            return _check_methods(C, '__await__', 'send', 'throw', 'close')
        return NotImplemented


Coroutine.register(coroutine)


class AsyncIterable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __aiter__(self):
        return AsyncIterator()

    @classmethod
    def __subclasshook__(cls, C):
        if cls is AsyncIterable:
            return _check_methods(C, "__aiter__")
        return NotImplemented


class AsyncIterator(AsyncIterable):

    __slots__ = ()

    @abstractmethod
    async def __anext__(self):
        """Return the next item or raise StopAsyncIteration when exhausted."""
        raise StopAsyncIteration

    def __aiter__(self):
        return self

    @classmethod
    def __subclasshook__(cls, C):
        if cls is AsyncIterator:
            return _check_methods(C, "__anext__", "__aiter__")
        return NotImplemented


class AsyncGenerator(AsyncIterator):

    __slots__ = ()

    async def __anext__(self):
        """Return the next item from the asynchronous generator.
        When exhausted, raise StopAsyncIteration.
        """
        return await self.asend(None)

    @abstractmethod
    async def asend(self, value):
        """Send a value into the asynchronous generator.
        Return next yielded value or raise StopAsyncIteration.
        """
        raise StopAsyncIteration

    @abstractmethod
    async def athrow(self, typ, val=None, tb=None):
        """Raise an exception in the asynchronous generator.
        Return next yielded value or raise StopAsyncIteration.
        """
        if val is None:
            if tb is None:
                raise typ
            val = typ()
        if tb is not None:
            val = val.with_traceback(tb)
        raise val

    async def aclose(self):
        """Raise GeneratorExit inside coroutine.
        """
        try:
            await self.athrow(GeneratorExit)
        except (GeneratorExit, StopAsyncIteration):
            pass
        else:
            raise RuntimeError("asynchronous generator ignored GeneratorExit")

    @classmethod
    def __subclasshook__(cls, C):
        if cls is AsyncGenerator:
            return _check_methods(C, '__aiter__', '__anext__',
                                  'asend', 'athrow', 'aclose')
        return NotImplemented


AsyncGenerator.register(async_generator)


class Iterable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __iter__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:
            return _check_methods(C, "__iter__")
        return NotImplemented


class Iterator(Iterable):

    __slots__ = ()

    @abstractmethod
    def __next__(self):
        'Return the next item from the iterator. When exhausted, raise StopIteration'
        raise StopIteration

    def __iter__(self):
        return self

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterator:
            return _check_methods(C, '__iter__', '__next__')
        return NotImplemented

Iterator.register(bytes_iterator)
Iterator.register(bytearray_iterator)
#Iterator.register(callable_iterator)
Iterator.register(dict_keyiterator)
Iterator.register(dict_valueiterator)
Iterator.register(dict_itemiterator)
Iterator.register(list_iterator)
Iterator.register(list_reverseiterator)
Iterator.register(range_iterator)
Iterator.register(longrange_iterator)
Iterator.register(set_iterator)
Iterator.register(str_iterator)
Iterator.register(tuple_iterator)
Iterator.register(zip_iterator)


class Reversible(Iterable):

    __slots__ = ()

    @abstractmethod
    def __reversed__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Reversible:
            return _check_methods(C, "__reversed__", "__iter__")
        return NotImplemented


class Generator(Iterator):

    __slots__ = ()

    def __next__(self):
        """Return the next item from the generator.
        When exhausted, raise StopIteration.
        """
        return self.send(None)

    @abstractmethod
    def send(self, value):
        """Send a value into the generator.
        Return next yielded value or raise StopIteration.
        """
        raise StopIteration

    @abstractmethod
    def throw(self, typ, val=None, tb=None):
        """Raise an exception in the generator.
        Return next yielded value or raise StopIteration.
        """
        if val is None:
            if tb is None:
                raise typ
            val = typ()
        if tb is not None:
            val = val.with_traceback(tb)
        raise val

    def close(self):
        """Raise GeneratorExit inside generator.
        """
        try:
            self.throw(GeneratorExit)
        except (GeneratorExit, StopIteration):
            pass
        else:
            raise RuntimeError("generator ignored GeneratorExit")

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Generator:
            return _check_methods(C, '__iter__', '__next__',
                                  'send', 'throw', 'close')
        return NotImplemented

Generator.register(generator)


class Sized(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __len__(self):
        return 0

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Sized:
            return _check_methods(C, "__len__")
        return NotImplemented


class Container(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __contains__(self, x):
        return False

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Container:
            return _check_methods(C, "__contains__")
        return NotImplemented

class Collection(Sized, Iterable, Container):

    __slots__ = ()

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Collection:
            return _check_methods(C,  "__len__", "__iter__", "__contains__")
        return NotImplemented

class Callable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __call__(self, *args, **kwds):
        return False

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Callable:
            return _check_methods(C, "__call__")
        return NotImplemented


### SETS ###


class Set(Collection):

    """A set is a finite, iterable container.

    This class provides concrete generic implementations of all
    methods except for __contains__, __iter__ and __len__.

    To override the comparisons (presumably for speed, as the
    semantics are fixed), redefine __le__ and __ge__,
    then the other operations will automatically follow suit.
    """

    __slots__ = ()

    def __le__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        if len(self) > len(other):
            return False
        for elem in self:
            if elem not in other:
                return False
        return True

    def __lt__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        return len(self) < len(other) and self.__le__(other)

    def __gt__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        return len(self) > len(other) and self.__ge__(other)

    def __ge__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        if len(self) < len(other):
            return False
        for elem in other:
            if elem not in self:
                return False
        return True

    def __eq__(self, other):
        if not isinstance(other, Set):
            return NotImplemented
        return len(self) == len(other) and self.__le__(other)

    @classmethod
    def _from_iterable(cls, it):
        '''Construct an instance of the class from any iterable input.

        Must override this method if the class constructor signature
        does not accept an iterable for an input.
        '''
        return cls(it)

    def __and__(self, other):
        if not isinstance(other, Iterable):
            return NotImplemented
        return self._from_iterable(value for value in other if value in self)

    __rand__ = __and__

    def isdisjoint(self, other):
        'Return True if two sets have a null intersection.'
        for value in other:
            if value in self:
                return False
        return True

    def __or__(self, other):
        if not isinstance(other, Iterable):
            return NotImplemented
        chain = (e for s in (self, other) for e in s)
        return self._from_iterable(chain)

    __ror__ = __or__

    def __sub__(self, other):
        if not isinstance(other, Set):
            if not isinstance(other, Iterable):
                return NotImplemented
            other = self._from_iterable(other)
        return self._from_iterable(value for value in self
                                   if value not in other)

    def __rsub__(self, other):
        if not isinstance(other, Set):
            if not isinstance(other, Iterable):
                return NotImplemented
            other = self._from_iterable(other)
        return self._from_iterable(value for value in other
                                   if value not in self)

    def __xor__(self, other):
        if not isinstance(other, Set):
            if not isinstance(other, Iterable):
                return NotImplemented
            other = self._from_iterable(other)
        return (self - other) | (other - self)

    __rxor__ = __xor__

    def _hash(self):
        """Compute the hash value of a set.

        Note that we don't define __hash__: not all sets are hashable.
        But if you define a hashable set type, its __hash__ should
        call this function.

        This must be compatible __eq__.

        All sets ought to compare equal if they contain the same
        elements, regardless of how they are implemented, and
        regardless of the order of the elements; so there's not much
        freedom for __eq__ or __hash__.  We match the algorithm used
        by the built-in frozenset type.
        """
        MAX = sys.maxsize
        MASK = 2 * MAX + 1
        n = len(self)
        h = 1927868237 * (n + 1)
        h &= MASK
        for x in self:
            hx = hash(x)
            h ^= (hx ^ (hx << 16) ^ 89869747)  * 3644798167
            h &= MASK
        h = h * 69069 + 907133923
        h &= MASK
        if h > MAX:
            h -= MASK + 1
        if h == -1:
            h = 590923713
        return h

Set.register(frozenset)


class MutableSet(Set):
    """A mutable set is a finite, iterable container.

    This class provides concrete generic implementations of all
    methods except for __contains__, __iter__, __len__,
    add(), and discard().

    To override the comparisons (presumably for speed, as the
    semantics are fixed), all you have to do is redefine __le__ and
    then the other operations will automatically follow suit.
    """

    __slots__ = ()

    @abstractmethod
    def add(self, value):
        """Add an element."""
        raise NotImplementedError

    @abstractmethod
    def discard(self, value):
        """Remove an element.  Do not raise an exception if absent."""
        raise NotImplementedError

    def remove(self, value):
        """Remove an element. If not a member, raise a KeyError."""
        if value not in self:
            raise KeyError(value)
        self.discard(value)

    def pop(self):
        """Return the popped value.  Raise KeyError if empty."""
        it = iter(self)
        try:
            value = next(it)
        except StopIteration:
            raise KeyError
        self.discard(value)
        return value

    def clear(self):
        """This is slow (creates N new iterators!) but effective."""
        try:
            while True:
                self.pop()
        except KeyError:
            pass

    def __ior__(self, it):
        for value in it:
            self.add(value)
        return self

    def __iand__(self, it):
        for value in (self - it):
            self.discard(value)
        return self

    def __ixor__(self, it):
        if it is self:
            self.clear()
        else:
            if not isinstance(it, Set):
                it = self._from_iterable(it)
            for value in it:
                if value in self:
                    self.discard(value)
                else:
                    self.add(value)
        return self

    def __isub__(self, it):
        if it is self:
            self.clear()
        else:
            for value in it:
                self.discard(value)
        return self

MutableSet.register(set)


### MAPPINGS ###


class Mapping(Collection):

    __slots__ = ()

    """A Mapping is a generic container for associating key/value
    pairs.

    This class provides concrete generic implementations of all
    methods except for __getitem__, __iter__, and __len__.

    """

    @abstractmethod
    def __getitem__(self, key):
        raise KeyError

    def get(self, key, default=None):
        'D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None.'
        try:
            return self[key]
        except KeyError:
            return default

    def __contains__(self, key):
        try:
            self[key]
        except KeyError:
            return False
        else:
            return True

    def keys(self):
        "D.keys() -> a set-like object providing a view on D's keys"
        return KeysView(self)

    def items(self):
        "D.items() -> a set-like object providing a view on D's items"
        return ItemsView(self)

    def values(self):
        "D.values() -> an object providing a view on D's values"
        return ValuesView(self)

    def __eq__(self, other):
        if not isinstance(other, Mapping):
            return NotImplemented
        return dict(self.items()) == dict(other.items())

    __reversed__ = None

Mapping.register(mappingproxy)


class MappingView(Sized):

    __slots__ = '_mapping',

    def __init__(self, mapping):
        self._mapping = mapping

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

    def __repr__(self):
        return '{0.__class__.__name__}({0._mapping!r})'.format(self)


class KeysView(MappingView, Set):

    __slots__ = ()

    @classmethod
    def _from_iterable(self, it):
        return set(it)

    def __contains__(self, key):
        return key in self._mapping

    def __iter__(self):
        yield from self._mapping

KeysView.register(dict_keys)


class ItemsView(MappingView, Set):

    __slots__ = ()

    @classmethod
    def _from_iterable(self, it):
        return set(it)

    def __contains__(self, item):
        key, value = item
        try:
            v = self._mapping[key]
        except KeyError:
            return False
        else:
            return v is value or v == value

    def __iter__(self):
        for key in self._mapping:
            yield (key, self._mapping[key])

ItemsView.register(dict_items)


class ValuesView(MappingView):

    __slots__ = ()

    def __contains__(self, value):
        for key in self._mapping:
            v = self._mapping[key]
            if v is value or v == value:
                return True
        return False

    def __iter__(self):
        for key in self._mapping:
            yield self._mapping[key]

ValuesView.register(dict_values)


class MutableMapping(Mapping):

    __slots__ = ()

    """A MutableMapping is a generic container for associating
    key/value pairs.

    This class provides concrete generic implementations of all
    methods except for __getitem__, __setitem__, __delitem__,
    __iter__, and __len__.

    """

    @abstractmethod
    def __setitem__(self, key, value):
        raise KeyError

    @abstractmethod
    def __delitem__(self, key):
        raise KeyError

    __marker = object()

    def pop(self, key, default=__marker):
        '''D.pop(k[,d]) -> v, remove specified key and return the corresponding value.
          If key is not found, d is returned if given, otherwise KeyError is raised.
        '''
        try:
            value = self[key]
        except KeyError:
            if default is self.__marker:
                raise
            return default
        else:
            del self[key]
            return value

    def popitem(self):
        '''D.popitem() -> (k, v), remove and return some (key, value) pair
           as a 2-tuple; but raise KeyError if D is empty.
        '''
        try:
            key = next(iter(self))
        except StopIteration:
            raise KeyError
        value = self[key]
        del self[key]
        return key, value

    def clear(self):
        'D.clear() -> None.  Remove all items from D.'
        try:
            while True:
                self.popitem()
        except KeyError:
            pass

    def update(*args, **kwds):
        ''' D.update([E, ]**F) -> None.  Update D from mapping/iterable E and F.
            If E present and has a .keys() method, does:     for k in E: D[k] = E[k]
            If E present and lacks .keys() method, does:     for (k, v) in E: D[k] = v
            In either case, this is followed by: for k, v in F.items(): D[k] = v
        '''
        if not args:
            raise TypeError("descriptor 'update' of 'MutableMapping' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('update expected at most 1 arguments, got %d' %
                            len(args))
        if args:
            other = args[0]
            if isinstance(other, Mapping):
                for key in other:
                    self[key] = other[key]
            elif hasattr(other, "keys"):
                for key in other.keys():
                    self[key] = other[key]
            else:
                for key, value in other:
                    self[key] = value
        for key, value in kwds.items():
            self[key] = value

    def setdefault(self, key, default=None):
        'D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D'
        try:
            return self[key]
        except KeyError:
            self[key] = default
        return default

MutableMapping.register(dict)


### SEQUENCES ###


class Sequence(Reversible, Collection):

    """All the operations on a read-only sequence.

    Concrete subclasses must override __new__ or __init__,
    __getitem__, and __len__.
    """

    __slots__ = ()

    @abstractmethod
    def __getitem__(self, index):
        raise IndexError

    def __iter__(self):
        i = 0
        try:
            while True:
                v = self[i]
                yield v
                i += 1
        except IndexError:
            return

    def __contains__(self, value):
        for v in self:
            if v is value or v == value:
                return True
        return False

    def __reversed__(self):
        for i in reversed(range(len(self))):
            yield self[i]

    def index(self, value, start=0, stop=None):
        '''S.index(value, [start, [stop]]) -> integer -- return first index of value.
           Raises ValueError if the value is not present.
        '''
        if start is not None and start < 0:
            start = max(len(self) + start, 0)
        if stop is not None and stop < 0:
            stop += len(self)

        i = start
        while stop is None or i < stop:
            try:
                v = self[i]
                if v is value or v == value:
                    return i
            except IndexError:
                break
            i += 1
        raise ValueError

    def count(self, value):
        'S.count(value) -> integer -- return number of occurrences of value'
        return sum(1 for v in self if v is value or v == value)

Sequence.register(tuple)
Sequence.register(str)
Sequence.register(range)
Sequence.register(memoryview)


class ByteString(Sequence):

    """This unifies bytes and bytearray.

    XXX Should add all their methods.
    """

    __slots__ = ()

ByteString.register(bytes)
ByteString.register(bytearray)


class MutableSequence(Sequence):

    __slots__ = ()

    """All the operations on a read-write sequence.

    Concrete subclasses must provide __new__ or __init__,
    __getitem__, __setitem__, __delitem__, __len__, and insert().

    """

    @abstractmethod
    def __setitem__(self, index, value):
        raise IndexError

    @abstractmethod
    def __delitem__(self, index):
        raise IndexError

    @abstractmethod
    def insert(self, index, value):
        'S.insert(index, value) -- insert value before index'
        raise IndexError

    def append(self, value):
        'S.append(value) -- append value to the end of the sequence'
        self.insert(len(self), value)

    def clear(self):
        'S.clear() -> None -- remove all items from S'
        try:
            while True:
                self.pop()
        except IndexError:
            pass

    def reverse(self):
        'S.reverse() -- reverse *IN PLACE*'
        n = len(self)
        for i in range(n//2):
            self[i], self[n-i-1] = self[n-i-1], self[i]

    def extend(self, values):
        'S.extend(iterable) -- extend sequence by appending elements from the iterable'
        for v in values:
            self.append(v)

    def pop(self, index=-1):
        '''S.pop([index]) -> item -- remove and return item at index (default last).
           Raise IndexError if list is empty or index is out of range.
        '''
        v = self[index]
        del self[index]
        return v

    def remove(self, value):
        '''S.remove(value) -- remove first occurrence of value.
           Raise ValueError if the value is not present.
        '''
        del self[self.index(value)]

    def __iadd__(self, values):
        self.extend(values)
        return self

MutableSequence.register(list)
MutableSequence.register(bytearray)  # Multiply inheriting, see ByteString

_collects_abc.py

 

 

2.3.使用isinstance而不是type

（1）语法：

isinstance(object, classinfo)

其中，object 是变量，classinfo 是类型即 (tuple,dict,int,float,list,bool等) 和 class类

若参数 object 是 classinfo 类的实例，或者 object 是 classinfo 类的子类的一个实例， 返回 True。
若 object 不是一个给定类型的的对象， 则返回结果总是False。

若 classinfo 不是一种数据类型或者由数据类型构成的元组，将引发一个 TypeError 异常。

（2）isinstance简单用法

>>> isinstance(1,int)
True
>>> 
>>> isinstance('1',str)
True
>>> 
>>> isinstance(1,list)
False

（3）type()与isinstance()的区别：

• 共同点：两者都可以判断对象类型
• 不同点：对于一个 class 类的子类对象类型判断，type就不行了，而 isinstance 可以。

class A:
    pass

class B(A):
    pass

b = B()

#判断b是不是B的类型
print(isinstance(b,B))        #True
# b是不是A的类型呢，也是的
#因为B继承A，isinstance内部会去检查继承链
print(isinstance(b,A))        #True

print(type(b) is B)           #True
#b指向了B()对象，虽然A是B的父类，但是A是另外一个对象，它们的id是不相等的
print(type(b) is A)           #False

 

2.4.类变量和实例变量

 python的类变量和实例变量，顾名思义，类变量是指跟类的变量，而实例变量，指跟类的具体实例相关联的变量

class A:
    #类变量
    bb = 11
    def __init__(self,x,y):
        #实例变量
        self.x = x
        self.y = y

a = A(2,3)
A.bb = 111111
print(a.x,a.y,a.bb)    # 2 3 111111
print(A.bb)            # 111111

a.bb = 2222     #实际上会在实例对象a里面新建一个属性bb
print(a.bb)          # 2222
print(A.bb)          # 111111

 

2.5.类和实例属性的查找顺序

 

class D:
    pass

class C(D):
    pass

class B(D):
    pass

class A(B,C):
    pass

#顺序：A,B,C,D
#__mro__,类的属性查找顺序
print(A.__mro__)      #(<class '__main__.A'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.D'>, <class 'object'>)

 

 

 

class D:
    pass

class E:
    pass

class C(E):
    pass

class B(D):
    pass

class A(B,C):
    pass

#顺序：A,B,D,C,E
#__mro__,类的属性查找顺序
print(A.__mro__)      

#(<class '__main__.A'>, <class '__main__.B'>, <class '__main__.D'>, <class '__main__.C'>, <class '__main__.E'>, <class 'object'>)

 

2.6.类方法，静态方法，和实例方法

实例：

class Date():
    #构造函数
    def __init__(self,year,month,day):
        self.year = year
        self.month = month
        self.day = day

    #实例方法
    def tomorrow(self):
        self.day += 1

    # 静态方法不用写self
    @staticmethod
    def parse_from_string(date_str):
        year, month, day = tuple(date_str.split("-"))
        # 静态方法不好的地方是采用硬编码，如果用类方法的话就不会了
        return Date(int(year), int(month), int(day))

    #类方法
    @classmethod
    def from_string(cls, date_str):
        year, month, day = tuple(date_str.split("-"))
        # cls：传进来的类,而不是像静态方法把类写死了
        return cls(int(year), int(month), int(day))

    def __str__(self):
        return '%s/%s/%s'%(self.year,self.month,self.day)

if __name__ == "__main__":
    new_day = Date(2018,5,9)
    #实例方法
    new_day.tomorrow()
    print(new_day)       #2018/5/10

    #静态方法
    date_str = '2018-05-09'
    new_day = Date.parse_from_string(date_str)
    print(new_day)       #2018/5/9

    # 类方法
    date_str = '2018-05-09'
    new_day = Date.from_string(date_str)
    print(new_day)  # 2018/5/9

 

2.7.python对象的自省机制

 在计算机编程中，自省是指一种能力：检查某些事物以确定它是什么、它知道什么以及它能做什么。自省向程序员提供了极大的灵活性和控制力。

class Person:
    '''人类'''
    name = "user"


class Student(Person):
    def __init__(self,school_name):
        self.school_name = school_name

if __name__ == "__main__":

    user = Student('仙剑')
    #通过 __dict__ 查询有哪些属性
    print(user.__dict__)        #{'school_name': '仙剑'}

    print(Person.__dict__)      #{'__module__': '__main__', '__doc__': '人类', 'name': 'user', '__dict__': <attribute '__dict__' of 'Person' objects>, '__weakref__': <attribute '__weakref__' of 'Person' objects>}

    print(Person.__doc__)       #人类

    #可以添加属性
    user.__dict__['school_addr'] = '北京'
    print(user.school_addr)     #北京


    #dir也可以查看属性，比__dict__功能更强大
    print(dir(user))
#['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'name', 'school_addr', 'school_name']

 

2.8.super函数

super执行的顺序

class A:
    def __init__(self):
        print('A')

class B(A):
    def __init__(self):
        print('B')
        super().__init__()


class C(A):
    def __init__(self):
        print('C')
        super().__init__()


class D(B,C):
    def __init__(self):
        print('D')
        super(D, self).__init__()

if __name__ == '__main__':
    print(D.__mro__)          #(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)
    d = D()
    
    
#执行结果
D
B
C
A

 

2.9.with语句（上下文管理器）

 

#上下文管理器
class Sample:
    def __enter__(self):
        print('enter')
        #获取资源
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        #释放资源
        print('exit')

    def do_something(self):
        print('doing something')

#会自动执行enter和exit方法
with Sample() as sample:
    sample.do_something()


# 运行结果
enter
doing something
exit