60數據庫5_元編程_ORM框架

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目錄

meta ?programming，元編程：... 1

ORM：... 5

字段類的實現：... 5

session類的實現：... 8

自定義Model類的實現：... 9

使用元類改造Model：... 10

引擎類：... 12

總結：... 13

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meta programming，元編程：

概念來自LISP和smaltalk；

我們寫程序是直接寫代碼，是否能用代碼來生成未來我們需要的代碼？這就是元編程；

用來生成代碼的程序稱為元程序meta program，編寫這種程序就稱為元編程meta programming；

py能通過反射實現元編程（元語言）；

在框架中用的多；

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type類：

type與object糾纏在一起，關系復雜；

所有自定義的類都是type的子類，即type類的實例；

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class type(object):

??? def __init__(cls, what, bases=None, dict=None): # known special case of type.__init__?? #what類名字，bases繼承列表，dict類對象（類屬性字典），這些與class定義的類相關

??????? """

??????? type(object_or_name, bases, dict)?? #借助type構造任何類，用代碼來生成代碼即元編程

??????? type(object) -> the object's type?? #返回對象的類型，如type(10)

??????? type(name, bases, dict) -> a new type?? #返回一個新的類型

??????? # (copied from class doc)

??????? """

??????? pass

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總結：

元類是制造類的工廠，是生成類的類；

定義一個元類，需要使用type(name,bases,dict)，也可以繼承type；

構造好元類，就可以在類定義時使用關鍵字參數metaclass指定元類，可使用最原始的metatype(name, bases, dict)方式構造一個元類；

元類的__new__()方法中，可獲取元類信息、當前類、基類、類變量信息；

元編程一般用于框架開發中；

開發中，除非明確知道自己在干什么，否則不要隨便使用元編程，99%的情況下用不到元類，可能有些程序員一輩子都不會使用元類；

Django、SQLAlchemy使用了元類，這樣我們使用起來很方便；

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例：

XClass = type('myclass', (object,), {'a': 100, 'b': 'string'})

print(XClass)

print(type(XClass))

print(XClass.__dict__)

print(XClass.mro())

輸出：

<class '__main__.myclass'>

<class 'type'>

{'b': 'string', '__dict__': <attribute '__dict__' of 'myclass' objects>, '__weakref__': <attribute '__weakref__' of 'myclass' objects>, 'a': 100, '__module__': '__main__', '__doc__': None}

[<class '__main__.myclass'>, <class 'object'>]

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例1：

def __init__(self):

??? self.x = 100

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def show(self):

??? print(self.__dict__)

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XClass = type('myclass', (object,), {'a': 100, 'b': 'string', '__init__': __init__, 'show': show})

print(XClass)

print(type(XClass))

print(XClass.__dict__)

print(XClass.mro())

輸出：

<class '__main__.myclass'>

<class 'type'>

{'a': 100, '__weakref__': <attribute '__weakref__' of 'myclass' objects>, '__dict__': <attribute '__dict__' of 'myclass' objects>, 'b': 'string', 'show': <function show at 0x0000000000801488>, '__module__': '__main__', '__doc__': None, '__init__': <function __init__ at 0x00000000008012F0>}

[<class '__main__.myclass'>, <class 'object'>]

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例2：

同例1；

class MyClass:

??? def __init__(self):

??????? print('init')

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??? def show(self):

??????? print('show')

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print(MyClass)

print(type(MyClass))

print(MyClass.__dict__)

print(MyClass.mro())

輸出：

<class '__main__.MyClass'>

<class 'type'>

{'show': <function MyClass.show at 0x00000000011B1598>, '__module__': '__main__', '__doc__': None, '__dict__': <attribute '__dict__' of 'MyClass' objects>, '__weakref__': <attribute '__weakref__' of 'MyClass' objects>, '__init__': <function MyClass.__init__ at 0x00000000011B1510>}

[<class '__main__.MyClass'>, <class 'object'>]

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例：

class XMeta(type):

??? # pass

??? def __new__(cls, *args, **kwargs):?? #__new__()可用作攔截；cls，不是普通的類，而是與type類等同，稱為元類；參數(cls,*args,**kwargs)等同于(cls,what,bases,dict)

??????? print(1, cls)?? #打印元類

??????? print(2, args)?? #what，打印三元組(what,bases,dict)

??????? print(3, kwargs)?? #bases，{}

??????? print(4, super())?? #dict

??????? print(5, type(cls))

??????? return super().__new__(cls, *args, **kwargs)

??????? # return 1

??????? # return super().__new__(cls, what, bases, dict)

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??? # def __new__(cls, name, bases, dict):

??? #???? print(1, cls)

??? #???? print(2, name)

??? #???? print(3, bases)

??? #???? print(4, dict)

??? #???? return super().__new__(cls, name, bases, dict)

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print(XMeta)

print(type(XMeta))

print(XMeta.__dict__)

# print(XMeta.mro())

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class A(metaclass=XMeta):?? #方式一，通過metaclass關鍵字指定元類，此處不是繼承而是替換元類

??? # pass

??? id = 100

??? def __init__(self):

??????? print('###A init###')

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A()

print(A)

print(type(A))

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class B(A):?? #方式二，通過繼承方式得到元類

??? # pass

??? def __init__(self):

??????? super().__init__()

??????? print('###B init###')

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B()

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C = XMeta('tom', (), {})?? #方式三，用元類構建其它類

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print(type(A), type(B))

輸出：

<class '__main__.XMeta'>

<class 'type'>

{'__doc__': None, '__module__': '__main__', '__new__': <staticmethod object at 0x000000000076E278>}

1 <class '__main__.XMeta'>

2 ('A', (), {'__init__': <function A.__init__ at 0x0000000000761598>, '__module__': '__main__', '__qualname__': 'A', 'id': 100})

3 {}

4 <super: <class 'XMeta'>, <XMeta object>>

5 <class 'type'>

###A init###

<class '__main__.A'>

<class '__main__.XMeta'>

1 <class '__main__.XMeta'>

2 ('B', (<class '__main__.A'>,), {'__init__': <function B.__init__ at 0x0000000000761730>, '__module__': '__main__', '__qualname__': 'B'})

3 {}

4 <super: <class 'XMeta'>, <XMeta object>>

5 <class 'type'>

###A init###

###B init###

1 <class '__main__.XMeta'>

2 ('tom', (), {})

3 {}

4 <super: <class 'XMeta'>, <XMeta object>>

5 <class 'type'>

<class '__main__.XMeta'> <class '__main__.XMeta'>

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ORM：

object relation map，對象關系映射，對象和關系之間的映射，使用面向對象的方式來操作DB；

關系模型和py對象之間的映射；

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table-->class?? #表映射為類

row-->object?? #行映射為實例

column-->property?? #字段映射為類屬性

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例：

實現ORM框架；

表由字段構成，表對應類，類屬性對應字段；

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字段類的實現：

字段特征有：name,column,type,pk,uk(unique key),index,nullable,default,auto_increment，所以字段可用類來描述；

字段類要提供對數據的校驗功能，如聲明字段是int類型，應要判斷數據是不是整型；

字段有多種類型，不同類型有差異，使用繼承的方式實現；

字段現定義為類屬性，而這個類屬性適合使用類來描述，即描述器；

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例：

class Field:

??? def __init__(self, name, column=None, pk=False, unique=False, index=False, nullable=True, default=None):

??????? self.name = name

??????? if column is None:

??????????? self.column = name

??????? else:

??????????? self.column = column

??????? self.pk = pk

??????? self.unique = unique

??????? self.index = index

??????? self.nullable = nullable

??????? self.default = default

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??? def validate(self, value):

??????? raise NotImplementedError

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??? def __get__(self, instance, owner):

??????? if instance is None:

??????????? return self

??????? return instance.__dict__[self.name]

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??? def __set__(self, instance, value):

??????? self.validate(value)

??????? instance.__dict__[self.name] = value

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??? def __str__(self):

??????? return '{} <{}>'.format(self.__class__.__name__, self.name)

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??? __repr__ = __str__

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class IntField(Field):

??? def __init__(self, name, column=None, pk=False, unique=False, index=False, nullable=True, default=None, auto_increment=False):

??????? super().__init__(name, column, pk, unique, index, nullable, default)

??????? self.auto_increment = auto_increment

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??? def validate(self, value):

??????? # pass

??????? if value is None:

??????????? if self.pk:

??????????????? raise TypeError('{} is pk, not None'.format(self.name))

??????????? if not self.nullable:

??????? ????????raise TypeError('{} required'.format(self.name))

??????? else:

??????????? if not isinstance(value, int):

??????????????? raise TypeError('{} should be integer'.format(self.name))

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class StringField(Field):

??? def __init__(self, name, column=None, pk=False, unique=False, index=False, nullable=True, default=None, length=False):

??????? super().__init__(name, column, pk, unique, index, nullable, default)

??????? self.length = length

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??? def validate(self, value):

??????? # pass

??????? if value is None:

??????????? if self.pk:

??????????????? raise TypeError('{} is pk, not None'.format(self.name))

??????????? if not self.nullable:

??????????????? raise TypeError('{} required'.format(self.name))

??????? else:

??????????? if not isinstance(value, str):

??????????????? raise TypeError('{} should be string'.format(self.name))

??????????? if len(value) > self.length:

??????????????? raise ValueError('{} is too long'.format(value))

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例：

Student類的操作對應表的CRUD操作，若使用pymysql，應用cursor對象的execute方法；

增加、修改數據自定義為save()方法，數據庫連接從外部傳入，這應是一個全局變量；

class Student:

??? id = IntField('id', 'id', True, nullable=False, auto_increment=True)

??? name = StringField('name', nullable=False, length=64)

??? age = IntField('age')

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??? def __init__(self, id, name, age):

??????? self.id = id

??????? self.name = name

??????? self.age = age

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??? def __str__(self):

??????? return 'Student({}, {}, {})'.format(self.id, self.name, self.age)

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??? __repr__ = __str__

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??? def save(self, conn:pymysql.connections.Connection):

??????? sql = 'insert into student (id, name, age) values (%s, %s, %s)'

??????? with conn as cursor:

??????????? cursor.execute(sql, (self.id, self.name, self.age))

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session類的實現：

每一次數據庫操作都是在一個會話中完成，將cursor的操作封裝到會話中；

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例：

class Session:

??? def __init__(self, conn: pymysql.connections.connection):

??????? self.conn = conn

??????? self.cursor = None

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??? def execute(self, query, *args):

??????? if self.cursor is None:

??????????? self.cursor = self.conn.cursor()

??????? self.cursor.execute(query, args)

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??? def __enter__(self):

??????? self.cursor = self.conn.cursor()

??????? return self

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??? def __exit__(self, exc_type, exc_val, exc_tb):

??????? self.cursor.close()

??????? if exc_type:

??????????? self.conn.rollback()

??????? else:

?????? ?????self.conn.commit()

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class Student:

??? id = IntField('id', 'id', True, nullable=False, auto_increment=True)

??? name = StringField('name', nullable=False, length=64)

??? age = IntField('age')

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??? def __init__(self, id, name, age):

??????? self.id = id

??????? self.name = name

??????? self.age = age

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??? def __str__(self):

??????? return 'Student({}, {}, {})'.format(self.id, self.name, self.age)

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??? __repr__ = __str__

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??? def save(self, session: Session):

??????? sql = 'insert into student (id, name, age) values (%s, %s, %s)'

??????? session.execute(sql, self.id, self.name, self.age)

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自定義Model類的實現：

Student這樣的類，若多建幾個，可發現千篇一律，每一個這樣的類，得定義一個名稱，對應不同的表，都要先定義好類屬性，再__init__()初始化值，而這些值正好是定義好的類屬性；

CRUD操作也一樣；

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設計一個Model類，增加一個__table__類屬性用來保存不同的表名；

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例：

class Model:

??? def save(self, session: Session):

??????? names = []

??????? values = []

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??????? for k, v in self.__class__.__dict__.items():

??????????? if isinstance(v, Field):

??????????????? if k in self.__dict__.keys():

??????????????????? names.append(k)

??????????????????? values.append(v)

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??????? query = 'insert into {} ({}) values({})'.format(

??????????? self.__table__,

??????????? ','.join(names),

??????????? ','.join(['%s']*len(values)))

??????? print(query)

???? ???print(values)

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class Student(Model):

??? __table__ = 'student'

??? id = IntField('id', 'id', True, nullable=False, auto_increment=True)

??? name = StringField('name', nullable=False, length=64)

??? age = IntField('age')

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??? def __init__(self, id, name, age):

??????? self.id = id

??????? self.name = name

??????? self.age = age

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??? def __str__(self):

??????? return 'Student({}, {}, {})'.format(self.id, self.name, self.age)

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??? __repr__ = __str__

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s = Student(1, 'tom', 20)

s.save(None)

輸出：

insert into student (name,age,id) values(%s,%s,%s)

[StringField <name>, IntField <age>, IntField <id>]

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使用元類改造Model：

通過元類編程，增加了mapping類變量，里面存儲著已經過濾出來的，字段類變量名=>Field對象映射；

例：

class ModelMeta(type):

??? def __new__(cls, name, bases, attrs: dict):?? #name類名，attrs類屬性字典

??????? if '__table__' not in attrs.keys():

??????????? attrs['__table__'] = name?? #默認添加表名為類名

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??????? mapping = {}?? #方便以后查詢屬性名和字段實例

??????? primarykey = []

??????? for k, v in attrs.items():?? #k，類變量名稱字符串；v，對象

??????????? if isinstance(v, Field):

????? ??????????print(k, v)

??????????????? v.name = k

??????????????? if v.column is None:

??????????????????? v.column = k?? #沒有給字段名

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??????????????? mapping[k] = v

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??????????????? if v.pk:

??????????????????? primarykey.append(v)

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??????? attrs['__mapping__'] = mapping?? #增加屬性

??????? attrs['__primary__'] = primarykey

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??????? return super().__new__(cls, name, bases, attrs)

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class Model(metaclass=ModelMeta):

??? def save(self, session: Session):

??????? names = []

??????? values = []

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??????? for k, v in self.__class__.__dict__.items():

??????????? if isinstance(v, Field):

??????????????? if k in self.__dict__.keys():

??????????????????? names.append(k)

??????????????????? values.append(v)

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??????? query = 'insert into {} ({}) values({})'.format(

??????????? self.__table__,

??????????? ','.join(names),

??????????? ','.join(['%s']*len(values)))

??????? print(query)

??????? print(values)

??????? # session.execute(query, *values)

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class Student(Model):

??? __table__ = 'student'?? #有了元類，此句可省

??? id = IntField('id', 'id', True, nullable=False, auto_increment=True)

??? name = StringField('name', nullable=False, length=64)

??? age = IntField('age')

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??? def __init__(self, id, name, age):

??????? self.id = id

??????? self.name = name

??????? self.age = age

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??? def __str__(self):

??????? return 'Student({}, {}, {})'.format(self.id, self.name, self.age)

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??? __repr__ = __str__

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s = Student(1, 'tom', 20)

s.save(None)

print(Student.__dict__)

print(Model.__dict__)

輸出：

insert into student (id,age,name) values(%s,%s,%s)

[IntField <id>, IntField <age>, StringField <name>]

{'__doc__': None, '__primary__': [IntField <id>], 'id': IntField <id>, '__init__': <function Student.__init__ at 0x0000000002DC7158>, '__table__': 'student', '__repr__': <function Student.__str__ at 0x0000000002DC71E0>, '__module__': '__main__', 'age': IntField <age>, '__str__': <function Student.__str__ at 0x0000000002DC71E0>, 'name': StringField <name>, '__mapping__': {'id': IntField <id>, 'age': IntField <age>, 'name': StringField <name>}}

{'__doc__': None, '__module__': '__main__', '__dict__': <attribute '__dict__' of 'Model' objects>, '__weakref__': <attribute '__weakref__' of 'Model' objects>, '__primary__': [], '__table__': 'Model', 'save': <function Model.save at 0x0000000002DC70D0>, '__mapping__': {}}

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引擎類：

實體類沒有提供數據庫連接，它也不應該提供，實例類就應只完成表和類的映射；

提供一個數據庫的包裝類：

1、負責數據庫連接；

2、負責CRUD操作，取代實體類的CRUD方法；

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例：

class Engine:

??? def __init__(self, *args, **kwargs):

??????? self.conn = pymysql.connect(*args, **kwargs)

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??? def save(self, instance: Student):

??????? names = []

??????? values = []

??????? for k, v in instance.__mapping__.items():

??????????? names.append('`{}`'.format(k))

??????????? values.append(instance.__dict__[k])

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??????? query = "insert into {} ({}) values ({})".format(

??????????? instance.__table__,

??????????? ','.join(names),

??????????? ','.join(['%s']*len(values))

??????????? )

??????? print(query)

??????? print(values)

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??????? # with self.conn as cursor:

??????? #???? with cursor:

??????? #? ???????cursor.execute(query, values)

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s = Student(1, 'tom', 20)

# s.save(None)

print(Student.__dict__)

print(Model.__dict__)

print(s.__dict__)

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engine = Engine('10.113.129.2', 'root', 'rootqazwsx', 'test1')

engine.save(s)

輸出：

id IntField <id>

age IntField <age>

name StringField <name>

{'__primary__': [IntField <id>], '__doc__': None, '__repr__': <function Student.__str__ at 0x0000000002DC81E0>, 'name': StringField <name>, '__table__': 'student', '__init__': <function Student.__init__ at 0x0000000002DC8158>, '__module__': '__main__', '__str__': <function Student.__str__ at 0x0000000002DC81E0>, 'id': IntField <id>, 'age': IntField <age>, '__mapping__': {'id': IntField <id>, 'age': IntField <age>, 'name': StringField <name>}}

{'save': <function Model.save at 0x0000000002DC80D0>, '__dict__': <attribute '__dict__' of 'Model' objects>, '__module__': '__main__', '__doc__': None, '__mapping__': {}, '__primary__': [], '__weakref__': <attribute '__weakref__' of 'Model' objects>, '__table__': 'Model'}

{'id': 1, 'name': 'tom', 'age': 20}

insert into student (`id`,`age`,`name`) values (%s,%s,%s)

[1, 20, 'tom']

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總結：

這是一個ORM框架的雛形，從這個例子就可以明白ORM框架的內部原理；

學習一個ORM框架：

1、看Model類如何描述表、屬性和字段如何映射；

2、增刪改查方法調用如何轉化為SQL語句并執行；

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