python - 在 python 中编写一个简单的 MongoDB 模块

Question

我已经开始编写一个供 mongodb 使用的简单模块。我是 python 的新手，我写的代码有问题:

import pymongo


class mongoDB():

    conn = object

    def __init__(self):
        global conn
        self.conn = pymongo.Connection("localhost",27017)

    def CreateCollection(self,name =""):
        self.dbCollection  = conn.name
        return self.dbCollection

if __name__ == '__main__':
    database = mongoDB
    collection = database.CreateCollection("Hello")

首先，如果您能发现并纠正我，我认为我的代码可能没有什么问题。我也一直收到这个错误:

collection = database.CreateCollection("Hello")
TypeError: unbound method CreateCollection() must be called with mongoDB      

instance as first argument (got str instance instead)

我希望能够在类的构造函数中创建连接，然后有一个创建集合并返回它的方法，以及另一个插入删除和更新实体的方法

Answer 1

所以，在语法方面你有很多问题。看起来您正在以不同的方式混合使用几个教程。所以，首先我会解释你的代码发生了什么，并解释为什么你会看到你所看到的:

import pymongo

class mongoDB():  # you don't need ()'s here - only if you are inheriting classes
                  # you could inherit from object here, which is a good practice
                  # by doing class mongoDb(object):, otherwise you can just take
                  # them out

    conn = object # here, you're defining a class member - global for all instances
                  # generally, you don't instantiate an object pointer like this,
                  # you would set it to None instead.  It won't fail doing this,
                  # but it's not "right"

    def __init__(self):
        # the __init__ method is the constructor method - this will 
        # allow you to initialize a particular instance of your class, represented
        # by the self argument.  This method is called when you call the class, i.e.
        # inst = mongoDb()

        # in this case, the conn variable is not a global.  Globals are defined
        # at the root module level - so in this example, only pymongo is a global
        # conn is a class member, and would be accessed by doing mongoDB.conn
        global conn

        # with that being said, you're initializing a local variable here called conn
        # that is not being stored anywhere - when this method finishes, this variable
        # will be cleaned up from memory, what you are thinking you're doing here
        # should be written as mongoDB.conn = pymongo.Connection("localhost", 27017)
        conn = pymongo.Connection("localhost",27017)

    def CreateCollection(name =""):
        # there is one of two things you are trying to do here - 1, access a class 
        # level member called conn, or 2, access an instance member called conn

        # depending on what you are going for, there are a couple of different ways 
        # to address it.

        # all methods for a class, by default, are instance methods - and all of them
        # need to take self as the first argument.  An instance method of a class
        # will always be called with the instance first.  Your error is caused because
        # you should declare the method as:

        # def CreateCollection(self, name = ""):

        # The alternative, is to define this method as a static method of the class -
        # which does not take an instance but applies to all instances of the class
        # to do that, you would add a @staticmethod decorator before the method.

        # either way, you're attempting to access the global variable "conn" here,
        # which again does not exist

        # the second problem with this, is that you are trying to take your variable
        # argument (name) and use it as a property.  What python is doing here, is
        # looking for a member variable called name from the conn object.  What you
        # are really trying to do is create a collection on the connection with the
        # inputed name

        # the pymongo class provides access to your collections via this method as a
        # convenience around the method, create_collection.  In the case where you
        # are using a variable to create the collection, you would call this by doing

        # conn.create_collection(name)

        # but again, that assumes conn is what you think it is, which it isn't
        dbCollection  = conn.name
        return dbCollection

if __name__ == '__main__':
    # here you are just creating a pointer to your class, not instantiating it
    # you are looking for:

    # database = mongoDB()
    database = mongoDB

    # this is your error, because of the afore mentioned lack of 'self' argument
    collection = database.CreateCollection("Hello")

我想看一下 Pep-8 (http://www.python.org/dev/peps/pep-0008/) 编码风格指南(非常有帮助)以了解如何编写代码以 Python 方式“流动”。

通过你的代码来解释发生了什么——这就是你最终想要做的:

import pymongo

class MongoDB: # Classes generally are camel-case, starting with uppercase
    def __init__(self, dbname):
        # the __init__ method is the class constructor, where you define
        # instance members.  We'll make conn an instance member rather
        # than a class level member
        self._conn = pymongo.Connection("localhost", 27017)
        self._db   = self._conn[dbname]

    # methods usually start with lowercase, and are either camel case (less desirable
    # by Python standards) or underscored (more desirable)
    # All instance methods require the 1st argument to be self (pointer to the
    # instance being affected)
    def createCollection(self, name=""):
        return self._db[name]

if __name__ == '__main__':
    # you want to initialize the class
    database   = MongoDB("Hello")
    collection = database.createCollection("MyTable")

鉴于此 - 编写此类包装器的目标是什么？同样可以写成:

import pymongo
conn       = pymongo.Connection('localhost', 27017)
database   = conn["Hello"]
collection = database["MyTable"]

如果您尝试创建一个更大的 API 来包裹 pymongo 数据库，那么我建议您查看一些已经构建的 ORM 模块。那里有一些 - 不能 100% 确定哪些可用于 MongoDB，但我使用的(我有偏见，我写的)称为 ORB，可以在 http://docs.projexsoftware.com/api/orb 找到