Question

我已经开始为mongodb编写一个简单的模块来使用。我是python的新手，我写的代码存在问题：

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，我认为我的代码可能没有什么问题，如果你能发现它并纠正我的话。此外，我一直在得到这个错误：

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，还有另一种插入删除和更新实体的方法

Answer 1

因此，语法方面你有很多问题。看起来你正在以不同的方式混合几个教程。所以，首先我将解释你的代码发生了什么，并解释你为什么看到你所看到的：

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")

我想看看Pep-8（http://www.python.org/dev/peps/pep-0008/）编码风格指南（非常有用），了解如何制作代码“流动”蟒蛇。

通过你的代码来解释发生了什么 - 这就是你最终要做的事情：

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"]

如果您正在尝试创建一个围绕pymongo数据库的更大的API，那么我建议您查看一些已经构建的ORM模块。有一些 - 不是100％确定哪些可用于MongoDB，但我使用的那个（我有偏见，我写了）被称为ORB，可以在http://docs.projexsoftware.com/api/orb

找到

Answer 2

这不是如何解决问题的具体答案，而是如何逐步完成您想要做的事情并解决出现的更简单问题的答案：

1）首先忘掉课程，而不是

2）使用Python命令行或IDLE，

等Python程序

3）通过编写调用打开MongoDB数据库来完成任务来建立目标。换句话说，在担心课程之前，编写最简单的代码来实现目标。

4）完成后，感觉良好继续前进，然后使用documentation编写测试类。我的链接是您能找到的众多链接之一。

5）我认为你的问题的一部分，但不是全部，你的课程设置不正确。我的班级 - 没有完全显示 - 如下：

class GlobalData:
    def set_xfer_date(self, value):
        self.xfer_date = value
        self.date_str = str(value)
        self.xfer_date_base = self.date_str[0:10] + " " + "00:00:00"

        # Now build tomorrow.
        self.xfer_date_tomorrow = datetime.today() + timedelta(1)
        self.date_str_tomorrow = str(self.xfer_date_tomorrow) 
        self.xfer_date_tomorrow = \
          self.date_str_tomorrow[0:10] + " " + "00:00:00" 

        self.section_totals = {}
        self.billable_reads = {}

    def set_xfer_fnam_out(self, value):
        self.xfer_fnam_out = value

    def set_xfer_dir_in(self, value):
        self.xfer_dir_in = value
    .
    .
    .

    def get_billable_reads(self):
        return self.billable_reads

我看到的一个问题是您没有使用self.来引用数据祝你好运。

在python中编写一个简单的MongoDB模块

2 个答案: