源码阅读之mongoengine（0）

最近工作上用到了mongodb，之前只是草草了解了一下。对于NoSQL的了解也不是太多。所以想趁机多学习一下。

工作的项目直接用了pymongo来操作直接操作mongodb。对于用惯了Djongo ORM的我来说，这种方式简直太不优雅。在网上找了一下，发现有一个mongoengine的 orm库。虽然项目暂时没办法改造。我就想先来研究一下这个库的源码。因为这是第一次把自己阅读源码的心得写出来。所以可能会比较啰嗦。请各位见谅！

在 __init__ 中，定义了__all__,在包级别就可以方便的导如所有模块的符号

#coding:utf-8
# Import submodules so that we can expose their __all__
from mongoengine import connection
from mongoengine import document
from mongoengine import errors
from mongoengine import fields
from mongoengine import queryset
from mongoengine import signals

# Import everything from each submodule so that it can be accessed via
# mongoengine, e.g. instead of `from mongoengine.connection import connect`,
# users can simply use `from mongoengine import connect`, or even
# `from mongoengine import *` and then `connect('testdb')`.
from mongoengine.connection import *
from mongoengine.document import *
from mongoengine.errors import *
from mongoengine.fields import *
from mongoengine.queryset import *
from mongoengine.signals import *

"""
定义了当你使用 from module import * 导入
某个模块的时候能导出的符号（这里代表变量，函数，类等）
"""
__all__ = (list(document.__all__) + list(fields.__all__) +
           list(connection.__all__) + list(queryset.__all__) +
           list(signals.__all__) + list(errors.__all__))

VERSION = (0, 11, 0)

def get_version():
    """Return the VERSION as a string, e.g. for VERSION == (0, 10, 7),
    return '0.10.7'.
    """
    """
    map()函数接收两个参数，
    一个是函数，一个是序列，map将传入的函数依次作用到序列的每个元素，
    并把结果作为新的list返回
    """
    return '.'.join(map(str, VERSION))

__version__ = get_version()

这个库非常小，而且也依赖了pymongo来操作数据库。看一下它的connection模块

from pymongo import MongoClient, ReadPreference, uri_parser
import six

from mongoengine.python_support import IS_PYMONGO_3

__all__ = ['MongoEngineConnectionError', 'connect', 'register_connection',
           'DEFAULT_CONNECTION_NAME']

DEFAULT_CONNECTION_NAME = 'default'

if IS_PYMONGO_3:
    READ_PREFERENCE = ReadPreference.PRIMARY
else:
    from pymongo import MongoReplicaSetClient
    READ_PREFERENCE = False

class MongoEngineConnectionError(Exception):
    """Error raised when the database connection can't be established or
    when a connection with a requested alias can't be retrieved.
    """
    pass

_connection_settings = {}
_connections = {}
_dbs = {}

def register_connection(alias, name=None, host=None, port=None,
                        read_preference=READ_PREFERENCE,
                        username=None, password=None,
                        authentication_source=None,
                        authentication_mechanism=None,
                        **kwargs):
    """Add a connection.

    :param alias: the name that will be used to refer to this connection
        throughout MongoEngine
    :param name: the name of the specific database to use
    :param host: the host name of the :program:`mongod` instance to connect to
    :param port: the port that the :program:`mongod` instance is running on
    :param read_preference: The read preference for the collection
       ** Added pymongo 2.1
    :param username: username to authenticate with
    :param password: password to authenticate with
    :param authentication_source: database to authenticate against
    :param authentication_mechanism: database authentication mechanisms.
        By default, use SCRAM-SHA-1 with MongoDB 3.0 and later,
        MONGODB-CR (MongoDB Challenge Response protocol) for older servers.
    :param is_mock: explicitly use mongomock for this connection
        (can also be done by using `mongomock://` as db host prefix)
    :param kwargs: allow ad-hoc parameters to be passed into the pymongo driver

    .. versionchanged:: 0.10.6 - added mongomock support
    """
    conn_settings = {
        'name': name or 'test',
        'host': host or 'localhost',
        'port': port or 27017,
        'read_preference': read_preference,
        'username': username,
        'password': password,
        'authentication_source': authentication_source,
        'authentication_mechanism': authentication_mechanism
    }

    # Handle uri style connections
    conn_host = conn_settings['host']
    # host can be a list or a string, so if string, force to a list
    if isinstance(conn_host, six.string_types):
        conn_host = [conn_host]

    resolved_hosts = []
    for entity in conn_host:

        # Handle Mongomock
        if entity.startswith('mongomock://'):
            conn_settings['is_mock'] = True
            # `mongomock://` is not a valid url prefix and must be replaced by `mongodb://`
            resolved_hosts.append(entity.replace('mongomock://', 'mongodb://', 1))

        # Handle URI style connections, only updating connection params which
        # were explicitly specified in the URI.
        elif '://' in entity:
            uri_dict = uri_parser.parse_uri(entity)
            resolved_hosts.append(entity)

            if uri_dict.get('database'):
                conn_settings['name'] = uri_dict.get('database')

            for param in ('read_preference', 'username', 'password'):
                if uri_dict.get(param):
                    conn_settings[param] = uri_dict[param]

            uri_options = uri_dict['options']
            if 'replicaset' in uri_options:
                conn_settings['replicaSet'] = True
            if 'authsource' in uri_options:
                conn_settings['authentication_source'] = uri_options['authsource']
            if 'authmechanism' in uri_options:
                conn_settings['authentication_mechanism'] = uri_options['authmechanism']
        else:
            resolved_hosts.append(entity)
    conn_settings['host'] = resolved_hosts

    # Deprecated parameters that should not be passed on
    kwargs.pop('slaves', None)
    kwargs.pop('is_slave', None)

    conn_settings.update(kwargs)
    _connection_settings[alias] = conn_settings

def disconnect(alias=DEFAULT_CONNECTION_NAME):
    """Close the connection with a given alias."""
    if alias in _connections:
        get_connection(alias=alias).close()
        del _connections[alias]
    if alias in _dbs:
        del _dbs[alias]

def get_connection(alias=DEFAULT_CONNECTION_NAME, reconnect=False):
    """Return a connection with a given alias."""

    # Connect to the database if not already connected
    if reconnect:
        disconnect(alias)

    # If the requested alias already exists in the _connections list, return
    # it immediately.
    if alias in _connections:
        return _connections[alias]

    # Validate that the requested alias exists in the _connection_settings.
    # Raise MongoEngineConnectionError if it doesn't.
    if alias not in _connection_settings:
        if alias == DEFAULT_CONNECTION_NAME:
            msg = 'You have not defined a default connection'
        else:
            msg = 'Connection with alias "%s" has not been defined' % alias
        raise MongoEngineConnectionError(msg)

    def _clean_settings(settings_dict):
        irrelevant_fields = set([
            'name', 'username', 'password', 'authentication_source',
            'authentication_mechanism'
        ])
        return {
            k: v for k, v in settings_dict.items()
            if k not in irrelevant_fields
        }

    # Retrieve a copy of the connection settings associated with the requested
    # alias and remove the database name and authentication info (we don't
    # care about them at this point).
    conn_settings = _clean_settings(_connection_settings[alias].copy())

    # Determine if we should use PyMongo's or mongomock's MongoClient.
    is_mock = conn_settings.pop('is_mock', False)
    if is_mock:
        try:
            import mongomock
        except ImportError:
            raise RuntimeError('You need mongomock installed to mock '
                               'MongoEngine.')
        connection_class = mongomock.MongoClient
    else:
        connection_class = MongoClient

        # Handle replica set connections
        if 'replicaSet' in conn_settings:

            # Discard port since it can't be used on MongoReplicaSetClient
            conn_settings.pop('port', None)

            # Discard replicaSet if it's not a string
            if not isinstance(conn_settings['replicaSet'], six.string_types):
                del conn_settings['replicaSet']

            # For replica set connections with PyMongo 2.x, use
            # MongoReplicaSetClient.
            # TODO remove this once we stop supporting PyMongo 2.x.
            if not IS_PYMONGO_3:
                connection_class = MongoReplicaSetClient
                conn_settings['hosts_or_uri'] = conn_settings.pop('host', None)

    # Iterate over all of the connection settings and if a connection with
    # the same parameters is already established, use it instead of creating
    # a new one.
    existing_connection = None
    connection_settings_iterator = (
        (db_alias, settings.copy())
        for db_alias, settings in _connection_settings.items()
    )
    for db_alias, connection_settings in connection_settings_iterator:
        connection_settings = _clean_settings(connection_settings)
        if conn_settings == connection_settings and _connections.get(db_alias):
            existing_connection = _connections[db_alias]
            break

    # If an existing connection was found, assign it to the new alias
    if existing_connection:
        _connections[alias] = existing_connection
    else:
        # Otherwise, create the new connection for this alias. Raise
        # MongoEngineConnectionError if it can't be established.
        try:
            _connections[alias] = connection_class(**conn_settings)
        except Exception as e:
            raise MongoEngineConnectionError(
                'Cannot connect to database %s :\n%s' % (alias, e))

    return _connections[alias]

def get_db(alias=DEFAULT_CONNECTION_NAME, reconnect=False):
    if reconnect:
        disconnect(alias)

    if alias not in _dbs:
        conn = get_connection(alias)
        conn_settings = _connection_settings[alias]
        db = conn[conn_settings['name']]
        auth_kwargs = {'source': conn_settings['authentication_source']}
        if conn_settings['authentication_mechanism'] is not None:
            auth_kwargs['mechanism'] = conn_settings['authentication_mechanism']
        # Authenticate if necessary
        if conn_settings['username'] and (conn_settings['password'] or
                                          conn_settings['authentication_mechanism'] == 'MONGODB-X509'):
            db.authenticate(conn_settings['username'], conn_settings['password'], **auth_kwargs)
        _dbs[alias] = db
    return _dbs[alias]

def connect(db=None, alias=DEFAULT_CONNECTION_NAME, **kwargs):
    """Connect to the database specified by the 'db' argument.

    Connection settings may be provided here as well if the database is not
    running on the default port on localhost. If authentication is needed,
    provide username and password arguments as well.

    Multiple databases are supported by using aliases.  Provide a separate
    `alias` to connect to a different instance of :program:`mongod`.

    .. versionchanged:: 0.6 - added multiple database support.
    """
    if alias not in _connections:
        register_connection(alias, db, **kwargs)

    return get_connection(alias)

# Support old naming convention
_get_connection = get_connection
_get_db = get_db