引擎是sqlalchemy的核心，不管是 sql core 還是orm的使用都需要依賴引擎的創建，為此我們研究下，引擎是如何創建的。
1 from sqlalchemy import create_engine 2 engine = create_engine('mysql+pymysql://root:x@127.0.0.1/test', 3 echo=True, # 設置為True，則輸出sql語句 4 pool_size=5, # 數據庫連接池初始化的容量 5 max_overflow=10, # 連接池最大溢出容量，該容量+初始容量=最大容量。超出會堵塞等待，等待時間為timeout參數值默認30 6 7 pool_recycle=7200 # 重連周期 8 )
create_engine 創建引擎對象，源代碼如下：
class PlainEngineStrategy(DefaultEngineStrategy):
"""Strategy for configuring a regular Engine."""
name = "plain"
engine_cls = base.Engine
PlainEngineStrategy()
這里有個參數 strategy：策略，一般情況默認是'plain',通過參數動態去實例策略類。我們看看對應默認的策略'plain'對應的類是哪個？
default_strategy = "plain"
def create_engine(*args, **kwargs):
strategy = kwargs.pop("strategy", default_strategy)
strategy = strategies.strategies[strategy]
return strategy.create(*args, **kwargs)
可以看到是PlainEngineStrategy(),接下來回到創建方法 strategy.create(*args, **kwargs),具體看看怎么創建的。
其實調用了父類DefaultEngineStrategy的方法create。
class DefaultEngineStrategy(EngineStrategy):
"""Base class for built-in strategies."""
def create(self, name_or_url, **kwargs):
# create url.URL object
u = url.make_url(name_or_url)
plugins = u._instantiate_plugins(kwargs)
u.query.pop("plugin", None)
kwargs.pop("plugins", None)
entrypoint = u._get_entrypoint()
dialect_cls = entrypoint.get_dialect_cls(u)
if kwargs.pop("_coerce_config", False):
def pop_kwarg(key, default=None):
value = kwargs.pop(key, default)
if key in dialect_cls.engine_config_types:
value = dialect_cls.engine_config_types[key](value)
return value
else:
pop_kwarg = kwargs.pop
dialect_args = {}
# consume dialect arguments from kwargs
for k in util.get_cls_kwargs(dialect_cls):
if k in kwargs:
dialect_args[k] = pop_kwarg(k)
dbapi = kwargs.pop("module", None)
if dbapi is None:
dbapi_args = {}
for k in util.get_func_kwargs(dialect_cls.dbapi):
if k in kwargs:
dbapi_args[k] = pop_kwarg(k)
dbapi = dialect_cls.dbapi(**dbapi_args)
dialect_args["dbapi"] = dbapi
for plugin in plugins:
plugin.handle_dialect_kwargs(dialect_cls, dialect_args)
# create dialect
dialect = dialect_cls(**dialect_args)
# assemble connection arguments
(cargs, cparams) = dialect.create_connect_args(u)
cparams.update(pop_kwarg("connect_args", {}))
cargs = list(cargs) # allow mutability
# look for existing pool or create
pool = pop_kwarg("pool", None)
if pool is None:
def connect(connection_record=None):
if dialect._has_events:
for fn in dialect.dispatch.do_connect:
connection = fn(
dialect, connection_record, cargs, cparams
)
if connection is not None:
return connection
return dialect.connect(*cargs, **cparams)
creator = pop_kwarg("creator", connect)
poolclass = pop_kwarg("poolclass", None)
if poolclass is None:
poolclass = dialect_cls.get_pool_class(u)
pool_args = {"dialect": dialect}
# consume pool arguments from kwargs, translating a few of
# the arguments
translate = {
"logging_name": "pool_logging_name",
"echo": "echo_pool",
"timeout": "pool_timeout",
"recycle": "pool_recycle",
"events": "pool_events",
"use_threadlocal": "pool_threadlocal",
"reset_on_return": "pool_reset_on_return",
"pre_ping": "pool_pre_ping",
"use_lifo": "pool_use_lifo",
}
for k in util.get_cls_kwargs(poolclass):
tk = translate.get(k, k)
if tk in kwargs:
pool_args[k] = pop_kwarg(tk)
for plugin in plugins:
plugin.handle_pool_kwargs(poolclass, pool_args)
pool = poolclass(creator, **pool_args)
else:
if isinstance(pool, poollib.dbapi_proxy._DBProxy):
pool = pool.get_pool(*cargs, **cparams)
else:
pool = pool
pool._dialect = dialect
# create engine.
engineclass = self.engine_cls
engine_args = {}
for k in util.get_cls_kwargs(engineclass):
if k in kwargs:
engine_args[k] = pop_kwarg(k)
_initialize = kwargs.pop("_initialize", True)
# all kwargs should be consumed
if kwargs:
raise TypeError(
"Invalid argument(s) %s sent to create_engine(), "
"using configuration %s/%s/%s. Please check that the "
"keyword arguments are appropriate for this combination "
"of components."
% (
",".join("'%s'" % k for k in kwargs),
dialect.__class__.__name__,
pool.__class__.__name__,
engineclass.__name__,
)
)
engine = engineclass(pool, dialect, u, **engine_args)
if _initialize:
do_on_connect = dialect.on_connect()
if do_on_connect:
def on_connect(dbapi_connection, connection_record):
conn = getattr(
dbapi_connection, "_sqla_unwrap", dbapi_connection
)
if conn is None:
return
do_on_connect(conn)
event.listen(pool, "first_connect", on_connect)
event.listen(pool, "connect", on_connect)
def first_connect(dbapi_connection, connection_record):
c = base.Connection(
engine, connection=dbapi_connection, _has_events=False
)
c._execution_options = util.immutabledict()
dialect.initialize(c)
dialect.do_rollback(c.connection)
event.listen(pool, "first_connect", first_connect, once=True)
dialect_cls.engine_created(engine)
if entrypoint is not dialect_cls:
entrypoint.engine_created(engine)
for plugin in plugins:
plugin.engine_created(engine)
return engine
我們逐一分析：
u = url.make_url(name_or_url) # 這個方法解析傳入的數據庫連接的uri信息，符合條件最終返回一個URL對象
plugins = u._instantiate_plugins(kwargs) # 插件初始化，
entrypoint = u._get_entrypoint() # 根據傳入url中的數據庫類型(mysql)和驅動庫(pymysql),來注冊插件，返回方言類
dialect_cls = entrypoint.get_dialect_cls(u) # 獲取Dialect類
這里需要說明下Dialect（方言類）的作用是用來定義數據庫和DBapi的行為
if kwargs.pop("_coerce_config", False):
def pop_kwarg(key, default=None):
value = kwargs.pop(key, default)
if key in dialect_cls.engine_config_types:
value = dialect_cls.engine_config_types[key](value)
return value
else:
pop_kwarg = kwargs.pop
dialect_args = {}
# consume dialect arguments from kwargs
for k in util.get_cls_kwargs(dialect_cls):
if k in kwargs:
dialect_args[k] = pop_kwarg(k)
這段代碼沒啥，創建出方言所需要的完整參數dialect_args
dbapi = kwargs.pop("module", None)
if dbapi is None:
dbapi_args = {}
for k in util.get_func_kwargs(dialect_cls.dbapi):
if k in kwargs:
dbapi_args[k] = pop_kwarg(k)
dbapi = dialect_cls.dbapi(**dbapi_args)
dialect_args["dbapi"] = dbapi
這段代碼 則是實例化dbpai對象。
# create dialect dialect = dialect_cls(**dialect_args)
開始實例化方言
(cargs, cparams) = dialect.create_connect_args(u)
cparams.update(pop_kwarg("connect_args", {}))
cargs = list(cargs) # allow mutability
創建連接所需要的參數
pool = pop_kwarg("pool", None)
if pool is None:
def connect(connection_record=None):
if dialect._has_events:
for fn in dialect.dispatch.do_connect:
connection = fn(
dialect, connection_record, cargs, cparams
)
if connection is not None:
return connection
return dialect.connect(*cargs, **cparams)
creator = pop_kwarg("creator", connect)
poolclass = pop_kwarg("poolclass", None)
if poolclass is None:
poolclass = dialect_cls.get_pool_class(u)
pool_args = {"dialect": dialect}
# consume pool arguments from kwargs, translating a few of
# the arguments
translate = {
"logging_name": "pool_logging_name",
"echo": "echo_pool",
"timeout": "pool_timeout",
"recycle": "pool_recycle",
"events": "pool_events",
"use_threadlocal": "pool_threadlocal",
"reset_on_return": "pool_reset_on_return",
"pre_ping": "pool_pre_ping",
"use_lifo": "pool_use_lifo",
}
for k in util.get_cls_kwargs(poolclass):
tk = translate.get(k, k)
if tk in kwargs:
pool_args[k] = pop_kwarg(tk)
for plugin in plugins:
plugin.handle_pool_kwargs(poolclass, pool_args)
pool = poolclass(creator, **pool_args)
else:
if isinstance(pool, poollib.dbapi_proxy._DBProxy):
pool = pool.get_pool(*cargs, **cparams)
else:
pool = pool
pool._dialect = dialect
創建連接池，默認創建pool.QueuePool
# create engine.
engineclass = self.engine_cls
engine_args = {}
for k in util.get_cls_kwargs(engineclass):
if k in kwargs:
engine_args[k] = pop_kwarg(k)
_initialize = kwargs.pop("_initialize", True)
# all kwargs should be consumed
if kwargs:
raise TypeError(
"Invalid argument(s) %s sent to create_engine(), "
"using configuration %s/%s/%s. Please check that the "
"keyword arguments are appropriate for this combination "
"of components."
% (
",".join("'%s'" % k for k in kwargs),
dialect.__class__.__name__,
pool.__class__.__name__,
engineclass.__name__,
)
)
engine = engineclass(pool, dialect, u, **engine_args)
從上面可以看出來，引擎的核心是連接池和方言，連接池負責連接的維護，方言負責數據的行為。
if _initialize:
do_on_connect = dialect.on_connect()
if do_on_connect:
def on_connect(dbapi_connection, connection_record):
conn = getattr(
dbapi_connection, "_sqla_unwrap", dbapi_connection
)
if conn is None:
return
do_on_connect(conn)
event.listen(pool, "first_connect", on_connect)
event.listen(pool, "connect", on_connect)
def first_connect(dbapi_connection, connection_record):
c = base.Connection(
engine, connection=dbapi_connection, _has_events=False
)
c._execution_options = util.immutabledict()
dialect.initialize(c)
dialect.do_rollback(c.connection)
event.listen(pool, "first_connect", first_connect, once=True)
第一次初始化連接並進行監聽
dialect_cls.engine_created(engine)
if entrypoint is not dialect_cls:
entrypoint.engine_created(engine)
for plugin in plugins:
plugin.engine_created(engine)
總結：
create_engine通過傳入的URI和相關參數，創建一個Engine，該引擎包含了方言（Dialect)和Pool，Dialect如中文名翻譯一樣，方言：作為不同的數據庫Mysql，Oracle，PostgreSQL等，會有不同的行為，Dialect就是用來操作不同數據庫的行為，對應接口調用dbapi操作。
而Pool作為數據庫連接池，用來管理數據庫連接，通過維護一個連接池，池子的大小，數量和生命周期，減少數據庫連接的頻繁切換，提高查詢等操作效率。