Release: 1.2.8 | Release Date: May 28, 2018

# SQLAlchemy 1.2 Documentation

# Connection Pooling

A connection pool is a standard technique used to maintain long running connections in memory for efficient re-use, as well as to provide management for the total number of connections an application might use simultaneously.

Particularly for server-side web applications, a connection pool is the standard way to maintain a “pool” of active database connections in memory which are reused across requests.

SQLAlchemy includes several connection pool implementations which integrate with the `Engine`. They can also be used directly for applications that want to add pooling to an otherwise plain DBAPI approach.

## Connection Pool Configuration

The `Engine` returned by the `create_engine()` function in most cases has a `QueuePool` integrated, pre-configured with reasonable pooling defaults. If you’re reading this section only to learn how to enable pooling - congratulations! You’re already done.

The most common `QueuePool` tuning parameters can be passed directly to `create_engine()` as keyword arguments: `pool_size`, `max_overflow`, `pool_recycle` and `pool_timeout`. For example:

    engine = create_engine('postgresql://me@localhost/mydb',
                           pool_size=20, max_overflow=0)

In the case of SQLite, the `SingletonThreadPool` or `NullPool` are selected by the dialect to provide greater compatibility with SQLite’s threading and locking model, as well as to provide a reasonable default behavior to SQLite “memory” databases, which maintain their entire dataset within the scope of a single connection.

All SQLAlchemy pool implementations have in common that none of them “pre create” connections - all implementations wait until first use before creating a connection. At that point, if no additional concurrent checkout requests for more connections are made, no additional connections are created. This is why it’s perfectly fine for `create_engine()` to default to using a `QueuePool` of size five without regard to whether or not the application really needs five connections queued up - the pool would only grow to that size if the application actually used five connections concurrently, in which case the usage of a small pool is an entirely appropriate default behavior.

## Switching Pool Implementations

The usual way to use a different kind of pool with `create_engine()` is to use the `poolclass` argument. This argument accepts a class imported from the `sqlalchemy.pool` module, and handles the details of building the pool for you. Common options include specifying `QueuePool` with SQLite:

    from sqlalchemy.pool import QueuePool
    engine = create_engine('sqlite:///file.db', poolclass=QueuePool)

Disabling pooling using `NullPool`:

    from sqlalchemy.pool import NullPool
    engine = create_engine(
              'postgresql+psycopg2://scott:tiger@localhost/test',
              poolclass=NullPool)

## Using a Custom Connection Function

All `Pool` classes accept an argument `creator` which is a callable that creates a new connection. `create_engine()` accepts this function to pass onto the pool via an argument of the same name:

    import sqlalchemy.pool as pool
    import psycopg2

    def getconn():
        c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
        # do things with 'c' to set up
        return c

    engine = create_engine('postgresql+psycopg2://', creator=getconn)

For most “initialize on connection” routines, it’s more convenient to use the `PoolEvents` event hooks, so that the usual URL argument to `create_engine()` is still usable. `creator` is there as a last resort for when a DBAPI has some form of `connect` that is not at all supported by SQLAlchemy.

## Constructing a Pool

To use a `Pool` by itself, the `creator` function is the only argument that’s required and is passed first, followed by any additional options:

    import sqlalchemy.pool as pool
    import psycopg2

    def getconn():
        c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
        return c

    mypool = pool.QueuePool(getconn, max_overflow=10, pool_size=5)

DBAPI connections can then be procured from the pool using the `Pool.connect()` function. The return value of this method is a DBAPI connection that’s contained within a transparent proxy:

    # get a connection
    conn = mypool.connect()

    # use it
    cursor = conn.cursor()
    cursor.execute("select foo")

The purpose of the transparent proxy is to intercept the `close()` call, such that instead of the DBAPI connection being closed, it is returned to the pool:

    # "close" the connection.  Returns
    # it to the pool.
    conn.close()

The proxy also returns its contained DBAPI connection to the pool when it is garbage collected, though it’s not deterministic in Python that this occurs immediately (though it is typical with cPython).

The `close()` step also performs the important step of calling the `rollback()` method of the DBAPI connection. This is so that any existing transaction on the connection is removed, not only ensuring that no existing state remains on next usage, but also so that table and row locks are released as well as that any isolated data snapshots are removed. This behavior can be disabled using the `reset_on_return` option of `Pool`.

A particular pre-created `Pool` can be shared with one or more engines by passing it to the `pool` argument of `create_engine()`:

    e = create_engine('postgresql://', pool=mypool)

## Pool Events

Connection pools support an event interface that allows hooks to execute upon first connect, upon each new connection, and upon checkout and checkin of connections. See `PoolEvents` for details.

## Dealing with Disconnects

The connection pool has the ability to refresh individual connections as well as its entire set of connections, setting the previously pooled connections as “invalid”. A common use case is allow the connection pool to gracefully recover when the database server has been restarted, and all previously established connections are no longer functional. There are two approaches to this.

### Disconnect Handling - Pessimistic

The pessimistic approach refers to emitting a test statement on the SQL connection at the start of each connection pool checkout, to test that the database connection is still viable. Typically, this is a simple statement like “SELECT 1”, but may also make use of some DBAPI-specific method to test the connection for liveness.

The approach adds a small bit of overhead to the connection checkout process, however is otherwise the most simple and reliable approach to completely eliminating database errors due to stale pooled connections. The calling application does not need to be concerned about organizing operations to be able to recover from stale connections checked out from the pool.

It is critical to note that the pre-ping approach does not accommodate for connections dropped in the middle of transactions or other SQL operations. If the database becomes unavailable while a transaction is in progress, the transaction will be lost and the database error will be raised. While the `Connection` object will detect a “disconnect” situation and recycle the connection as well as invalidate the rest of the connection pool when this condition occurs, the individual operation where the exception was raised will be lost, and it’s up to the application to either abandon the operation, or retry the whole transaction again.

Pessimistic testing of connections upon checkout is achievable by using the `Pool.pre_ping` argument, available from `create_engine()` via the `create_engine.pool_pre_ping` argument:

    engine = create_engine("mysql+pymysql://user:pw@host/db", pool_pre_ping=True)

The “pre ping” feature will normally emit SQL equivalent to “SELECT 1” each time a connection is checked out from the pool; if an error is raised that is detected as a “disconnect” situation, the connection will be immediately recycled, and all other pooled connections older than the current time are invalidated, so that the next time they are checked out, they will also be recycled before use.

If the database is still not available when “pre ping” runs, then the initial connect will fail and the error for failure to connect will be propagated normally. In the uncommon situation that the database is available for connections, but is not able to respond to a “ping”, the “pre_ping” will try up to three times before giving up, propagating the database error last received.

Note

the “SELECT 1” emitted by “pre-ping” is invoked within the scope of the connection pool / dialect, using a very short codepath for minimal Python latency. As such, this statement is not logged in the SQL echo output, and will not show up in SQLAlchemy’s engine logging.

New in version 1.2: Added “pre-ping” capability to the `Pool` class.

#### Custom / Legacy Pessimistic Ping

Before `create_engine.pool_pre_ping` was added, the “pre-ping” approach historically has been performed manually using the `ConnectionEvents.engine_connect()` engine event. The most common recipe for this is below, for reference purposes in case an application is already using such a recipe, or special behaviors are needed:

    from sqlalchemy import exc
    from sqlalchemy import event
    from sqlalchemy import select

    some_engine = create_engine(...)

    @event.listens_for(some_engine, "engine_connect")
    def ping_connection(connection, branch):
        if branch:
            # "branch" refers to a sub-connection of a connection,
            # we don't want to bother pinging on these.
            return

        # turn off "close with result".  This flag is only used with
        # "connectionless" execution, otherwise will be False in any case
        save_should_close_with_result = connection.should_close_with_result
        connection.should_close_with_result = False

        try:
            # run a SELECT 1.   use a core select() so that
            # the SELECT of a scalar value without a table is
            # appropriately formatted for the backend
            connection.scalar(select([1]))
        except exc.DBAPIError as err:
            # catch SQLAlchemy's DBAPIError, which is a wrapper
            # for the DBAPI's exception.  It includes a .connection_invalidated
            # attribute which specifies if this connection is a "disconnect"
            # condition, which is based on inspection of the original exception
            # by the dialect in use.
            if err.connection_invalidated:
                # run the same SELECT again - the connection will re-validate
                # itself and establish a new connection.  The disconnect detection
                # here also causes the whole connection pool to be invalidated
                # so that all stale connections are discarded.
                connection.scalar(select([1]))
            else:
                raise
        finally:
            # restore "close with result"
            connection.should_close_with_result = save_should_close_with_result

The above recipe has the advantage that we are making use of SQLAlchemy’s facilities for detecting those DBAPI exceptions that are known to indicate a “disconnect” situation, as well as the `Engine` object’s ability to correctly invalidate the current connection pool when this condition occurs and allowing the current `Connection` to re-validate onto a new DBAPI connection.

### Disconnect Handling - Optimistic

When pessimistic handling is not employed, as well as when the database is shutdown and/or restarted in the middle of a connection’s period of use within a transaction, the other approach to dealing with stale / closed connections is to let SQLAlchemy handle disconnects as they occur, at which point all connections in the pool are invalidated, meaning they are assumed to be stale and will be refreshed upon next checkout. This behavior assumes the `Pool` is used in conjunction with a `Engine`. The `Engine` has logic which can detect disconnection events and refresh the pool automatically.

When the `Connection` attempts to use a DBAPI connection, and an exception is raised that corresponds to a “disconnect” event, the connection is invalidated. The `Connection` then calls the `Pool.recreate()` method, effectively invalidating all connections not currently checked out so that they are replaced with new ones upon next checkout. This flow is illustrated by the code example below:

    from sqlalchemy import create_engine, exc
    e = create_engine(...)
    c = e.connect()

    try:
        # suppose the database has been restarted.
        c.execute("SELECT * FROM table")
        c.close()
    except exc.DBAPIError, e:
        # an exception is raised, Connection is invalidated.
        if e.connection_invalidated:
            print("Connection was invalidated!")

    # after the invalidate event, a new connection
    # starts with a new Pool
    c = e.connect()
    c.execute("SELECT * FROM table")

The above example illustrates that no special intervention is needed to refresh the pool, which continues normally after a disconnection event is detected. However, one database exception is raised, per each connection that is in use while the database unavailability event occurred. In a typical web application using an ORM Session, the above condition would correspond to a single request failing with a 500 error, then the web application continuing normally beyond that. Hence the approach is “optimistic” in that frequent database restarts are not anticipated.

#### Setting Pool Recycle

An additional setting that can augment the “optimistic” approach is to set the pool recycle parameter. This parameter prevents the pool from using a particular connection that has passed a certain age, and is appropriate for database backends such as MySQL that automatically close connections that have been stale after a particular period of time:

    from sqlalchemy import create_engine
    e = create_engine("mysql://scott:tiger@localhost/test", pool_recycle=3600)

Above, any DBAPI connection that has been open for more than one hour will be invalidated and replaced, upon next checkout. Note that the invalidation only occurs during checkout - not on any connections that are held in a checked out state. `pool_recycle` is a function of the `Pool` itself, independent of whether or not an `Engine` is in use.

### More on Invalidation

The `Pool` provides “connection invalidation” services which allow both explicit invalidation of a connection as well as automatic invalidation in response to conditions that are determined to render a connection unusable.

“Invalidation” means that a particular DBAPI connection is removed from the pool and discarded. The `.close()` method is called on this connection if it is not clear that the connection itself might not be closed, however if this method fails, the exception is logged but the operation still proceeds.

When using a `Engine`, the `Connection.invalidate()` method is the usual entrypoint to explicit invalidation. Other conditions by which a DBAPI connection might be invalidated include:

- a DBAPI exception such as `OperationalError`, raised when a method like `connection.execute()` is called, is detected as indicating a so-called “disconnect” condition. As the Python DBAPI provides no standard system for determining the nature of an exception, all SQLAlchemy dialects include a system called `is_disconnect()` which will examine the contents of an exception object, including the string message and any potential error codes included with it, in order to determine if this exception indicates that the connection is no longer usable. If this is the case, the `_ConnectionFairy.invalidate()` method is called and the DBAPI connection is then discarded.
- When the connection is returned to the pool, and calling the `connection.rollback()` or `connection.commit()` methods, as dictated by the pool’s “reset on return” behavior, throws an exception. A final attempt at calling `.close()` on the connection will be made, and it is then discarded.
- When a listener implementing `PoolEvents.checkout()` raises the `DisconnectionError` exception, indicating that the connection won’t be usable and a new connection attempt needs to be made.

All invalidations which occur will invoke the `PoolEvents.invalidate()` event.

## Using Connection Pools with Multiprocessing

It’s critical that when using a connection pool, and by extension when using an `Engine` created via `create_engine()`, that the pooled connections are not shared to a forked process. TCP connections are represented as file descriptors, which usually work across process boundaries, meaning this will cause concurrent access to the file descriptor on behalf of two or more entirely independent Python interpreter states.

There are two approaches to dealing with this.

The first is, either create a new `Engine` within the child process, or upon an existing `Engine`, call `Engine.dispose()` before the child process uses any connections. This will remove all existing connections from the pool so that it makes all new ones. Below is a simple version using `multiprocessing.Process`, but this idea should be adapted to the style of forking in use:

    eng = create_engine("...")

    def run_in_process():
      eng.dispose()

      with eng.connect() as conn:
          conn.execute("...")

    p = Process(target=run_in_process)

The next approach is to instrument the `Pool` itself with events so that connections are automatically invalidated in the subprocess. This is a little more magical but probably more foolproof:

    from sqlalchemy import event
    from sqlalchemy import exc
    import os

    eng = create_engine("...")

    @event.listens_for(engine, "connect")
    def connect(dbapi_connection, connection_record):
        connection_record.info['pid'] = os.getpid()

    @event.listens_for(engine, "checkout")
    def checkout(dbapi_connection, connection_record, connection_proxy):
        pid = os.getpid()
        if connection_record.info['pid'] != pid:
            connection_record.connection = connection_proxy.connection = None
            raise exc.DisconnectionError(
                    "Connection record belongs to pid %s, "
                    "attempting to check out in pid %s" %
                    (connection_record.info['pid'], pid)
            )

Above, we use an approach similar to that described in Disconnect Handling - Pessimistic to treat a DBAPI connection that originated in a different parent process as an “invalid” connection, coercing the pool to recycle the connection record to make a new connection.

## API Documentation - Available Pool Implementations

class `sqlalchemy.pool.``Pool`(creator, recycle=-1, echo=None, use_threadlocal=False, logging_name=None, reset_on_return=True, listeners=None, events=None, dialect=None, pre_ping=False, _dispatch=None)

  Bases: `sqlalchemy.log.Identified`

  Abstract base class for connection pools.

  `__init__`(creator, recycle=-1, echo=None, use_threadlocal=False, logging_name=None, reset_on_return=True, listeners=None, events=None, dialect=None, pre_ping=False, _dispatch=None)

    Construct a Pool.

    Parameters:

    - creator – a callable function that returns a DB-API connection object. The function will be called with parameters.
    - recycle – If set to non -1, number of seconds between connection recycling, which means upon checkout, if this timeout is surpassed the connection will be closed and replaced with a newly opened connection. Defaults to -1.
    - logging_name – String identifier which will be used within the “name” field of logging records generated within the “sqlalchemy.pool” logger. Defaults to a hexstring of the object’s id.
    - echo – If True, connections being pulled and retrieved from the pool will be logged to the standard output, as well as pool sizing information. Echoing can also be achieved by enabling logging for the “sqlalchemy.pool” namespace. Defaults to False.
    - use_threadlocal –

      If set to True, repeated calls to `connect()` within the same application thread will be guaranteed to return the same connection object, if one has already been retrieved from the pool and has not been returned yet. Offers a slight performance advantage at the cost of individual transactions by default. The `Pool.unique_connection()` method is provided to return a consistently unique connection to bypass this behavior when the flag is set.

      Warning

      The `Pool.use_threadlocal` flag does not affect the behavior of `Engine.connect()`. `Engine.connect()` makes use of the `Pool.unique_connection()` method which does not use thread local context. To produce a `Connection` which refers to the `Pool.connect()` method, use `Engine.contextual_connect()`.

      Note that other SQLAlchemy connectivity systems such as `Engine.execute()` as well as the orm `Session` make use of `Engine.contextual_connect()` internally, so these functions are compatible with the `Pool.use_threadlocal` setting.

      See also

      Using the Threadlocal Execution Strategy - contains detail on the “threadlocal” engine strategy, which provides a more comprehensive approach to “threadlocal” connectivity for the specific use case of using `Engine` and `Connection` objects directly.

    - reset_on_return –

      Determine steps to take on connections as they are returned to the pool. reset_on_return can have any of these values:

      - `"rollback"` - call rollback() on the connection, to release locks and transaction resources. This is the default value. The vast majority of use cases should leave this value set.
      - `True` - same as ‘rollback’, this is here for backwards compatibility.
      - `"commit"` - call commit() on the connection, to release locks and transaction resources. A commit here may be desirable for databases that cache query plans if a commit is emitted, such as Microsoft SQL Server. However, this value is more dangerous than ‘rollback’ because any data changes present on the transaction are committed unconditionally.
      - `None` - don’t do anything on the connection. This setting should only be made on a database that has no transaction support at all, namely MySQL MyISAM. By not doing anything, performance can be improved. This setting should never be selected for a database that supports transactions, as it will lead to deadlocks and stale state.
      - `"none"` - same as `None`

        New in version 0.9.10.

      - `False` - same as None, this is here for backwards compatibility.

      Changed in version 0.7.6: `Pool.reset_on_return` accepts `"rollback"` and `"commit"` arguments.

    - events – a list of 2-tuples, each of the form `(callable, target)` which will be passed to `event.listen()` upon construction. Provided here so that event listeners can be assigned via `create_engine()` before dialect-level listeners are applied.
    - listeners – Deprecated. A list of `PoolListener`-like objects or dictionaries of callables that receive events when DB-API connections are created, checked out and checked in to the pool. This has been superseded by `listen()`.
    - dialect –

      a `Dialect` that will handle the job of calling rollback(), close(), or commit() on DBAPI connections. If omitted, a built-in “stub” dialect is used. Applications that make use of `create_engine()` should not use this parameter as it is handled by the engine creation strategy.

      New in version 1.1: - `dialect` is now a public parameter to the `Pool`.

    - pre_ping –

      if True, the pool will emit a “ping” (typically “SELECT 1”, but is dialect-specific) on the connection upon checkout, to test if the connection is alive or not. If not, the connection is transparently re-connected and upon success, all other pooled connections established prior to that timestamp are invalidated. Requires that a dialect is passed as well to interpret the disconnection error.

      New in version 1.2.

  `connect`()

    Return a DBAPI connection from the pool.

    The connection is instrumented such that when its `close()` method is called, the connection will be returned to the pool.

  `dispose`()

    Dispose of this pool.

    This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.

    See also the `Pool.recreate()` method.

  `recreate`()

    Return a new `Pool`, of the same class as this one and configured with identical creation arguments.

    This method is used in conjunction with `dispose()` to close out an entire `Pool` and create a new one in its place.

  `unique_connection`()

    Produce a DBAPI connection that is not referenced by any thread-local context.

    This method is equivalent to `Pool.connect()` when the `Pool.use_threadlocal` flag is not set to True. When `Pool.use_threadlocal` is True, the `Pool.unique_connection()` method provides a means of bypassing the threadlocal context.

class `sqlalchemy.pool.``QueuePool`(creator, pool_size=5, max_overflow=10, timeout=30, **kw)

  Bases: `sqlalchemy.pool.Pool`

  A `Pool` that imposes a limit on the number of open connections.

  `QueuePool` is the default pooling implementation used for all `Engine` objects, unless the SQLite dialect is in use.

  `__init__`(creator, pool_size=5, max_overflow=10, timeout=30, **kw)

    Construct a QueuePool.

    Parameters:

    - creator – a callable function that returns a DB-API connection object, same as that of `Pool.creator`.
    - pool_size – The size of the pool to be maintained, defaults to 5. This is the largest number of connections that will be kept persistently in the pool. Note that the pool begins with no connections; once this number of connections is requested, that number of connections will remain. `pool_size` can be set to 0 to indicate no size limit; to disable pooling, use a `NullPool` instead.
    - max_overflow – The maximum overflow size of the pool. When the number of checked-out connections reaches the size set in pool_size, additional connections will be returned up to this limit. When those additional connections are returned to the pool, they are disconnected and discarded. It follows then that the total number of simultaneous connections the pool will allow is pool_size + max_overflow, and the total number of “sleeping” connections the pool will allow is pool_size. max_overflow can be set to -1 to indicate no overflow limit; no limit will be placed on the total number of concurrent connections. Defaults to 10.
    - timeout – The number of seconds to wait before giving up on returning a connection. Defaults to 30.
    - **kw – Other keyword arguments including `Pool.recycle`, `Pool.echo`, `Pool.reset_on_return` and others are passed to the `Pool` constructor.

  `connect`()
    inherited from the `connect()` method of `Pool`

    Return a DBAPI connection from the pool.

    The connection is instrumented such that when its `close()` method is called, the connection will be returned to the pool.

  `unique_connection`()
    inherited from the `unique_connection()` method of `Pool`

    Produce a DBAPI connection that is not referenced by any thread-local context.

    This method is equivalent to `Pool.connect()` when the `Pool.use_threadlocal` flag is not set to True. When `Pool.use_threadlocal` is True, the `Pool.unique_connection()` method provides a means of bypassing the threadlocal context.

class `sqlalchemy.pool.``SingletonThreadPool`(creator, pool_size=5, **kw)

  Bases: `sqlalchemy.pool.Pool`

  A Pool that maintains one connection per thread.

  Maintains one connection per each thread, never moving a connection to a thread other than the one which it was created in.

  Warning

  the `SingletonThreadPool` will call `.close()` on arbitrary connections that exist beyond the size setting of `pool_size`, e.g. if more unique thread identities than what `pool_size` states are used. This cleanup is non-deterministic and not sensitive to whether or not the connections linked to those thread identities are currently in use.

  `SingletonThreadPool` may be improved in a future release, however in its current status it is generally used only for test scenarios using a SQLite `:memory:` database and is not recommended for production use.

  Options are the same as those of `Pool`, as well as:

  Parameters: | pool_size – The number of threads in which to maintain connections at once. Defaults to five.

  `SingletonThreadPool` is used by the SQLite dialect automatically when a memory-based database is used. See SQLite.

  `__init__`(creator, pool_size=5, **kw)

    Construct a Pool.

    Parameters:

    - creator – a callable function that returns a DB-API connection object. The function will be called with parameters.
    - recycle – If set to non -1, number of seconds between connection recycling, which means upon checkout, if this timeout is surpassed the connection will be closed and replaced with a newly opened connection. Defaults to -1.
    - logging_name – String identifier which will be used within the “name” field of logging records generated within the “sqlalchemy.pool” logger. Defaults to a hexstring of the object’s id.
    - echo – If True, connections being pulled and retrieved from the pool will be logged to the standard output, as well as pool sizing information. Echoing can also be achieved by enabling logging for the “sqlalchemy.pool” namespace. Defaults to False.
    - use_threadlocal –

      If set to True, repeated calls to `connect()` within the same application thread will be guaranteed to return the same connection object, if one has already been retrieved from the pool and has not been returned yet. Offers a slight performance advantage at the cost of individual transactions by default. The `Pool.unique_connection()` method is provided to return a consistently unique connection to bypass this behavior when the flag is set.

      Warning

      The `Pool.use_threadlocal` flag does not affect the behavior of `Engine.connect()`. `Engine.connect()` makes use of the `Pool.unique_connection()` method which does not use thread local context. To produce a `Connection` which refers to the `Pool.connect()` method, use `Engine.contextual_connect()`.

      Note that other SQLAlchemy connectivity systems such as `Engine.execute()` as well as the orm `Session` make use of `Engine.contextual_connect()` internally, so these functions are compatible with the `Pool.use_threadlocal` setting.

      See also

      Using the Threadlocal Execution Strategy - contains detail on the “threadlocal” engine strategy, which provides a more comprehensive approach to “threadlocal” connectivity for the specific use case of using `Engine` and `Connection` objects directly.

    - reset_on_return –

      Determine steps to take on connections as they are returned to the pool. reset_on_return can have any of these values:

      - `"rollback"` - call rollback() on the connection, to release locks and transaction resources. This is the default value. The vast majority of use cases should leave this value set.
      - `True` - same as ‘rollback’, this is here for backwards compatibility.
      - `"commit"` - call commit() on the connection, to release locks and transaction resources. A commit here may be desirable for databases that cache query plans if a commit is emitted, such as Microsoft SQL Server. However, this value is more dangerous than ‘rollback’ because any data changes present on the transaction are committed unconditionally.
      - `None` - don’t do anything on the connection. This setting should only be made on a database that has no transaction support at all, namely MySQL MyISAM. By not doing anything, performance can be improved. This setting should never be selected for a database that supports transactions, as it will lead to deadlocks and stale state.
      - `"none"` - same as `None`

        New in version 0.9.10.

      - `False` - same as None, this is here for backwards compatibility.

      Changed in version 0.7.6: `Pool.reset_on_return` accepts `"rollback"` and `"commit"` arguments.

    - events – a list of 2-tuples, each of the form `(callable, target)` which will be passed to `event.listen()` upon construction. Provided here so that event listeners can be assigned via `create_engine()` before dialect-level listeners are applied.
    - listeners – Deprecated. A list of `PoolListener`-like objects or dictionaries of callables that receive events when DB-API connections are created, checked out and checked in to the pool. This has been superseded by `listen()`.
    - dialect –

      a `Dialect` that will handle the job of calling rollback(), close(), or commit() on DBAPI connections. If omitted, a built-in “stub” dialect is used. Applications that make use of `create_engine()` should not use this parameter as it is handled by the engine creation strategy.

      New in version 1.1: - `dialect` is now a public parameter to the `Pool`.

    - pre_ping –

      if True, the pool will emit a “ping” (typically “SELECT 1”, but is dialect-specific) on the connection upon checkout, to test if the connection is alive or not. If not, the connection is transparently re-connected and upon success, all other pooled connections established prior to that timestamp are invalidated. Requires that a dialect is passed as well to interpret the disconnection error.

      New in version 1.2.

class `sqlalchemy.pool.``AssertionPool`(*args, **kw)

  Bases: `sqlalchemy.pool.Pool`

  A `Pool` that allows at most one checked out connection at any given time.

  This will raise an exception if more than one connection is checked out at a time. Useful for debugging code that is using more connections than desired.

  Changed in version 0.7: `AssertionPool` also logs a traceback of where the original connection was checked out, and reports this in the assertion error raised.

class `sqlalchemy.pool.``NullPool`(creator, recycle=-1, echo=None, use_threadlocal=False, logging_name=None, reset_on_return=True, listeners=None, events=None, dialect=None, pre_ping=False, _dispatch=None)

  Bases: `sqlalchemy.pool.Pool`

  A Pool which does not pool connections.

  Instead it literally opens and closes the underlying DB-API connection per each connection open/close.

  Reconnect-related functions such as `recycle` and connection invalidation are not supported by this Pool implementation, since no connections are held persistently.

  Changed in version 0.7: `NullPool` is used by the SQlite dialect automatically when a file-based database is used. See SQLite.

class `sqlalchemy.pool.``StaticPool`(creator, recycle=-1, echo=None, use_threadlocal=False, logging_name=None, reset_on_return=True, listeners=None, events=None, dialect=None, pre_ping=False, _dispatch=None)

  Bases: `sqlalchemy.pool.Pool`

  A Pool of exactly one connection, used for all requests.

  Reconnect-related functions such as `recycle` and connection invalidation (which is also used to support auto-reconnect) are not currently supported by this Pool implementation but may be implemented in a future release.

class `sqlalchemy.pool.``_ConnectionFairy`(dbapi_connection, connection_record, echo)

  Proxies a DBAPI connection and provides return-on-dereference support.

  This is an internal object used by the `Pool` implementation to provide context management to a DBAPI connection delivered by that `Pool`.

  The name “fairy” is inspired by the fact that the `_ConnectionFairy` object’s lifespan is transitory, as it lasts only for the length of a specific DBAPI connection being checked out from the pool, and additionally that as a transparent proxy, it is mostly invisible.

  See also

  `_ConnectionRecord`

  `_connection_record` = None

    A reference to the `_ConnectionRecord` object associated with the DBAPI connection.

    This is currently an internal accessor which is subject to change.

  `connection` = None

    A reference to the actual DBAPI connection being tracked.

  `cursor`(*args, **kwargs)

    Return a new DBAPI cursor for the underlying connection.

    This method is a proxy for the `connection.cursor()` DBAPI method.

  `detach`()

    Separate this connection from its Pool.

    This means that the connection will no longer be returned to the pool when closed, and will instead be literally closed. The containing ConnectionRecord is separated from the DB-API connection, and will create a new connection when next used.

    Note that any overall connection limiting constraints imposed by a Pool implementation may be violated after a detach, as the detached connection is removed from the pool’s knowledge and control.

  `info`

    Info dictionary associated with the underlying DBAPI connection referred to by this `ConnectionFairy`, allowing user-defined data to be associated with the connection.

    The data here will follow along with the DBAPI connection including after it is returned to the connection pool and used again in subsequent instances of `_ConnectionFairy`. It is shared with the `_ConnectionRecord.info` and `Connection.info` accessors.

    The dictionary associated with a particular DBAPI connection is discarded when the connection itself is discarded.

  `invalidate`(e=None, soft=False)

    Mark this connection as invalidated.

    This method can be called directly, and is also called as a result of the `Connection.invalidate()` method. When invoked, the DBAPI connection is immediately closed and discarded from further use by the pool. The invalidation mechanism proceeds via the `_ConnectionRecord.invalidate()` internal method.

    Parameters:

    - e – an exception object indicating a reason for the invalidation.
    - soft –

      if True, the connection isn’t closed; instead, this connection will be recycled on next checkout.

      New in version 1.0.3.

    See also

    More on Invalidation

  `is_valid`

    Return True if this `_ConnectionFairy` still refers to an active DBAPI connection.

  `record_info`

    Info dictionary associated with the `_ConnectionRecord container referred to by this :class:`.ConnectionFairy`.

    Unlike the `_ConnectionFairy.info` dictionary, the lifespan of this dictionary is persistent across connections that are disconnected and/or invalidated within the lifespan of a `_ConnectionRecord`.

    New in version 1.1.

class `sqlalchemy.pool.``_ConnectionRecord`(pool, connect=True)

  Internal object which maintains an individual DBAPI connection referenced by a `Pool`.

  The `_ConnectionRecord` object always exists for any particular DBAPI connection whether or not that DBAPI connection has been “checked out”. This is in contrast to the `_ConnectionFairy` which is only a public facade to the DBAPI connection while it is checked out.

  A `_ConnectionRecord` may exist for a span longer than that of a single DBAPI connection. For example, if the `_ConnectionRecord.invalidate()` method is called, the DBAPI connection associated with this `_ConnectionRecord` will be discarded, but the `_ConnectionRecord` may be used again, in which case a new DBAPI connection is produced when the `Pool` next uses this record.

  The `_ConnectionRecord` is delivered along with connection pool events, including `PoolEvents.connect()` and `PoolEvents.checkout()`, however `_ConnectionRecord` still remains an internal object whose API and internals may change.

  See also

  `_ConnectionFairy`

  `connection` = None

    A reference to the actual DBAPI connection being tracked.

    May be `None` if this `_ConnectionRecord` has been marked as invalidated; a new DBAPI connection may replace it if the owning pool calls upon this `_ConnectionRecord` to reconnect.

  `info`

    The `.info` dictionary associated with the DBAPI connection.

    This dictionary is shared among the `_ConnectionFairy.info` and `Connection.info` accessors.

    Note

    The lifespan of this dictionary is linked to the DBAPI connection itself, meaning that it is discarded each time the DBAPI connection is closed and/or invalidated. The `_ConnectionRecord.record_info` dictionary remains persistent throughout the lifespan of the `_ConnectionRecord` container.

  `invalidate`(e=None, soft=False)

    Invalidate the DBAPI connection held by this `_ConnectionRecord`.

    This method is called for all connection invalidations, including when the `_ConnectionFairy.invalidate()` or `Connection.invalidate()` methods are called, as well as when any so-called “automatic invalidation” condition occurs.

    Parameters:

    - e – an exception object indicating a reason for the invalidation.
    - soft –

      if True, the connection isn’t closed; instead, this connection will be recycled on next checkout.

      New in version 1.0.3.

    See also

    More on Invalidation

  `record_info`

    An “info’ dictionary associated with the connection record itself.

    Unlike the `_ConnectionRecord.info` dictionary, which is linked to the lifespan of the DBAPI connection, this dictionary is linked to the lifespan of the `_ConnectionRecord` container itself and will remain persisent throughout the life of the `_ConnectionRecord`.

    New in version 1.1.

## Pooling Plain DB-API Connections

Any PEP 249 DB-API module can be “proxied” through the connection pool transparently. Usage of the DB-API is exactly as before, except the `connect()` method will consult the pool. Below we illustrate this with `psycopg2`:

    import sqlalchemy.pool as pool
    import psycopg2 as psycopg

    psycopg = pool.manage(psycopg)

    # then connect normally
    connection = psycopg.connect(database='test', username='scott',
                                 password='tiger')

This produces a `_DBProxy` object which supports the same `connect()` function as the original DB-API module. Upon connection, a connection proxy object is returned, which delegates its calls to a real DB-API connection object. This connection object is stored persistently within a connection pool (an instance of `Pool`) that corresponds to the exact connection arguments sent to the `connect()` function.

The connection proxy supports all of the methods on the original connection object, most of which are proxied via `__getattr__()`. The `close()` method will return the connection to the pool, and the `cursor()` method will return a proxied cursor object. Both the connection proxy and the cursor proxy will also return the underlying connection to the pool after they have both been garbage collected, which is detected via weakref callbacks (`__del__` is not used).

Additionally, when connections are returned to the pool, a `rollback()` is issued on the connection unconditionally. This is to release any locks still held by the connection that may have resulted from normal activity.

By default, the `connect()` method will return the same connection that is already checked out in the current thread. This allows a particular connection to be used in a given thread without needing to pass it around between functions. To disable this behavior, specify `use_threadlocal=False` to the `manage()` function.

`sqlalchemy.pool.``manage`(module, **params)

  Return a proxy for a DB-API module that automatically pools connections.

  Given a DB-API 2.0 module and pool management parameters, returns a proxy for the module that will automatically pool connections, creating new connection pools for each distinct set of connection arguments sent to the decorated module’s connect() function.

  Parameters:

  - module – a DB-API 2.0 database module
  - poolclass – the class used by the pool module to provide pooling. Defaults to `QueuePool`.
  - **params – will be passed through to poolclass

`sqlalchemy.pool.``clear_managers`()

  Remove all current DB-API 2.0 managers.

  All pools and connections are disposed.