27. Appendix B: Differences between python-oracledb Thin and Thick Modes

By default, python-oracledb runs in a ‘Thin’ mode which connects directly to Oracle Database. This mode does not need Oracle Client libraries. However, some additional functionality is available when python-oracledb uses them. Python-oracledb is said to be in ‘Thick’ mode when Oracle Client libraries are used. See Enabling python-oracledb Thick mode. Both modes have comprehensive functionality supporting the Python Database API v2.0 Specification.

This section details the differences between the python-oracledb Thin and Thick modes. Also see the summary feature comparison table in Appendix A: Oracle Database Features Supported by python-oracledb.

27.1. Connection Handling Differences between Thin and Thick Modes

Python-oracledb can create connections in either a Thin mode or a Thick mode. However, only one of these modes can be used in each Python process:

  • By default, python-oracledb runs in a Thin mode which connects directly to Oracle Database.

  • If oracledb.init_oracle_client() loads Oracle Client libraries before any standalone connection or pool is created, then the python-oracledb mode becomes Thick. The client libraries handle communication with Oracle Database. See Enabling python-oracledb Thick mode.

  • If an application opens a connection or creates a pool and then calls oracledb.init_oracle_client(), an error will occur.

  • Once a connection or pool has been opened, or init_oracle_client() has been called, you cannot change the mode.


The parameters of connection and pool creation functions oracledb.connect() and oracledb.create_pool() are now keyword and not positional in both Thin and Thick modes. This change makes the python-oracledb driver compliant with the Python Database API specification PEP 249. The old usage will cause an error, see Common Connection Errors.

27.1.1. Connections to a Local Database

In Thin mode, there is no concept of a local database. Bequeath connections cannot be made since no Oracle Client libraries are used. The Thin mode does not de-reference environment variables such as ORACLE_SID, TWO_TASK, or LOCAL (the latter is specific to Windows). A connection string, or equivalent, must always be used.

27.1.2. Oracle Net Services and Client Configuration Files

In the python-oracledb Thin mode:

  • The location of any tnsnames.ora files must explicitly be passed to the application.

  • Any sqlnet.ora file will not be read. Instead, pass an equivalent setting when connecting.

  • There is no support for oraaccess.xml since there are no Oracle Client libraries.

See Optional Oracle Net Configuration Files and Optional Oracle Client Configuration File for more information.

27.1.3. Connection Strings

The python-oracledb Thin mode accepts connection strings in the same formats as the Oracle Client libraries used by Thick mode does, but not all Oracle Net keywords will be supported.

The following table lists the parameters that are recognized in Thin mode either in Easy Connect Strings or in Full Connect Descriptor Strings that are either explicitly passed or referred to by a tnsnames.ora alias. All unrecognized parameters are ignored. The connection parameters shown can be used in oracledb.connect(), oracledb.create_pool(), oracledb.ConnectParams(), and oracledb.PoolParams().

Table 27.1 Oracle Net Keywords Supported in the python-oracledb Thin Mode

Oracle Net Keyword

Equivalent Connection Parameter




If specified, this value is used for any verification. Otherwise, the hostname will be used.



In Thin mode parsing the parameter supports case insensitive on/yes/true values similar to the Thick mode. Any other value is treated as disabling it.



Used in Easy Connect Strings. It is same as MY_WALLET_DIRECTORY in a connect descriptor.



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes



No relevant notes

In python-oracledb Thin mode, using the POOL_CONNECTION_CLASS or POOL_PURITY parameters in a connection string is similar to setting the equivalent attributes when creating a connection or connection pool.

In python-oracledb Thick mode, the POOL_CONNECTION_CLASS or POOL_PURITY values will only work when connected to Oracle Database 21c, or later. Note if POOL_PURITY=SELF is used in a connect string, then python-oracledb Thick mode applications will ignore the action to drop the session when attempting to remove an unusable connections from a pool in some uncommon error cases. It is recommended to avoid using POOL_PURITY=SELF in a connect string with python-oracledb Thick mode. Instead, code the python-oracledb Thick mode application to explicitly specify the purity and connection class as attributes.

The ENABLE=BROKEN connect descriptor option is not supported in python-oracledb Thin mode. Use expire_time instead.

If a name is given as a connect string, then the python-oracledb Thin mode will consider it as a Net Service Name and not as the minimal Easy Connect string of a hostname. The given connect string will be looked up in a tnsnames.ora file. This is different from the python-oracledb Thick mode. If supporting a bare name as a hostname is important to you in the python-oracledb Thin mode, then you can alter the connection string to include a port number such as hostname:1521 or a protocol such as tcp://hostname.

27.1.4. Token Based Authentication

In the python-oracledb Thin mode:

27.1.5. Transport Layer Security (TLS) Support

When connecting with mutual TLS (mTLS) also known as two-way TLS, for example to Oracle Autonomous Database in Oracle Cloud using a wallet, the certificate must be in the correct format.

For the python-oracledb Thin mode, the certificate must be in a Privacy Enhanced Mail (PEM) ewallet.pem file. In python-oracledb Thick mode the certificate must be in a cwallet.sso file. See Connecting to Oracle Cloud Autonomous Databases for more information.

27.1.6. Native Network Encryption and Checksumming

The python-oracledb Thin mode does not support connections using Oracle Database native network encryption or checksumming. You can enable TLS instead of using native network encryption. If native network encryption or checksumming are required, then use python-oracledb in the Thick mode. See Enabling python-oracledb Thick mode.

For example, if you use python-oracledb Thin mode and try to connect to the Oracle Cloud Infrastructure (OCI) Oracle Base Database where by default native network encryption is set to REQUIRED in the sqlnet.ora file of the OCI Oracle Base Database server, the connection will fail with an error like:

DPY-4011: the database or network closed the connection


DPY-6000: cannot connect to database. Listener refused connection.
(Similar to ORA-12660)

27.2. Connection Pooling Differences between Thin and Thick Modes

Python-oracledb introduced the ConnectionPool Object class to replace SessionPool. A new oracledb.create_pool() method is now the recommended way to create a connection pool. The use of the equivalent SessionPool() constructor is deprecated.

The create_pool() method in the python-oracledb Thin mode differs from the python-oracledb Thick mode in the following ways:

  • Not all the parameters of the oracledb.create_pool() method are applicable to both python-oracledb modes. Each mode ignores unrecognized parameters. The parameters that are ignored in Thin mode include events, tag, matchanytag, appcontext, shardingkey, supershardingkey, and handle parameters. The parameters that are ignored in the Thick mode include wallet_password, disable_oob, config_dir, and debug_jdwp parameters.

  • The python-oracledb Thin mode only suppports homogeneous pools.

  • The python-oracledb Thin mode creates connections in a daemon thread and so oracledb.create_pool() returns before any or all minimum number of connections are created. As soon as the pool is created, the ConnectionPool.opened attribute will not be equal to ConnectionPool.min. The opened attribute will increase to the minimum value over a short time as the connections are established. Note that this behavior may also be true of recent versions of the Oracle Call Interface (OCI) Session Pool used in the Thin mode.

    If the new getmode default value of POOL_GETMODE_WAIT is used, then this behavior will not be an issue. With this new default value, any immediate ConnectionPool.acquire() calls will wait for the connections to be created by the daemon thread. This improves the application start up time when compared to the python-oracledb Thick mode and cx_Oracle 8.3 driver, where oracledb.create_pool() will not return control to the application until all pool.min connections have been created.

    If the old default value POOL_GETMODE_NOWAIT is required, then the application could check if ConnectionPool.opened has reached ConnectionPool.min and then continue with application start up.

  • In python-oracledb Thick mode, when you close a connection pool with the parameter force=True, the underlying Oracle Client libraries wait for the current SQL executions to complete before closing the connections. All of the connections are then dropped from the pool and the pool is closed. Closing the pool in python-oracledb Thick mode could stop responding indefinitely, depending on the network and Oracle Net Services timeout parameters. This is also applicable to the cx_Oracle 8.3 driver. In python-oracledb Thin mode, the parameter force=True disconnects each connection’s socket using a background thread, leaving the database to clean up its end of the connections.

  • In python-oracledb Thin mode, the cclass parameter value is not used to tag connections in the application connection pool. It is only used for Database Resident Connection Pooling (DRCP).

  • In python-oracledb Thin mode, the connection pool supports all the connection mode privileges.

    The python-oracledb Thick mode only supports the AUTH_MODE_SYSDBA privilege.

27.3. Supported Database Data Types in Thin and Thick Modes

The python-oracledb Thin and Thick modes support different Oracle database data types. See Supported Oracle Database Data Types.

27.4. Query Metadata in Thin and Thick Modes

In python-oracledb Thin mode, Cursor.description metadata can distinguish the ROWID and UROWID database types. The UROWID database type shows the new value DB_TYPE_UROWID and the database type ROWID uses the existing value DB_TYPE_ROWID.

In python-oracledb Thick mode, the value DB_TYPE_ROWID is shown for both ROWID and UROWID database types. In python-oracledb Thick and Thin modes, comparison with the type oracledb.ROWID (defined in the Python DB API) will match both ROWID and UROWID database types.

27.5. Statement Caching in Thin and Thick Modes

The statement cache implemented in the python-oracledb Thin mode is capable of determining when different database data types are used for the same bind variables when a statement is re-executed. This capability is not supported in the Oracle Client libraries that are used in python-oracledb Thick mode. Note changing the type of bind variables for the same SQL text is inappropriate and gives indeterminate results in both modes.

27.6. Error Handling in Thin and Thick Modes

The python-oracledb Thin and Thick modes handle some errors differently. See Error Handling in Thin and Thick Modes.

27.7. Globalization in Thin and Thick Modes

All NLS environment variables, and the ORA_TZFILE environment variable, are ignored by the python-oracledb Thin mode. Use Python’s capabilities instead.

The python-oracledb Thin mode can only use NCHAR, NVARCHAR2, and NCLOB data when Oracle Database’s secondary character set is AL16UTF16.

See Character Sets and Globalization.

27.8. Tracing in Thin and Thick Modes

In the python-oracledb Thin mode, low level tracing is different because there are no Oracle Client libraries. See Tracing python-oracledb.