3. Initializing python-oracledb

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. Both modes have comprehensive functionality supporting the Python Database API v2.0 Specification.

3.1. Enabling python-oracledb Thick mode

Changing from the default Thin mode to the Thick mode requires the addition of a call to oracledb.init_oracle_client() as shown in sections below. Other small code updates may also be required, see Appendix B: Differences between python-oracledb Thin and Thick Modes . If you are upgrading a cx_Oracle application to python-oracledb, then refer to Upgrading from cx_Oracle 8.3 to python-oracledb for changes that may be needed.

Note

To use python-oracledb in Thick mode you must call init_oracle_client() in the application. It must be called before any standalone connection or connection pool is created. If a connection or pool is first created, then the Thick mode cannot be enabled.

All connections in an application use the same mode.

Once the Thick mode is enabled, you cannot go back to Thin mode except by restarting the application.

Enabling the python-oracledb Thick mode loads Oracle Client libraries which handle communication to your database. The Oracle Client libraries need to be installed separately. See Installing python-oracledb.

Fig. 3.1 Architecture of the python-oracledb driver in Thick mode

You can validate python-oracledb is running in Thick mode by querying the CLIENT_DRIVER column of V$SESSION_CONNECT_INFO and verifying the value of the column begins with python-oracledb thk. See Finding the Python-oracledb Mode.

3.1.1. Setting the Oracle Client Library Directory

When init_oracle_client() is called, python-oracledb dynamically loads Oracle Client libraries using a search heuristic. The libraries can be:

• in an installation of Oracle Instant Client

• or in a full Oracle Client installation

• or in an Oracle Database installation (if Python is running on the same machine as the database).

See Installing python-oracledb for information about installing Oracle Client libraries.

The versions of Oracle Client libraries and Oracle Database do not have to be the same. For certified configurations see Oracle Support’s Doc ID 207303.1.

Note

If Oracle Client libraries cannot be loaded then init_oracle_client() will raise an error DPI-1047: Oracle Client library cannot be loaded. To resolve this, review the platform-specific instructions below or see Runtime Error Troubleshooting. Alternatively remove the call to init_oracle_client() and use Thin mode. The features supported by Thin mode can be found in Appendix B: Differences between python-oracledb Thin and Thick Modes.

3.1.1.1. Setting the Oracle Client Library Directory on Windows

On Windows, python-oracledb Thick mode can be enabled as follows:

• By passing the lib_dir parameter in a call to init_oracle_client(), for example:

  import oracledb
  
  oracledb.init_oracle_client(lib_dir=r"C:\instantclient_19_14")
  

  This directory should contain the libraries from an unzipped Instant Client ‘Basic’ or ‘Basic Light’ package. If you pass the library directory from a full client or database installation, such as Oracle Database “XE” Express Edition, then you will need to have previously set your environment to use that same software installation. Otherwise, files such as message files will not be located and you may have library version clashes. On Windows, when the path contains backslashes, use a ‘raw’ string like r"C:\instantclient_19_14".

  If the Oracle Client libraries cannot be loaded from lib_dir, then an exception is raised.

• By calling init_oracle_client() without passing a lib_dir parameter:

  import oracledb
  
  oracledb.init_oracle_client()
  

  In this case, Oracle Client libraries are first looked for in the directory where the python-oracledb binary module is installed. This directory should contain the libraries from an unzipped Instant Client ‘Basic’ or ‘Basic Light’ package.

  If the libraries are not found there, the search looks at the directories on the system library search path, for example, the PATH environment variable.

  If the Oracle Client libraries cannot be loaded, then an exception is raised.

3.1.1.2. Setting the Oracle Client Library Directory on macOS

On macOS, python-oracledb Thick mode can be enabled as follows:

• By passing the lib_dir parameter in a call to init_oracle_client(), for example:

  import oracledb
  
  oracledb.init_oracle_client(lib_dir="/Users/your_username/Downloads/instantclient_19_8")
  

  This directory should contain the libraries from an unzipped Instant Client ‘Basic’ or ‘Basic Light’ package. If the Oracle Client libraries cannot be loaded from lib_dir, then an exception is raised.

• By calling init_oracle_client() without passing a lib_dir parameter:

  import oracledb
  
  oracledb.init_oracle_client()
  

  In this case, the Oracle Client libraries are first looked for in the directory where the python-oracledb Thick mode binary module is installed. This directory should contain the libraries from an unzipped Instant Client ‘Basic’ or ‘Basic Light’ package, or a symbolic link to the main Oracle Client library if Instant Client is in a different directory.

  You can find the directory containing the Thick mode binary module by calling the python CLI without specifying a Python script, executing import oracledb, and then typing oracledb at the prompt. For example this might show /Users/yourname/.pyenv/versions/3.9.6/lib/python3.9/site-packages/oracledb/__init__.py. After checking that /Users/yourname/.pyenv/versions/3.9.6/lib/python3.9/site-packages/oracledb contains the binary module thick_impl.cpython-39-darwin.so you could then run these commands in a terminal window:

  CLIENT_DIR=~/Downloads/instantclient_19_8
  DPY_DIR=~/.pyenv/versions/3.9.6/lib/python3.9/site-packages/oracledb
  ln -s $CLIENT_DIR/libclntsh.dylib $DPY_DIR
  

  This can be automated in Python with:

  CLIENT_DIR = "~/Downloads/instantclient_19_8"
  LIB_NAME = "libclntsh.dylib"
  
  import os
  import oracledb
  
  target_dir = oracledb.__path__[0]
  os.symlink(os.path.join(CLIENT_DIR, LIB_NAME),
             os.path.join(target_dir, LIB_NAME))
  

  If python-oracledb does not find the Oracle Client library in that directory, the directories on the system library search path may be used, for example, ~/lib/ and /usr/local/lib, or in $DYLD_LIBRARY_PATH. These paths will vary with macOS version and Python version. Any value in DYLD_LIBRARY_PATH will not propagate to a sub-shell.

  If the Oracle Client libraries cannot be loaded, then an exception is raised.

3.1.1.3. Setting the Oracle Client Library Directory on Linux and Related Platforms

On Linux and related platforms, python-oracledb Thick mode can be enabled as follows:

• By calling init_oracle_client() without passing a lib_dir parameter:

  import oracledb
  
  oracledb.init_oracle_client()
  

  Oracle Client libraries are looked for in the operating system library search path, such as configured with ldconfig or set in the environment variable LD_LIBRARY_PATH. On some UNIX platforms an OS specific equivalent, such as LIBPATH or SHLIB_PATH is used instead of LD_LIBRARY_PATH.

  If libraries are not found in the system library search path, then $ORACLE_HOME/lib will be used. Note that the environment variable ORACLE_HOME should only ever be set when you have a full database installation or full client installation (such as installed with the Oracle GUI installer). It should not be set if you are using Oracle Instant Client. The ORACLE_HOME variable, and other necessary variables, should be set before starting Python. See Oracle Environment Variables for python-oracledb Thick Mode.

  If the Oracle Client libraries cannot be loaded, then an exception is raised.

Ensure that the Python process has directory and file access permissions for the Oracle Client libraries. On some platforms OS restrictions may prevent the opening of Oracle Client libraries installed in unsafe paths, such as from a user directory. On Linux ensure a libclntsh.so file exists. On macOS ensure a libclntsh.dylib file exists. Python-oracledb Thick mode will not directly load libclntsh.*.XX.1 files in lib_dir or from the directory where the python-oracledb binary module is available. Note that other libraries used by libclntsh* are also required.

3.1.1.4. Example Calling oracledb.init_oracle_client()

Oracle Client Libraries are loaded when oracledb.init_oracle_client() is called. In some environments, applications can use the lib_dir parameter to specify the directory containing the Oracle Client libraries. Otherwise, the system library search path should contain the relevant library directory before Python is invoked.

For example, if the Oracle Instant Client Libraries are in C:\oracle\instantclient_19_17 on Windows or $HOME/Downloads/instantclient_19_8 on macOS (Intel x86), then you can use:

import oracledb
import os
import platform

d = None  # default suitable for Linux
if platform.system() == "Darwin" and platform.machine() == "x86_64":
    d = os.environ.get("HOME")+"/Downloads/instantclient_19_8")
elif platform.system() == "Windows":
    d = r"C:\oracle\instantclient_19_17"
oracledb.init_oracle_client(lib_dir=d)

The use of a ‘raw’ string r"..." on Windows means that backslashes are treated as directory separators.

Note that if you set lib_dir on Linux and related platforms, you must still have configured the system library search path to include that directory before starting Python.

On any operating system, if you set lib_dir to the library directory of a full database or full client installation, you will need to have previously set the Oracle environment, for example by setting the ORACLE_HOME environment variable. Otherwise, you will get errors like ORA-1804. You should set this along with other Oracle environment variables before starting Python as shown in Oracle Environment Variables for python-oracledb Thick Mode.

Tracing Oracle Client Libraries Loading

To trace the loading of Oracle Client libraries, the environment variable DPI_DEBUG_LEVEL can be set to 64 before starting Python. At a Windows command prompt, this could be done with:

set DPI_DEBUG_LEVEL=64

On Linux and macOS, you might use:

export DPI_DEBUG_LEVEL=64

When your python-oracledb application is run, logging output is shown.

3.2. Optional Oracle Net Configuration Files

Optional Oracle Net configuration files may be read by python-oracledb. These files affect connections and applications. The common files are:

• tnsnames.ora: A configuration file that defines databases addresses for establishing connections. See Net Service Name for Connection Strings.

• sqlnet.ora: A profile configuration file that may contain information on features such as connection failover, network encryption, logging, and tracing. The files should be in a directory accessible to Python, not on the database server host. See Oracle Net Services Reference for more information.

  Note

  The sqlnet.ora file is only supported in the python-oracledb Thick mode. See Enabling python-oracledb Thick mode.

  In the python-oracledb Thin mode, many of the equivalent settings can be defined as connection time parameters, for example by using the ConnectParams Class.

python-oracledb Thin mode

In python-oracledb Thin mode applications, you specify the directory that contains the tnsnames.ora file by:

• setting the TNS_ADMIN environment variable to the directory containing the file

• setting defaults.config_dir to the directory containing the file

• setting the config_dir parameter to the directory containing the file when connecting or creating a connection pool.

For example:

import oracledb

oracledb.defaults.config_dir = "/opt/oracle/config"

Note

In Thin mode, you must explicitly set the directory because traditional “default” locations such as the Instant Client network/admin/ subdirectory, or $ORACLE_HOME/network/admin/, or $ORACLE_BASE/homes/XYZ/network/admin/ (in a read-only Oracle Database home) are not automatically looked in.

python-oracledb Thick mode

In python-oracledb Thick mode, the files are loaded from default locations (shown below), from the directory also specified in the $TNS_ADMIN environment variable, or from the directory specified as a parameter in the oracledb.init_oracle_client() call. For example, if the file /opt/oracle/config/tnsnames.ora should be used, you can call:

import oracledb
import sys

try:
    oracledb.init_oracle_client(config_dir="/opt/oracle/config")
except Exception as err:
    print("Whoops!")
    print(err)
    sys.exit(1)

Note

In python-oracledb Thick mode, once an application has created its first connection, trying to change the configuration directory will not have any effect.

If init_oracle_client() is called to enable Thick mode but config_dir is not specified, then default directories are searched for the configuration files. They include:

• $TNS_ADMIN

• /opt/oracle/instantclient_19_14/network/admin if Instant Client is in /opt/oracle/instantclient_19_14.

• /usr/lib/oracle/19.14/client64/lib/network/admin if Oracle 19.6 Instant Client RPMs are used on Linux.

• $ORACLE_HOME/network/admin if python-oracledb Thick is using libraries from a database installation.

Note that the Easy Connect Syntax for Connection Strings can set many common configuration options without needing tnsnames.ora or sqlnet.ora files.

The section Network Configuration has additional information about Oracle Net configuration.

3.3. Optional Oracle Client Configuration File

When python-oracledb uses Oracle Client libraries version 12.1 or later, an optional client parameter file called oraaccess.xml can be used to configure some behaviors of those libraries, such as statement caching and prefetching. This can be useful if the application cannot be altered. The file is read from the same directory as the Optional Oracle Net Configuration Files.

Note

The oraaccess.xml file is only supported in the python-oracledb Thick mode. See Enabling python-oracledb Thick mode.

A sample oraaccess.xml file that sets the Oracle client ‘prefetch’ value to 1000 rows. This value affects every SQL query in the application:

<?xml version="1.0"?>
 <oraaccess xmlns="http://xmlns.oracle.com/oci/oraaccess"
  xmlns:oci="http://xmlns.oracle.com/oci/oraaccess"
  schemaLocation="http://xmlns.oracle.com/oci/oraaccess
  http://xmlns.oracle.com/oci/oraaccess.xsd">
  <default_parameters>
    <prefetch>
      <rows>1000</rows>
    </prefetch>
  </default_parameters>
</oraaccess>

Prefetching is the number of additional rows that the underlying Oracle Client library fetches whenever python-oracledb Thick requests query data from the database. Prefetching is a tuning option to maximize data transfer efficiency and minimize round-trips to the database. The prefetch size does not affect when or how many rows are returned by the Thick mode to the application. The cache management is transparently handled by the Oracle Client libraries. Note that standard Thick mode fetch tuning is done using Cursor.arraysize, but changing the prefetch value can be useful in some cases such as when modifying the application is not feasible.

The oraaccess.xml file has other uses including:

• Changing the value of Fast Application Notification FAN events which affects notifications and Runtime Load Balancing (RLB).

• Configuring Client Result Caching parameters

• Turning on Client Statement Cache Auto-tuning

Refer to the documentation on oraaccess.xml for more details.

3.4. Oracle Environment Variables for python-oracledb Thick Mode

Some common environment variables that influence python-oracledb are shown below. The variables that may be needed depend on how Python is installed, how you connect to the database, and what optional settings are desired. It is recommended to set Oracle variables in the environment before calling Python. However, they may also be set in the application with os.putenv() before the first connection is established. System environment variables like LD_LIBRARY_PATH must be set before Python starts.

Note

These variables, with the exception of TNS_ADMIN, are only supported in the python-oracledb Thick mode. See Enabling python-oracledb Thick mode.

Table 3.1 Common Oracle environment variables

Oracle Environment Variables	Purpose
LD_LIBRARY_PATH	The library search path for platforms like Linux should include the Oracle libraries, for example $ORACLE_HOME/lib or /opt/instantclient_19_3. This variable is not needed if the libraries are located by an alternative method, such as with ldconfig. On other UNIX platforms, you may need to set an OS specific equivalent such as LIBPATH or SHLIB_PATH.
PATH	The library search path for Windows should include the location where OCI.DLL is found. Not needed if you set lib_dir in a call to oracledb.init_oracle_client()
TNS_ADMIN	The directory of optional Oracle Client configuration files such as tnsnames.ora and sqlnet.ora. Not needed if the configuration files are in a default location or if config_dir was not used in oracledb.init_oracle_client(). See Optional Oracle Net Configuration Files.
ORA_SDTZ	The default session time zone.
ORA_TZFILE	The name of the Oracle time zone file to use. See below.
ORACLE_HOME	The directory containing the Oracle Database software. The directory and various configuration files must be readable by the Python process. This variable should not be set if you are using Oracle Instant Client.
NLS_LANG	Determines the ‘national language support’ globalization options for python-oracledb. Note that from cx_Oracle 8, the character set component is ignored and only the language and territory components of NLS_LANG are used. The character set can instead be specified during connection or connection pool creation. See Character Sets and Globalization.
NLS_DATE_FORMAT, NLS_TIMESTAMP_FORMAT	Often set in Python applications to force a consistent date format independent of the locale. The variables are ignored if the environment variable NLS_LANG is not set.

Oracle Instant Client includes a small and big time zone file, for example timezone_32.dat and timezlrg_32.dat. The versions can be shown by running the utility genezi -v located in the Instant Client directory. The small file contains only the most commonly used time zones. By default, the larger timezlrg_n.dat file is used. If you want to use the smaller timezone_n.dat file, then set the ORA_TZFILE environment variable to the name of the file without any directory prefix. For example export ORA_TZFILE=timezone_32.dat. With Oracle Instant Client 12.2 or later, you can also use an external time zone file. Create a subdirectory oracore/zoneinfo under the Instant Client directory, and move the file into it. Then set ORA_TZFILE to the file name, without any directory prefix. The genezi -v utility will show the time zone file in use.

If python-oracledb Thick mode is using Oracle Client libraries from an Oracle Database or full Oracle Client software installation (such as installed with Oracle’s GUI installer), and you want to use a non-default time zone file, then set ORA_TZFILE to the file name with a directory prefix. For example: export ORA_TZFILE=/opt/oracle/myconfig/timezone_31.dat.

The Oracle Database documentation contains more information about time zone files, see Choosing a Time Zone File.

3.5. Other python-oracledb Thick Mode Initialization

The oracledb.init_oracle_client() function allows driver_name and error_url parameters to be set. These are useful for applications whose end-users are not aware that python-oracledb is being used. An example of setting the parameters is:

import oracledb
import sys

try:
    oracledb.init_oracle_client(driver_name="My Great App : 3.1.4",
                                error_url="https://example.com/MyInstallInstructions.html")
except Exception as err:
    print("Whoops!")
    print(err)
    sys.exit(1)

The convention for driver_name is to separate the product name from the product version by a colon and single blank characters. The value will be shown in Oracle Database views like V$SESSION_CONNECT_INFO. If this parameter is not specified, then a value like “python-oracledb thk : 1.0.0” is shown, see Finding the Python-oracledb Mode.

The error_url string will be shown in the exception raised if init_oracle_client() cannot load the Oracle Client libraries. This allows applications that use python-oracledb in Thick mode to refer users to application-specific installation instructions. If this value is not specified, then the Installing python-oracledb URL is used.

3.6. Changing from python-oracledb Thick Mode to python-oracledb Thin Mode

Changing an application that currently uses Thin mode requires the removal of calls to oracledb.init_oracle_client() and an application restart. Other small changes may be required.

All connections in a python-oracledb application must use the same mode.

If you have been using python-oracledb in Thick mode, you can use Thin mode by:

1. Reviewing Appendix A: Oracle Database Features Supported by python-oracledb and Appendix B: Differences between python-oracledb Thin and Thick Modes for code changes that may be needed. Also read Toggling between Drivers.

2. Removing all calls to oracledb.init_oracle_client() from the application.

3. Make other necessary changes identified in step 1.

4. When you are satisfied, you can optionally remove Oracle Client libraries. For example, delete your Oracle Instant Client directory.

You can validate the python-oracledb mode by querying the CLIENT_DRIVER column of V$SESSION_CONNECT_INFO and verifying if the value of the column begins with python-oracledb thn. See Finding the Python-oracledb Mode.