The module interface respects the standard defined in the DB API 2.0.
Create a new database session and return a new connection object.
The connection parameters can be specified either as a libpq connection string using the dsn parameter:
conn = psycopg2.connect("dbname=test user=postgres password=secret")
or using a set of keyword arguments:
conn = psycopg2.connect(database="test", user="postgres", password="secret")
The two call styles are mutually exclusive: you cannot specify connection parameters as keyword arguments together with a connection string; only the parameters not needed for the database connection (i.e. connection_factory, cursor_factory, and async) are supported together with the dsn argument.
The basic connection parameters are:
Any other connection parameter supported by the client library/server can be passed either in the connection string or as keywords. The PostgreSQL documentation contains the complete list of the supported parameters. Also note that the same parameters can be passed to the client library using environment variables.
Using the connection_factory parameter a different class or connections factory can be specified. It should be a callable object taking a dsn string argument. See Connection and cursor factories for details. If a cursor_factory is specified, the connection’s cursor_factory is set to it. If you only need customized cursors you can use this parameter instead of subclassing a connection.
Using async=True an asynchronous connection will be created: see Asynchronous support to know about advantages and limitations.
Changed in version 2.4.3: any keyword argument is passed to the connection. Previously only the basic parameters (plus sslmode) were supported as keywords.
Changed in version 2.5: added the cursor_factory parameter.
DB API extension
The parameters connection_factory and async are Psycopg extensions to the DB API 2.0.
In compliance with the DB API 2.0, the module makes informations about errors available through the following exceptions:
Exception raised for important warnings like data truncations while inserting, etc. It is a subclass of the Python StandardError.
Exception that is the base class of all other error exceptions. You can use this to catch all errors with one single except statement. Warnings are not considered errors and thus not use this class as base. It is a subclass of the Python StandardError.
String representing the error message returned by the backend, None if not available.
String representing the error code returned by the backend, None if not available. The errorcodes module contains symbolic constants representing PostgreSQL error codes.
>>> try: ... cur.execute("SELECT * FROM barf") ... except psycopg2.Error as e: ... pass >>> e.pgcode '42P01' >>> print e.pgerror ERROR: relation "barf" does not exist LINE 1: SELECT * FROM barf ^
Exception raised for errors that are related to the database interface rather than the database itself. It is a subclass of Error.
Exception raised for errors that are related to the database. It is a subclass of Error.
Exception raised for errors that are due to problems with the processed data like division by zero, numeric value out of range, etc. It is a subclass of DatabaseError.
Exception raised for errors that are related to the database’s operation and not necessarily under the control of the programmer, e.g. an unexpected disconnect occurs, the data source name is not found, a transaction could not be processed, a memory allocation error occurred during processing, etc. It is a subclass of DatabaseError.
Exception raised when the relational integrity of the database is affected, e.g. a foreign key check fails. It is a subclass of DatabaseError.
Exception raised when the database encounters an internal error, e.g. the cursor is not valid anymore, the transaction is out of sync, etc. It is a subclass of DatabaseError.
Exception raised for programming errors, e.g. table not found or already exists, syntax error in the SQL statement, wrong number of parameters specified, etc. It is a subclass of DatabaseError.
Exception raised in case a method or database API was used which is not supported by the database, e.g. requesting a rollback() on a connection that does not support transaction or has transactions turned off. It is a subclass of DatabaseError.
DB API extension
Psycopg may raise a few other, more specialized, exceptions: currently QueryCanceledError and TransactionRollbackError are defined. These exceptions are not exposed by the main psycopg2 module but are made available by the extensions module. All the additional exceptions are subclasses of standard DB API 2.0 exceptions, so trapping them specifically is not required.
This is the exception inheritance layout:
StandardError |__ Warning |__ Error |__ InterfaceError |__ DatabaseError |__ DataError |__ OperationalError | |__ psycopg2.extensions.QueryCanceledError | |__ psycopg2.extensions.TransactionRollbackError |__ IntegrityError |__ InternalError |__ ProgrammingError |__ NotSupportedError
This section is mostly copied verbatim from the DB API 2.0 specification. While these objects are exposed in compliance to the DB API, Psycopg offers very accurate tools to convert data between Python and PostgreSQL formats. See Adapting new Python types to SQL syntax and Type casting of SQL types into Python objects
Many databases need to have the input in a particular format for binding to an operation’s input parameters. For example, if an input is destined for a DATE column, then it must be bound to the database in a particular string format. Similar problems exist for “Row ID” columns or large binary items (e.g. blobs or RAW columns). This presents problems for Python since the parameters to the .execute*() method are untyped. When the database module sees a Python string object, it doesn’t know if it should be bound as a simple CHAR column, as a raw BINARY item, or as a DATE.
To overcome this problem, a module must provide the constructors defined below to create objects that can hold special values. When passed to the cursor methods, the module can then detect the proper type of the input parameter and bind it accordingly.
A Cursor Object’s description attribute returns information about each of the result columns of a query. The type_code must compare equal to one of Type Objects defined below. Type Objects may be equal to more than one type code (e.g. DATETIME could be equal to the type codes for date, time and timestamp columns; see the Implementation Hints below for details).
The module exports the following constructors and singletons:
This function constructs an object holding a date value.
This function constructs an object holding a time value.
This function constructs an object holding a time stamp value.
This function constructs an object holding a date value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).
This function constructs an object holding a time value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).
This function constructs an object holding a time stamp value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).
This function constructs an object capable of holding a binary (long) string value.
All the adapters returned by the module level factories (Binary, Date, Time, Timestamp and the *FromTicks variants) expose the wrapped object (a regular Python object such as datetime) in an adapted attribute.
This type object is used to describe columns in a database that are string-based (e.g. CHAR).
This type object is used to describe (long) binary columns in a database (e.g. LONG, RAW, BLOBs).
This type object is used to describe numeric columns in a database.
This type object is used to describe date/time columns in a database.
This type object is used to describe the “Row ID” column in a database.