pq – libpq wrapper module

Psycopg is built around the libpq, the PostgreSQL client library, which performs most of the network communications and returns query results in C structures.

The low-level functions of the library are exposed by the objects in the psycopg.pq module.

pq module implementations

There are actually several implementations of the module, all offering the same interface. Current implementations are:

  • python: a pure-python implementation, implemented using the ctypes module. It is less performing than the others, but it doesn’t need a C compiler to install. It requires the libpq installed in the system.

  • c: a C implementation of the libpq wrapper (more precisely, implemented in Cython). It is much better performing than the python implementation, however it requires development packages installed on the client machine. It can be installed using the c extra, i.e. running pip install psycopg[c].

  • binary: a pre-compiled C implementation, bundled with all the required libraries. It is the easiest option to deal with, fast to install and it should require no development tool or client library, however it may be not available for every platform. You can install it using the binary extra, i.e. running pip install psycopg[binary].

The implementation currently used is available in the __impl__ module constant.

At import time, Psycopg 3 will try to use the best implementation available and will fail if none is usable. You can force the use of a specific implementation by exporting the env var PSYCOPG_IMPL: importing the library will fail if the requested implementation is not available:

$ PSYCOPG_IMPL=c python -c "import psycopg"
Traceback (most recent call last):
   ...
ImportError: couldn't import requested psycopg 'c' implementation: No module named 'psycopg_c'

Module content

psycopg.pq.__impl__: str = 'python'

The currently loaded implementation of the psycopg.pq package.

Possible values include python, c, binary.

The choice of implementation is automatic but can be forced setting the PSYCOPG_IMPL env var.

psycopg.pq.version()

Return the version number of the libpq currently loaded.

The number is in the same format of server_version.

Certain features might not be available if the libpq library used is too old.

Return type

int

See also

the PQlibVersion() function

psycopg.pq.__build_version__: Optional[int] = None

The libpq version the C package was built with.

A number in the same format of server_version representing the libpq used to build the speedup module (c, binary) if available. None if __impl__ is python.

Certain features might not be available if the built version is too old.

psycopg.pq.error_message(obj, encoding='utf8')

Return an error message from a PGconn or PGresult.

The return value is a str (unlike pq data which is usually bytes): use the connection encoding if available, otherwise the encoding parameter as a fallback for decoding. Don’t raise exceptions on decoding errors.

Return type

str

Objects wrapping libpq structures and functions

TODO

finish documentation

class psycopg.pq.PGconn(pgconn_ptr)

Python representation of a libpq connection.

pgconn_ptr: Optional[int]

The pointer to the underlying PGconn structure, as integer.

None if the connection is closed.

The value can be used to pass the structure to libpq functions which psycopg doesn’t (currently) wrap, either in C or in Python using FFI libraries such as ctypes.

get_cancel()

Create an object with the information needed to cancel a command.

See PQgetCancel for details.

Return type

PGcancel

class psycopg.pq.PGresult(pgresult_ptr)

Python representation of a libpq result.

pgresult_ptr: Optional[int]

The pointer to the underlying PGresult structure, as integer.

None if the result was cleared.

The value can be used to pass the structure to libpq functions which psycopg doesn’t (currently) wrap, either in C or in Python using FFI libraries such as ctypes.

class psycopg.pq.Conninfo

Utility object to manipulate connection strings.

class psycopg.pq.Escaping(conn=None)

Utility object to escape strings for SQL interpolation.

class psycopg.pq.PGcancel(pgcancel_ptr)

Token to cancel the current operation on a connection.

Created by PGconn.get_cancel().

free()

Free the data structure created by PQgetCancel().

Automatically invoked by __del__().

See PQfreeCancel() for details.

cancel()

Requests that the server abandon processing of the current command.

See PQcancel() for details.

Enumerations

class psycopg.pq.ConnStatus(value)

Current status of the connection.

Other possible status are only seen during the connection phase.

See also

PQstatus() returns this value.

OK = 0
BAD = 1
class psycopg.pq.PollingStatus(value)

The status of the socket during a connection.

If READING or WRITING you may select before polling again.

See also

PQconnectPoll for a description of these states.

FAILED = 0
READING = 1
WRITING = 2
OK = 3
class psycopg.pq.TransactionStatus(value)

The transaction status of a connection.

See also

PQtransactionStatus for a description of these states.

IDLE = 0
ACTIVE = 1
INTRANS = 2
INERROR = 3
UNKNOWN = 4
class psycopg.pq.ExecStatus(value)

The status of a command.

See also

PQresultStatus for a description of these states.

EMPTY_QUERY = 0
COMMAND_OK = 1
TUPLES_OK = 2
COPY_OUT = 3
COPY_IN = 4
BAD_RESPONSE = 5
NONFATAL_ERROR = 6
FATAL_ERROR = 7
COPY_BOTH = 8
SINGLE_TUPLE = 9
class psycopg.pq.Format(value)

Enum representing the format of a query argument or return value.

These values are only the ones managed by the libpq. psycopg may also support automatically-chosen values: see psycopg.adapt.PyFormat.

TEXT = 0
BINARY = 1
class psycopg.pq.DiagnosticField(value)

Fields in an error report.

Available attributes:

SEVERITY
SEVERITY_NONLOCALIZED
SQLSTATE
MESSAGE_PRIMARY
MESSAGE_DETAIL
MESSAGE_HINT
STATEMENT_POSITION
INTERNAL_POSITION
INTERNAL_QUERY
CONTEXT
SCHEMA_NAME
TABLE_NAME
COLUMN_NAME
DATATYPE_NAME
CONSTRAINT_NAME
SOURCE_FILE
SOURCE_LINE
SOURCE_FUNCTION

See also

PQresultErrorField for a description of these values.

class psycopg.pq.Ping(value)

Response from a ping attempt.

See also

PQpingParams for a description of these values.

OK = 0
REJECT = 1
NO_RESPONSE = 2
NO_ATTEMPT = 3