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FAQs grouped in themes.

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Daniele Varrazzo 2010-04-14 15:40:02 +01:00
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@ -7,6 +7,9 @@ Here are a few gotchas you may encounter using `psycopg2`. Feel free to
suggest new entries!
Problems with transactions handling
-----------------------------------
.. cssclass:: faq
Why does `!psycopg2` leave database sessions "idle in transaction"?
@ -22,6 +25,27 @@ Why does `!psycopg2` leave database sessions "idle in transaction"?
connection in :ref:`autocommit <autocommit>` mode to avoid a new
transaction to be started at the first command.
I receive the error *current transaction is aborted, commands ignored until end of transaction block* and can't do anything else!
There was a problem *in the previous* command to the database, which
resulted in an error. The database will not recover automatically from
this condition: you must run a `~connection.rollback()` before sending
new commands to the session (if this seems too harsh, remember that
PostgreSQL supports nested transactions using the |SAVEPOINT|_ command).
.. |SAVEPOINT| replace:: :sql:`SAVEPOINT`
.. _SAVEPOINT: http://www.postgresql.org/docs/8.4/static/sql-savepoint.html
Why do i get the error *current transaction is aborted, commands ignored until end of transaction block* when I use `!multiprocessing` (or any other forking system) and not when use `!threading`?
Psycopg's connections can't be shared across processes (but are thread
safe). If you are forking the Python process ensure to create a new
connection in each forked child.
Problems with type conversions
------------------------------
.. cssclass:: faq
Why does `!cursor.execute()` raise the exception *can't adapt*?
Psycopg converts Python objects in a SQL string representation by looking
at the object class. The exception is raised when you are trying to pass
@ -47,21 +71,6 @@ I try to execute a query but it fails with the error *not all arguments converte
>>> cur.execute("INSERT INTO foo VALUES (%s)", ("bar")) # WRONG
>>> cur.execute("INSERT INTO foo VALUES (%s)", ("bar",)) # correct
I receive the error *current transaction is aborted, commands ignored until end of transaction block* and can't do anything else!
There was a problem *in the previous* command to the database, which
resulted in an error. The database will not recover automatically from
this condition: you must run a `~connection.rollback()` before sending
new commands to the session (if this seems too harsh, remember that
PostgreSQL supports nested transactions using the |SAVEPOINT|_ command).
.. |SAVEPOINT| replace:: :sql:`SAVEPOINT`
.. _SAVEPOINT: http://www.postgresql.org/docs/8.4/static/sql-savepoint.html
Why do i get the error *current transaction is aborted, commands ignored until end of transaction block* when I use `!multiprocessing` (or any other forking system) and not when use `!threading`?
Psycopg's connections can't be shared across processes (but are thread
safe). If you are forking the Python process ensure to create a new
connection in each forked child.
My database is Unicode, but I receive all the strings as UTF-8 `str`. Can I receive `unicode` objects instead?
The following magic formula will do the trick::
@ -84,13 +93,11 @@ Psycopg converts :sql:`decimal`\/\ :sql:`numeric` database types into Python `!D
documentation. If you find `!psycopg2.extensions.DECIMAL` not avalable, use
`!psycopg2._psycopg.DECIMAL` instead.
I can't compile `!psycopg2`: the compiler says *error: Python.h: No such file or directory*. What am I missing?
You need to install a Python development package: it is usually called
``python-dev``.
I can't compile `!psycopg2`: the compiler says *error: libpq-fe.h: No such file or directory*. What am I missing?
You need to install the development version of the libpq: the package is
usually called ``libpq-dev``.
Best practices
--------------
.. cssclass:: faq
When should I save and re-use a cursor as opposed to creating a new one as needed?
Cursors are lightweight objects and creating lots of them should not pose
@ -116,3 +123,18 @@ What are the advantages or disadvantages of using named cursors?
the backend. The advantage is that data is fetched one chunk at a time:
using small `~cursor.fetchmany()` values it is possible to use very
little memory on the client and to skip or discard parts of the result set.
Problems compiling Psycopg from source
--------------------------------------
.. cssclass:: faq
I can't compile `!psycopg2`: the compiler says *error: Python.h: No such file or directory*. What am I missing?
You need to install a Python development package: it is usually called
``python-dev``.
I can't compile `!psycopg2`: the compiler says *error: libpq-fe.h: No such file or directory*. What am I missing?
You need to install the development version of the libpq: the package is
usually called ``libpq-dev``.