Psycopg2 threaded connection pool example

Here are a few gotchas you may encounter using psycopg2. Feel free to suggest new entries! Psycopg normally starts a new transaction the first time a query is executed, e. The transaction is not closed until an explicit commit or rollback. If you are writing a long-living program, you should probably make sure to call one of the transaction closing methods before leaving the connection unused for a long time which may also be a few seconds, depending on the concurrency level in your database.

Alternatively you can use a connection in autocommit mode to avoid a new transaction to be started at the first command. All Python objects are converted by Psycopg in their SQL representation, so they get passed to the query as strings.

See Passing parameters to SQL queries. Psycopg always require positional arguments to be passed as a sequence, even when the query takes a single parameter. And remember that to make a single item tuple in Python you need a comma! See Unicode handling for the gory details. From psycopg 2. See Unicode handling for an example. If you find psycopg2. See JSON adaptation for further details.

Automatic conversion of jsonb values is supported from Psycopg release 2. For previous versions you can register the json typecaster on the jsonb oids which are known and not suppsed to change in future PostgreSQL versions :.

The arguments in the execute methods can only represent data to pass to the query: they cannot represent a table or field name:. If you want to build a query dynamically you can use the objects exposed by the psycopg2.

PostgreSQL 9. The problem is solved in Psycopg 2. For previous Psycopg releases, three options to solve the problem are:.

Normally the interactive shell becomes unresponsive to Ctrl-C when running a query. Starting from psycopg 2. For previous versions, you can use this implementation. Have you first checked if your question is answered already in the documentation?

If your question is about installing psycopg, have you checked the install FAQ and the install docs? Have you googled for your error message? You will only get piro grumpy. 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 rollback before sending new commands to the session if this seems too harsh, remember that PostgreSQL supports nested transactions using the SAVEPOINT command.

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?

If you are forking the Python process make sure to create a new connection in each forked child. See Thread and process safety for further informations.Want to edit, but don't see an edit button when logged in? Click here. Psycopg2 is a fairly mature driver for interacting with PostgreSQL from the Python scripting language.

It is written in C and provides to efficiently perform the full range of SQL operations against Postgres databases. This page deals with the 2nd version of the driver, not much older psycopg driver.

I will not assume you are perfectly familiar with Python in the below examples; therefore I have provided an excessive amount of comments and each example is a full-fledged script instead of just a snippet. Connect to the Postgres Database using authentication. Catch and print a connection error if one occurs. This example shows how to connect to a database, and then obtain and use a cursor object to retrieve records from a table.

In this example we will perform a select just like we did above but this time we will return columns as a Python Dictionary so column names are present. We will also use Psycopg2's prinf-style variable replacement, as well as a different fetch method to return a row fetchone.

If you have an extremely large result set to retrieve from your database, or you would like to iterate through a tables records without first retrieving the entire table a cursor is exactly what you need.

A cursor keeps the database connection open and retrieves database records 1 by 1 as you request them. Jump to: navigationsearch. In this case there is only 1 variable. Note that in python you specify a tuple with one item in it by placing a comma after the first variable and surrounding it in parentheses.

Note that even though we've returned the columns by name we can still access columns by numeric index as well - which is really nice. DictCursor cursor. Category : Python. Views Page Discussion View source History. Personal tools Log in. Contents 1 Overview 1.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. If nothing happens, download GitHub Desktop and try again. If nothing happens, download Xcode and try again.

If nothing happens, download the GitHub extension for Visual Studio and try again. Then, interact with your Psycopg cursor like you normally would:. Feel free to take a look at tests.

psycopg2 threaded connection pool example

The test suite was developed using Docker Compose to ensure that a fresh database is created and linked automagically on each run. Skip to content. Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. Sign up. Conveniences for establishing a threaded psycopg2 pool.

Python Dockerfile. Python Branch: master. Find file. Sign in Sign up. Go back. Launching Xcode If nothing happens, download Xcode and try again. Latest commit Fetching latest commit…. FLDB Postgres database connection manager, backed by a threaded connection pool.

You signed in with another tab or window. Reload to refresh your session. You signed out in another tab or window.This API has been designed to encourage and maintain the similarity between the Python modules that are used to access databases. This module is available on pypi. Use the following pip command to install Psycopg2. You can resolve this error by setting pypi. In this section, we will learn how to connect to PostgreSQL through python using psycopg2.

You need to know the following detail of PostgreSQL to perform the connection.

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If you have not created any database, I advise you to create one before proceeding further. We can execute such functions from Python. Founder of PYnative. Follow me on Twitter. All the best for your future Python endeavors! Free Coding Exercises for Python Developers. Exercises cover Python Basics, Data structure, Data analytics and more. Note : — As you can see we define a create table query and passed it to the cursor.

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We use cookies to ensure that you have the best possible experience on our website. Accept Learn More.SQLAlchemy and Postgres are a very popular choice for python applications needing a database. The default pooling implementation is the QueuePool. When an engine is instantiated, a QueuePool is also instantiated.

Transaction Handling with Psycopg2

A connection is established using the underlying engine and added to the pool. This means that if no concurrent connections are needed, only one connection will ever be open in the database.

Threads in python share memory: so if an object is modified in a thread, this change is reflected in the other. This means that 2 threads requesting a connection simultaneously will checkout 2 different connections. By extension, an engine will also be thread-safe. Within a web application, it is common practice to associate the management of a session with the lifetime of the web request.

However, both the Connection object and the Session object are not thread-safe. There is although an important thing to consider: when the process is forked, the default multiprocessing mode on Unix, every object in memory at that point is copied. Some WSGI servers have a configuration where the wsgi app is created before the worker processes are forked.

Here is a sample app that uses flask. This script is imported before the forking takes place.

psycopg2 threaded connection pool example

The connection gets released in the worker that received the request, and postgres discards it. But the other worker still has a reference to it. Ultimately, the important fact is that you must not use the same connections from multiple threads or processes. David Caron Blog. The connection created is now part of the pool. OperationalError: psycopg2. OperationalError server closed the connection unexpectedly This probably means the server terminated abnormally before or while processing the request.Create a new database session and return a new connection object.

The connection parameters can be specified as a libpq connection string using the dsn parameter:. Note that either the dsn or at least one connection-related keyword argument is required. 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.

It should be a callable object taking a dsn string argument. See Connection and cursor factories for details. If you only need customized cursors you can use this parameter instead of subclassing a connection. Changed in version 2. Previously only the basic parameters plus sslmode were supported as keywords. For psycopg2 is 2. Integer constant stating the level of thread safety the interface supports.

For psycopg2 is 2i. See Thread and process safety for details. String constant stating the type of parameter marker formatting expected by the interface. For psycopg2 is pyformat. See also Passing parameters to SQL queries. Exception raised for important warnings like data truncations while inserting, etc. 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. 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. The cursor the exception was raised from; None if not applicable.

A Diagnostics object containing further information about the error. The pgerrorpgcodecursorand diag attributes are Psycopg extensions. 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. 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.Want to edit, but don't see an edit button when logged in? Click here. There are any number of programming languages available for you to use with PostgreSQL.

One such language is Python and it happens to be one of my favored languages. I use it for almost all hacking that I do.

psycopg2 threaded connection pool example

Well to be honest it is because I am not that great of a programmer. I am a database administrator and operating system consultant by trade. Python ensures that the code that I write is readable by other more talented programmers 6 months from when I stopped working on it. My driver of choice when doing so is called Psycopg.

Recently Psycopg2 has been under heavy development and is currently in Beta 4. It is said that this will be the last Beta. Like the first release of Pyscopg the driver is designed to be lightweight, fast.

The following article discusses how to connect to PostgreSQL with Psycopg2 and also illustrates some of the nice features that come with the driver. The test platform for this article is Psycopg2, Python 2. It is designed for multi-threaded applications and manages its own connection pool.

Other interesting features of the adapter are that if you are using the PostgreSQL array data type, Psycopg will automatically convert a result using that data type to a Python list. The following discusses specific use of Psycopg. It does not try to implement a lot of Object Orientated goodness but to provide clear and concise syntactical examples of uses the driver with PostgreSQL. Making the initial connection:.

The above will import the adapter and try to connect to the database.


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