.. currentmodule:: psycopg .. _module-usage: Basic module usage ================== The basic Psycopg usage is common to all the database adapters implementing the `DB-API`__ protocol. Other database adapters, such as the builtin `sqlite3` or `psycopg2`, have roughly the same pattern of interaction. .. __: https://www.python.org/dev/peps/pep-0249/ .. index:: pair: Example; Usage .. _usage: Main objects in Psycopg 3 ------------------------- Here is an interactive session showing some of the basic commands: .. code:: python # Note: the module name is psycopg, not psycopg3 import psycopg # Connect to an existing database with psycopg.connect("dbname=test user=postgres") as conn: # Open a cursor to perform database operations with conn.cursor() as cur: # Execute a command: this creates a new table cur.execute(""" CREATE TABLE test ( id serial PRIMARY KEY, num integer, data text) """) # Pass data to fill a query placeholders and let Psycopg perform # the correct conversion (no SQL injections!) cur.execute( "INSERT INTO test (num, data) VALUES (%s, %s)", (100, "abc'def")) # Query the database and obtain data as Python objects. cur.execute("SELECT * FROM test") cur.fetchone() # will return (1, 100, "abc'def") # You can use `cur.fetchmany()`, `cur.fetchall()` to return a list # of several records, or even iterate on the cursor for record in cur: print(record) # Make the changes to the database persistent conn.commit() In the example you can see some of the main objects and methods and how they relate to each other: - The function `~Connection.connect()` creates a new database session and returns a new `Connection` instance. `AsyncConnection.connect()` creates an `asyncio` connection instead. - The `~Connection` class encapsulates a database session. It allows to: - create new `~Cursor` instances using the `~Connection.cursor()` method to execute database commands and queries, - terminate transactions using the methods `~Connection.commit()` or `~Connection.rollback()`. - The class `~Cursor` allows interaction with the database: - send commands to the database using methods such as `~Cursor.execute()` and `~Cursor.executemany()`, - retrieve data from the database, iterating on the cursor or using methods such as `~Cursor.fetchone()`, `~Cursor.fetchmany()`, `~Cursor.fetchall()`. - Using these objects as context managers (i.e. using `!with`) will make sure to close them and free their resources at the end of the block (notice that :ref:`this is different from psycopg2 `). .. seealso:: A few important topics you will have to deal with are: - :ref:`query-parameters`. - :ref:`types-adaptation`. - :ref:`transactions`. Shortcuts --------- The pattern above is familiar to `!psycopg2` users. However, Psycopg 3 also exposes a few simple extensions which make the above pattern leaner: - the `Connection` objects exposes an `~Connection.execute()` method, equivalent to creating a cursor, calling its `~Cursor.execute()` method, and returning it. .. code:: # In Psycopg 2 cur = conn.cursor() cur.execute(...) # In Psycopg 3 cur = conn.execute(...) - The `Cursor.execute()` method returns `!self`. This means that you can chain a fetch operation, such as `~Cursor.fetchone()`, to the `!execute()` call: .. code:: # In Psycopg 2 cur.execute(...) record = cur.fetchone() cur.execute(...) for record in cur: ... # In Psycopg 3 record = cur.execute(...).fetchone() for record in cur.execute(...): ... Using them together, in simple cases, you can go from creating a connection to using a result in a single expression: .. code:: print(psycopg.connect(DSN).execute("SELECT now()").fetchone()[0]) # 2042-07-12 18:15:10.706497+01:00 .. index:: pair: Connection; `!with` .. _with-connection: Connection context ------------------ Psycopg 3 `Connection` can be used as a context manager: .. code:: python with psycopg.connect() as conn: ... # use the connection # the connection is now closed When the block is exited, if there is a transaction open, it will be committed. If an exception is raised within the block the transaction is rolled back. In both cases the connection is closed. It is roughly the equivalent of: .. code:: python conn = psycopg.connect() try: ... # use the connection except BaseException: conn.rollback() else: conn.commit() finally: conn.close() .. note:: This behaviour is not what `!psycopg2` does: in `!psycopg2` :ref:`there is no final close() ` and the connection can be used in several `!with` statements to manage different transactions. This behaviour has been considered non-standard and surprising so it has been replaced by the more explicit `~Connection.transaction()` block. Note that, while the above pattern is what most people would use, `connect()` doesn't enter a block itself, but returns an "un-entered" connection, so that it is still possible to use a connection regardless of the code scope and the developer is free to use (and responsible for calling) `~Connection.commit()`, `~Connection.rollback()`, `~Connection.close()` as and where needed. .. warning:: If a connection is just left to go out of scope, the way it will behave with or without the use of a `!with` block is different: - if the connection is used without a `!with` block, the server will find a connection closed INTRANS and roll back the current transaction; - if the connection is used with a `!with` block, there will be an explicit COMMIT and the operations will be finalised. You should use a `!with` block when your intention is just to execute a set of operations and then committing the result, which is the most usual thing to do with a connection. If your connection life cycle and transaction pattern is different, and want more control on it, the use without `!with` might be more convenient. See :ref:`transactions` for more information. `AsyncConnection` can be also used as context manager, using ``async with``, but be careful about its quirkiness: see :ref:`async-with` for details. Adapting pyscopg to your program -------------------------------- The above :ref:`pattern of use ` only shows the default behaviour of the adapter. Psycopg can be customised in several ways, to allow the smoothest integration between your Python program and your PostgreSQL database: - If your program is concurrent and based on `asyncio` instead of on threads/processes, you can use :ref:`async connections and cursors `. - If you want to customise the objects that the cursor returns, instead of receiving tuples, you can specify your :ref:`row factories `. - If you want to customise how Python values and PostgreSQL types are mapped into each other, beside the :ref:`basic type mapping `, you can :ref:`configure your types `.