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.. include:: tutorial_nav_include.rst

.. rst-class:: core-header, orm-addin

.. _tutorial_engine:

Establishing Connectivity - the Engine
==========================================

.. container:: orm-header

    **Welcome ORM and Core readers alike!**

    Every SQLAlchemy application that connects to a database needs to use
    an :class:`_engine.Engine`.  This short section is for everyone.

The start of any SQLAlchemy application is an object called the
:class:`_engine.Engine`.   This object acts as a central source of connections
to a particular database, providing both a factory as well as a holding
space called a :ref:`connection pool <pooling_toplevel>` for these database
connections.   The engine is typically a global object created just
once for a particular database server, and is configured using a URL string
which will describe how it should connect to the database host or backend.

For this tutorial we will use an in-memory-only SQLite database. This is an
easy way to test things without needing to have an actual pre-existing database
set up.  The :class:`_engine.Engine` is created by using the
:func:`_sa.create_engine` function:

.. sourcecode:: pycon+sql

    >>> from sqlalchemy import create_engine
    >>> engine = create_engine("sqlite+pysqlite:///:memory:", echo=True)

The main argument to :class:`_sa.create_engine`
is a string URL, above passed as the string ``"sqlite+pysqlite:///:memory:"``.
This string indicates to the :class:`_engine.Engine` three important
facts:

1. What kind of database are we communicating with?   This is the ``sqlite``
   portion above, which links in SQLAlchemy to an object known as the
   :term:`dialect`.

2. What :term:`DBAPI` are we using?  The Python :term:`DBAPI` is a third party
   driver that SQLAlchemy uses to interact with a particular database.  In
   this case, we're using the name ``pysqlite``, which in modern Python
   use is the `sqlite3 <https://docs.python.org/library/sqlite3.html>`_ standard
   library interface for SQLite. If omitted, SQLAlchemy will use a default
   :term:`DBAPI` for the particular database selected.

3. How do we locate the database?   In this case, our URL includes the phrase
   ``/:memory:``, which is an indicator to the ``sqlite3`` module that we
   will be using an **in-memory-only** database.   This kind of database
   is perfect for experimenting as it does not require any server nor does
   it need to create new files.

.. sidebar:: Lazy Connecting

    The :class:`_engine.Engine`, when first returned by :func:`_sa.create_engine`,
    has not actually tried to connect to the database yet; that happens
    only the first time it is asked to perform a task against the database.
    This is a software design pattern known as :term:`lazy initialization`.

We have also specified a parameter :paramref:`_sa.create_engine.echo`, which
will instruct the :class:`_engine.Engine` to log all of the SQL it emits to a
Python logger that will write to standard out.   This flag is a shorthand way
of setting up
:ref:`Python logging more formally <dbengine_logging>` and is useful for
experimentation in scripts.   Many of the SQL examples will include this
SQL logging output beneath a ``[SQL]`` link that when clicked, will reveal
the full SQL interaction.

