.. _overview_toplevel:
.. _overview:

========
Overview
========

The SQLAlchemy SQL Toolkit and Object Relational Mapper
is a comprehensive set of tools for working with
databases and Python. It has several distinct areas of
functionality which can be used individually or combined
together. Its major components are illustrated below,
with component dependencies organized into layers:

.. image:: sqla_arch_small.png

Above, the two most significant front-facing portions of
SQLAlchemy are the **Object Relational Mapper** and the
**SQL Expression Language**. SQL Expressions can be used
independently of the ORM. When using the ORM, the SQL
Expression language remains part of the public facing API
as it is used within object-relational configurations and
queries.

.. _doc_overview:

Documentation Overview
======================

The documentation is separated into three sections: :ref:`orm_toplevel`,
:ref:`core_toplevel`, and :ref:`dialect_toplevel`.

In :ref:`orm_toplevel`, the Object Relational Mapper is introduced and fully
described. New users should begin with the :ref:`ormtutorial_toplevel`. If you
want to work with higher-level SQL which is constructed automatically for you,
as well as management of Python objects, proceed to this tutorial.

In :ref:`core_toplevel`, the breadth of SQLAlchemy's SQL and database
integration and description services are documented, the core of which is the
SQL Expression language. The SQL Expression Language is a toolkit all its own,
independent of the ORM package, which can be used to construct manipulable SQL
expressions which can be programmatically constructed, modified, and executed,
returning cursor-like result sets. In contrast to the ORM's domain-centric
mode of usage, the expression language provides a schema-centric usage
paradigm. New users should begin here with :ref:`sqlexpression_toplevel`.
SQLAlchemy engine, connection, and pooling services are also described in
:ref:`core_toplevel`.

In :ref:`dialect_toplevel`, reference documentation for all provided
database and DBAPI backends is provided.

Code Examples
=============

Working code examples, mostly regarding the ORM, are included in the
SQLAlchemy distribution. A description of all the included example
applications is at :ref:`examples_toplevel`.

There is also a wide variety of examples involving both core SQLAlchemy
constructs as well as the ORM on the wiki.  See
`Theatrum Chemicum <http://www.sqlalchemy.org/trac/wiki/UsageRecipes>`_.

.. _installation:

Installation Guide
==================

Supported Platforms
-------------------

SQLAlchemy has been tested against the following platforms:

* cPython since version 2.7, through the 2.xx series
* cPython version 3, throughout all 3.xx series
* `PyPy <http://pypy.org/>`_ 2.1 or greater

.. versionchanged:: 1.2
   Python 2.7 is now the minimum Python version supported.

Platforms that don't currently have support include Jython and IronPython.
Jython has been supported in the past and may be supported in future
releases as well, depending on the state of Jython itself.

Supported Installation Methods
-------------------------------

SQLAlchemy installation is via standard Python methodologies that are
based on `setuptools <http://pypi.python.org/pypi/setuptools/>`_, either
by referring to ``setup.py`` directly or by using
`pip <http://pypi.python.org/pypi/pip/>`_ or other setuptools-compatible
approaches.

.. versionchanged:: 1.1 setuptools is now required by the setup.py file;
   plain distutils installs are no longer supported.

Install via pip
---------------

When ``pip`` is available, the distribution can be
downloaded from PyPI and installed in one step::

    pip install SQLAlchemy

This command will download the latest **released** version of SQLAlchemy from the `Python
Cheese Shop <http://pypi.python.org/pypi/SQLAlchemy>`_ and install it to your system.

In order to install the latest **prerelease** version, such as ``1.2.0b1``,
pip requires that the ``--pre`` flag be used::

    pip install --pre SQLAlchemy

Where above, if the most recent version is a prerelease, it will be installed
instead of the latest released version.


Installing using setup.py
----------------------------------

Otherwise, you can install from the distribution using the ``setup.py`` script::

    python setup.py install

.. _c_extensions:

Installing the C Extensions
----------------------------------

SQLAlchemy includes C extensions which provide an extra speed boost for
dealing with result sets.   The extensions are supported on both the 2.xx
and 3.xx series of cPython.

``setup.py`` will automatically build the extensions if an appropriate platform is
detected. If the build of the C extensions fails due to a missing compiler or
other issue, the setup process will output a warning message and re-run the
build without the C extensions upon completion, reporting final status.

To run the build/install without even attempting to compile the C extensions,
the ``DISABLE_SQLALCHEMY_CEXT`` environment variable may be specified.  The
use case for this is either for special testing circumstances, or in the rare
case of compatibility/build issues not overcome by the usual "rebuild"
mechanism::

  export DISABLE_SQLALCHEMY_CEXT=1; python setup.py install

.. versionchanged:: 1.1 The legacy ``--without-cextensions`` flag has been
   removed from the installer as it relies on deprecated features of
   setuptools.



Installing a Database API
----------------------------------

SQLAlchemy is designed to operate with a :term:`DBAPI` implementation built for a
particular database, and includes support for the most popular databases.
The individual database sections in :doc:`/dialects/index` enumerate
the available DBAPIs for each database, including external links.

Checking the Installed SQLAlchemy Version
------------------------------------------

This documentation covers SQLAlchemy version 1.2. If you're working on a
system that already has SQLAlchemy installed, check the version from your
Python prompt like this:

.. sourcecode:: python+sql

     >>> import sqlalchemy
     >>> sqlalchemy.__version__ # doctest: +SKIP
     1.2.0

.. _migration:

1.1 to 1.2 Migration
=====================

Notes on what's changed from 1.1 to 1.2 is available here at :doc:`changelog/migration_12`.
