`psycopg2.sql` -- SQL string composition
========================================

.. sectionauthor:: Daniele Varrazzo <daniele.varrazzo@gmail.com>

.. module:: psycopg2.sql

.. versionadded:: 2.7

The module contains objects and functions useful to generate SQL dynamically,
in a convenient and safe way. SQL identifiers (e.g. names of tables and
fields) cannot be passed to the `~cursor.execute()` method like query
arguments::

    # This will not work
    table_name = 'my_table'
    cur.execute("insert into %s values (%s, %s)", [table_name, 10, 20])

The SQL query should be composed before the arguments are merged, for
instance::

    # This works, but it is not optimal
    table_name = 'my_table'
    cur.execute(
        "insert into %s values (%%s, %%s)" % table_name,
        [10, 20])

This sort of works, but it is an accident waiting to happen: the table name
may be an invalid SQL literal and need quoting; even more serious is the
security problem in case the table name comes from an untrusted source. The
name should be escaped using `~psycopg2.extensions.quote_ident()`::

    # This works, but it is not optimal
    table_name = 'my_table'
    cur.execute(
        "insert into %s values (%%s, %%s)" % ext.quote_ident(table_name, cur),
        [10, 20])

This is now safe, but it somewhat ad-hoc. In case, for some reason, it is
necessary to include a value in the query string (as opposite as in a value)
the merging rule is still different (`~psycopg2.extensions.adapt()` should be
used...). It is also still relatively dangerous: if `!quote_ident()` is
forgotten somewhere, the program will usually work, but will eventually crash
in the presence of a table or field name with containing characters to escape,
or will present a potentially exploitable weakness.

The objects exposed by the `!psycopg2.sql` module allow generating SQL
statements on the fly, separating clearly the variable parts of the statement
from the query parameters::

    from psycopg2 import sql

    cur.execute(
        sql.SQL("insert into {} values (%s, %s)")
            .format(sql.Identifier('my_table')),
        [10, 20])


Module usage
------------

Usually you should express the template of your query as an `SQL` instance
with `{}`\-style placeholders and use `~SQL.format()` to merge the variable
parts into them, all of which must be `Composable` subclasses. You can still
have `%s`\ -style placeholders in your query and pass values to
`~cursor.execute()`: such value placeholders will be untouched by
`!format()`::

    query = sql.SQL("select {field} from {table} where {pkey} = %s").format(
        field=sql.Identifier('my_name'),
        table=sql.Identifier('some_table'),
        pkey=sql.Identifier('id'))

The resulting object is meant to be passed directly to cursor methods such as
`~cursor.execute()`, `~cursor.executemany()`, `~cursor.copy_expert()`, but can
also be used to compose a query as a Python string, using the
`~Composable.as_string()` method::

    cur.execute(query, (42,))

If part of your query is a variable sequence of arguments, such as a
comma-separated list of field names, you can use the `SQL.join()` method to
pass them to the query::

    query = sql.SQL("select {fields} from {table}").format(
        fields=sql.SQL(',').join([
            sql.Identifier('field1'),
            sql.Identifier('field2'),
            sql.Identifier('field3'),
        ]),
        table=sql.Identifier('some_table'))


`!sql` objects
--------------

The `!sql` objects are in the following inheritance hierarchy:

|   `Composable`: the base class exposing the common interface
|   ``|__`` `SQL`: a literal snippet of an SQL query
|   ``|__`` `Identifier`: a PostgreSQL identifier or dot-separated sequence of identifiers
|   ``|__`` `Literal`: a value hardcoded into a query
|   ``|__`` `Placeholder`: a `%s`\ -style placeholder whose value will be added later e.g. by `~cursor.execute()`
|   ``|__`` `Composed`: a sequence of `!Composable` instances.


.. autoclass:: Composable

    .. automethod:: as_string


.. autoclass:: SQL

    .. autoattribute:: string

    .. automethod:: format

    .. automethod:: join


.. autoclass:: Identifier

    .. versionchanged:: 2.8
        added support for multiple strings.

    .. autoattribute:: strings

        .. versionadded:: 2.8
            previous verions only had a `!string` attribute. The attribute
            still exists but is deprecate and will only work if the
            `!Identifier` wraps a single string.

.. autoclass:: Literal

    .. autoattribute:: wrapped


.. autoclass:: Placeholder

    .. autoattribute:: name


.. autoclass:: Composed

    .. autoattribute:: seq

    .. automethod:: join
