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`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),
[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])
The objects exposed by the `!sql` module can be used to compose a query as a
Python string (using the `~Composable.as_string()` method) or passed directly
to cursor methods such as `~cursor.execute()`, `~cursor.executemany()`,
`~cursor.copy_expert()`.
.. 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
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