psycopg2.sql -- SQL string composition

module psycopg2.sql

New in version 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 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 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 (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 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 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 execute(), executemany(), copy_expert(), but can also be used to compose a query as a Python string, using the 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 execute()
|__ Composed: a sequence of Composable instances.

class Composable

method as_string

class SQL

attribute string

method format

method join

class Identifier

Changed in version 2.8. added support for multiple strings.

attribute strings

New in version 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.

class Literal

attribute wrapped

class Placeholder

attribute name

class Composed

attribute seq

method join