psycopg2.sql – SQL string composition — Psycopg 2.7.3.2 documentation

# `psycopg2.sql` – SQL string composition

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),
        [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])

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

class `psycopg2.sql.` `Composable`(wrapped)

  Abstract base class for objects that can be used to compose an SQL string.

  `Composable` objects can be passed directly to `execute()`, `executemany()`, `copy_expert()` in place of the query string.

  `Composable` objects can be joined using the `+` operator: the result will be a `Composed` instance containing the objects joined. The operator `*` is also supported with an integer argument: the result is a `Composed` instance containing the left argument repeated as many times as requested.

  `as_string`(context)

    Return the string value of the object.

    Parameters: | context (`connection` or `cursor`) – the context to evaluate the string into.

    The method is automatically invoked by `execute()`, `executemany()`, `copy_expert()` if a `Composable` is passed instead of the query string.

class `psycopg2.sql.` `SQL`(string)

  A `Composable` representing a snippet of SQL statement.

  `SQL` exposes `join()` and `format()` methods useful to create a template where to merge variable parts of a query (for instance field or table names).

  The string doesn’t undergo any form of escaping, so it is not suitable to represent variable identifiers or values: you should only use it to pass constant strings representing templates or snippets of SQL statements; use other objects such as `Identifier` or `Literal` to represent variable parts.

  Example:

      >>> query = sql.SQL("select {0} from {1}").format(
      ...    sql.SQL(', ').join([sql.Identifier('foo'), sql.Identifier('bar')]),
      ...    sql.Identifier('table'))
      >>> print(query.as_string(conn))
      select "foo", "bar" from "table"

  `string`

    The string wrapped by the `SQL` object.

  `format`(*args, **kwargs)

    Merge `Composable` objects into a template.

    Parameters: | args (Composable) – parameters to replace to numbered (`{0}`, `{1}`) or auto-numbered (`{}`) placeholders kwargs (Composable) – parameters to replace to named (`{name}`) placeholders
    Returns: | the union of the `SQL` string with placeholders replaced
    Return type: | `Composed`

    The method is similar to the Python `str.format()` method: the string template supports auto-numbered (`{}`, only available from Python 2.7), numbered (`{0}`, `{1}`...), and named placeholders (`{name}`), with positional arguments replacing the numbered placeholders and keywords replacing the named ones. However placeholder modifiers (`{0!r}`, `{0:<10}`) are not supported. Only `Composable` objects can be passed to the template.

    Example:

        >>> print(sql.SQL("select * from {} where {} = %s")
        ...     .format(sql.Identifier('people'), sql.Identifier('id'))
        ...     .as_string(conn))
        select * from "people" where "id" = %s

        >>> print(sql.SQL("select * from {tbl} where {pkey} = %s")
        ...     .format(tbl=sql.Identifier('people'), pkey=sql.Identifier('id'))
        ...     .as_string(conn))
        select * from "people" where "id" = %s

  `join`(seq)

    Join a sequence of `Composable`.

    Parameters: | seq (iterable of `Composable`) – the elements to join.

    Use the `SQL` object’s string to separate the elements in seq. Note that `Composed` objects are iterable too, so they can be used as argument for this method.

    Example:

        >>> snip = sql.SQL(', ').join(
        ...     sql.Identifier(n) for n in ['foo', 'bar', 'baz'])
        >>> print(snip.as_string(conn))
        "foo", "bar", "baz"

class `psycopg2.sql.` `Identifier`(string)

  A `Composable` representing an SQL identifer.

  Identifiers usually represent names of database objects, such as tables or fields. PostgreSQL identifiers follow different rules than SQL string literals for escaping (e.g. they use double quotes instead of single).

  Example:

      >>> t1 = sql.Identifier("foo")
      >>> t2 = sql.Identifier("ba'r")
      >>> t3 = sql.Identifier('ba"z')
      >>> print(sql.SQL(', ').join([t1, t2, t3]).as_string(conn))
      "foo", "ba'r", "ba""z"

  `string`

    The string wrapped by the `Identifier`.

class `psycopg2.sql.` `Literal`(wrapped)

  A `Composable` representing an SQL value to include in a query.

  Usually you will want to include placeholders in the query and pass values as `execute()` arguments. If however you really really need to include a literal value in the query you can use this object.

  The string returned by `as_string()` follows the normal adaptation rules for Python objects.

  Example:

      >>> s1 = sql.Literal("foo")
      >>> s2 = sql.Literal("ba'r")
      >>> s3 = sql.Literal(42)
      >>> print(sql.SQL(', ').join([s1, s2, s3]).as_string(conn))
      'foo', 'ba''r', 42

  `wrapped`

    The object wrapped by the `Literal`.

class `psycopg2.sql.` `Placeholder`(name=None)

  A `Composable` representing a placeholder for query parameters.

  If the name is specified, generate a named placeholder (e.g. `%(name)s`), otherwise generate a positional placeholder (e.g. `%s`).

  The object is useful to generate SQL queries with a variable number of arguments.

  Examples:

      >>> names = ['foo', 'bar', 'baz']

      >>> q1 = sql.SQL("insert into table ({}) values ({})").format(
      ...     sql.SQL(', ').join(map(sql.Identifier, names)),
      ...     sql.SQL(', ').join(sql.Placeholder() * len(names)))
      >>> print(q1.as_string(conn))
      insert into table ("foo", "bar", "baz") values (%s, %s, %s)

      >>> q2 = sql.SQL("insert into table ({}) values ({})").format(
      ...     sql.SQL(', ').join(map(sql.Identifier, names)),
      ...     sql.SQL(', ').join(map(sql.Placeholder, names)))
      >>> print(q2.as_string(conn))
      insert into table ("foo", "bar", "baz") values (%(foo)s, %(bar)s, %(baz)s)

  `name`

    The name of the `Placeholder`.

class `psycopg2.sql.` `Composed`(seq)

  A `Composable` object made of a sequence of `Composable`.

  The object is usually created using `Composable` operators and methods. However it is possible to create a `Composed` directly specifying a sequence of `Composable` as arguments.

  Example:

      >>> comp = sql.Composed(
      ...     [sql.SQL("insert into "), sql.Identifier("table")])
      >>> print(comp.as_string(conn))
      insert into "table"

  `Composed` objects are iterable (so they can be used in `SQL.join` for instance).

  `seq`

    The list of the content of the `Composed`.

  `join`(joiner)

    Return a new `Composed` interposing the joiner with the `Composed` items.

    The joiner must be a `SQL` or a string which will be interpreted as an `SQL`.

    Example:

        >>> fields = sql.Identifier('foo') + sql.Identifier('bar')  # a Composed
        >>> print(fields.join(', ').as_string(conn))
        "foo", "bar"