Adding sql module documentation

doc/src/index.rst

@@ -44,6 +44,7 @@ Psycopg 2 is both Unicode and Python 3 friendly.
    advanced
    extensions
    extras
+   sql
    tz
    pool
    errorcodes

lib/sql.py

@@ -31,8 +31,29 @@ from psycopg2 import extensions as ext
 
 
 class Composable(object):
-    """Base class for objects that can be used to compose an SQL string."""
+    """
+    Abstract base class for objects that can be used to compose an SQL string.
+
+    Composables can be passed directly to `~cursor.execute()` and
+    `~cursor.executemany()`.
+
+    Composables 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.
+
+    .. automethod:: as_string
+    """
     def as_string(self, conn_or_curs):
+        """
+        Return the string value of the object.
+
+        The object is evaluated in the context of the *conn_or_curs* argument.
+
+        The function is automatically invoked by `~cursor.execute()` and
+        `~cursor.executemany()` if a `!Composable` is passed instead of the
+        query string.
+        """
         raise NotImplementedError
 
     def __add__(self, other):
@@ -43,8 +64,26 @@ class Composable(object):
         else:
             return NotImplemented
 
+    def __mul__(self, n):
+        return Composed([self] * n)
+
 
 class Composed(Composable):
+    """
+    A `Composable` object obtained concatenating a sequence of `Composable`.
+
+    The object is usually created using `compose()` and the `Composable`
+    operators. However it is possible to create a `!Composed` directly
+    specifying a sequence of `Composable` as arguments.
+
+    Example::
+
+        >>> sql.Composed([sql.SQL("insert into "), sql.Identifier("table")]) \\
+        ...     .as_string(conn)
+        'insert into "table"'
+
+    .. automethod:: join
+    """
     def __init__(self, seq):
         self._seq = []
         for i in seq:
@@ -70,10 +109,20 @@ class Composed(Composable):
         else:
             return NotImplemented
 
-    def __mul__(self, n):
-        return Composed(self._seq * n)
-
     def join(self, 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
+            >>> fields.join(', ').as_string(conn)
+            '"foo", "bar"'
+
+        """
         if isinstance(joiner, basestring):
             joiner = SQL(joiner)
         elif not isinstance(joiner, SQL):
@@ -93,10 +142,15 @@ class Composed(Composable):
 
 
 class SQL(Composable):
-    def __init__(self, wrapped):
-        if not isinstance(wrapped, basestring):
+    """
+    A `Composable` representing a snippet of SQL string to be included verbatim.
+
+    .. automethod:: join
+    """
+    def __init__(self, string):
+        if not isinstance(string, basestring):
             raise TypeError("SQL values must be strings")
-        self._wrapped = wrapped
+        self._wrapped = string
 
     def __repr__(self):
         return "sql.SQL(%r)" % (self._wrapped,)
@@ -104,10 +158,21 @@ class SQL(Composable):
     def as_string(self, conn_or_curs):
         return self._wrapped
 
-    def __mul__(self, n):
-        return Composed([self] * n)
-
     def join(self, seq):
+        """
+        Join a sequence of `Composable` or a `Composed` and return a `!Composed`.
+
+        Use the object value to separate the *seq* elements.
+
+        Example::
+
+            >>> snip - sql.SQL(', ').join(map(sql.Identifier, ['foo', 'bar', 'baz']))
+            >>> snip.as_string(conn)
+            '"foo", "bar", "baz"'
+        """
+        if isinstance(seq, Composed):
+            seq = seq._seq
+
         rv = []
         it = iter(seq)
         try:
@@ -123,15 +188,21 @@ class SQL(Composable):
 
 
 class Identifier(Composable):
-    def __init__(self, wrapped):
-        if not isinstance(wrapped, basestring):
+    """
+    A `Composable` representing an SQL identifer.
+
+    Identifiers usually represent names of database objects, such as tables
+    or fields. They follow `different rules`__ than SQL string literals for
+    escaping (e.g. they use double quotes).
+
+    .. __: https://www.postgresql.org/docs/current/static/sql-syntax-lexical.html# \
+        SQL-SYNTAX-IDENTIFIERS
+    """
+    def __init__(self, string):
+        if not isinstance(string, basestring):
             raise TypeError("SQL identifiers must be strings")
 
-        self._wrapped = wrapped
-
-    @property
-    def wrapped(self):
-        return self._wrapped
+        self._wrapped = string
 
     def __repr__(self):
         return "sql.Identifier(%r)" % (self._wrapped,)
@@ -141,6 +212,11 @@ class Identifier(Composable):
 
 
 class Literal(Composable):
+    """
+    Represent an SQL value to be included in a query.
+
+    The object follows the normal :ref:`adaptation rules <python-types-adaptation>`
+    """
     def __init__(self, wrapped):
         self._wrapped = wrapped
 
@@ -166,11 +242,31 @@ class Literal(Composable):
 
         return rv
 
-    def __mul__(self, n):
-        return Composed([self] * n)
-
 
 class Placeholder(Composable):
+    """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::
+
+        >>> sql.compose("insert into table (%s) values (%s)", [
+        ...     sql.SQL(', ').join(map(sql.Identifier, names)),
+        ...     sql.SQL(', ').join(sql.Placeholder() * 3)
+        ... ]).as_string(conn)
+        'insert into table ("foo", "bar", "baz") values (%s, %s, %s)'
+
+        >>> sql.compose("insert into table (%s) values (%s)", [
+        ...     sql.SQL(', ').join(map(sql.Identifier, names)),
+        ...     sql.SQL(', ').join(map(sql.Placeholder, names))
+        ... ]).as_string(conn)
+        'insert into table ("foo", "bar", "baz") values (%(foo)s, %(bar)s, %(baz)s)'
+    """
+
     def __init__(self, name=None):
         if isinstance(name, basestring):
             if ')' in name:
@@ -185,9 +281,6 @@ class Placeholder(Composable):
         return "sql.Placeholder(%r)" % (
             self._name if self._name is not None else '',)
 
-    def __mul__(self, n):
-        return Composed([self] * n)
-
     def as_string(self, conn_or_curs):
         if self._name is not None:
             return "%%(%s)s" % self._name
@@ -204,7 +297,37 @@ re_compose = re.compile("""
     """, re.VERBOSE)
 
 
-def compose(sql, args=()):
+def compose(sql, args=None):
+    """
+    Merge an SQL string with some variable parts.
+
+    The *sql* string can contain placeholders such as `%s` or `%(name)s`.
+    If the string must contain a literal ``%`` symbol use ``%%``. Note that,
+    unlike `~cursor.execute()`, the replacement ``%%`` |=>| ``%`` is *always*
+    performed, even if there is no argument.
+
+    .. |=>| unicode:: 0x21D2 .. double right arrow
+
+    *args* must be a sequence or mapping (according to the placeholder style)
+    of `Composable` instances.
+
+    The value returned is a `Composed` instance obtained replacing the
+    arguments to the query placeholders.
+
+    Example::
+
+        >>> query = sql.compose(
+        ...    "select %s from %s", [
+        ...        sql.SQL(', ').join(
+        ...            [sql.Identifier('foo'), sql.Identifier('bar')]),
+        ...        sql.Identifier('table')])
+        >>> query.as_string(conn)
+        select "foo", "bar" from "table"'
+
+    """
+    if args is None:
+        args = ()
+
     phs = list(re_compose.finditer(sql))
 
     # check placeholders consistent

tests/test_sql.py

@@ -208,6 +208,11 @@ class SQLTests(ConnectingTestCase):
         self.assert_(isinstance(obj, sql.Composed))
         self.assertEqual(obj.as_string(self.conn), '"foo", bar, 42')
 
+        obj = sql.SQL(", ").join(
+            sql.Composed([sql.Identifier('foo'), sql.SQL('bar'), sql.Literal(42)]))
+        self.assert_(isinstance(obj, sql.Composed))
+        self.assertEqual(obj.as_string(self.conn), '"foo", bar, 42')
+
 
 class ComposedTest(ConnectingTestCase):
     def test_class(self):