operations.py 文件源码

def drop_constraint(self, name, table_name, type_=None, schema=None):
        """Drop a constraint of the given name, typically via DROP CONSTRAINT.

        :param name: name of the constraint.
        :param table_name: table name.

         .. versionchanged:: 0.5.0
            The ``tablename`` parameter is now named ``table_name``.
            As this is a positional argument, the old name is no
            longer present.

        :param ``type_``: optional, required on MySQL.  can be
         'foreignkey', 'primary', 'unique', or 'check'.

         .. versionchanged:: 0.5.0
            The ``type`` parameter is now named ``type_``.  The old name
            ``type`` will remain for backwards compatibility.

         .. versionadded:: 0.3.6 'primary' qualfier to enable
            dropping of MySQL primary key constraints.

        :param schema: Optional schema name to operate within.

         .. versionadded:: 0.4.0

        """

        t = self._table(table_name, schema=schema)
        types = {
            'foreignkey': lambda name: sa_schema.ForeignKeyConstraint(
                                [], [], name=name),
            'primary': sa_schema.PrimaryKeyConstraint,
            'unique': sa_schema.UniqueConstraint,
            'check': lambda name: sa_schema.CheckConstraint("", name=name),
            None: sa_schema.Constraint
        }
        try:
            const = types[type_]
        except KeyError:
            raise TypeError("'type' can be one of %s" %
                        ", ".join(sorted(repr(x) for x in types)))

        const = const(name=name)
        t.append_constraint(const)
        self.impl.drop_constraint(const)