Source

sqlalchemy / lib / sqlalchemy / sql / visitors.py

# sql/visitors.py
# Copyright (C) 2005-2013 the SQLAlchemy authors and contributors <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Visitor/traversal interface and library functions.

SQLAlchemy schema and expression constructs rely on a Python-centric
version of the classic "visitor" pattern as the primary way in which
they apply functionality.  The most common use of this pattern
is statement compilation, where individual expression classes match
up to rendering methods that produce a string result.   Beyond this,
the visitor system is also used to inspect expressions for various
information and patterns, as well as for usage in
some kinds of expression transformation.  Other kinds of transformation
use a non-visitor traversal system.

For many examples of how the visit system is used, see the
sqlalchemy.sql.util and the sqlalchemy.sql.compiler modules.
For an introduction to clause adaption, see
http://techspot.zzzeek.org/2008/01/23/expression-transformations/

"""

from collections import deque
from .. import util
import operator
from .. import exc

__all__ = ['VisitableType', 'Visitable', 'ClauseVisitor',
    'CloningVisitor', 'ReplacingCloningVisitor', 'iterate',
    'iterate_depthfirst', 'traverse_using', 'traverse',
    'traverse_depthfirst',
    'cloned_traverse', 'replacement_traverse']


class VisitableType(type):
    """Metaclass which assigns a `_compiler_dispatch` method to classes
    having a `__visit_name__` attribute.

    The _compiler_dispatch attribute becomes an instance method which
    looks approximately like the following::

        def _compiler_dispatch (self, visitor, **kw):
            '''Look for an attribute named "visit_" + self.__visit_name__
            on the visitor, and call it with the same kw params.'''
            visit_attr = 'visit_%s' % self.__visit_name__
            return getattr(visitor, visit_attr)(self, **kw)

    Classes having no __visit_name__ attribute will remain unaffected.
    """
    def __init__(cls, clsname, bases, clsdict):
        if clsname != 'Visitable' and \
                hasattr(cls, '__visit_name__'):
            _generate_dispatch(cls)

        super(VisitableType, cls).__init__(clsname, bases, clsdict)


def _generate_dispatch(cls):
    """Return an optimized visit dispatch function for the cls
    for use by the compiler.
    """
    if '__visit_name__' in cls.__dict__:
        visit_name = cls.__visit_name__
        if isinstance(visit_name, str):
            # There is an optimization opportunity here because the
            # the string name of the class's __visit_name__ is known at
            # this early stage (import time) so it can be pre-constructed.
            getter = operator.attrgetter("visit_%s" % visit_name)

            def _compiler_dispatch(self, visitor, **kw):
                try:
                    meth = getter(visitor)
                except AttributeError:
                    raise exc.UnsupportedCompilationError(visitor, cls)
                else:
                    return meth(self, **kw)
        else:
            # The optimization opportunity is lost for this case because the
            # __visit_name__ is not yet a string. As a result, the visit
            # string has to be recalculated with each compilation.
            def _compiler_dispatch(self, visitor, **kw):
                visit_attr = 'visit_%s' % self.__visit_name__
                try:
                    meth = getattr(visitor, visit_attr)
                except AttributeError:
                    raise exc.UnsupportedCompilationError(visitor, cls)
                else:
                    return meth(self, **kw)

        _compiler_dispatch.__doc__ = \
          """Look for an attribute named "visit_" + self.__visit_name__
            on the visitor, and call it with the same kw params.
            """
        cls._compiler_dispatch = _compiler_dispatch


class Visitable(util.with_metaclass(VisitableType, object)):
    """Base class for visitable objects, applies the
    ``VisitableType`` metaclass.

    """


class ClauseVisitor(object):
    """Base class for visitor objects which can traverse using
    the traverse() function.

    """

    __traverse_options__ = {}

    def traverse_single(self, obj, **kw):
        for v in self._visitor_iterator:
            meth = getattr(v, "visit_%s" % obj.__visit_name__, None)
            if meth:
                return meth(obj, **kw)

    def iterate(self, obj):
        """traverse the given expression structure, returning an iterator
        of all elements.

        """
        return iterate(obj, self.__traverse_options__)

    def traverse(self, obj):
        """traverse and visit the given expression structure."""

        return traverse(obj, self.__traverse_options__, self._visitor_dict)

    @util.memoized_property
    def _visitor_dict(self):
        visitors = {}

        for name in dir(self):
            if name.startswith('visit_'):
                visitors[name[6:]] = getattr(self, name)
        return visitors

    @property
    def _visitor_iterator(self):
        """iterate through this visitor and each 'chained' visitor."""

        v = self
        while v:
            yield v
            v = getattr(v, '_next', None)

    def chain(self, visitor):
        """'chain' an additional ClauseVisitor onto this ClauseVisitor.

        the chained visitor will receive all visit events after this one.

        """
        tail = list(self._visitor_iterator)[-1]
        tail._next = visitor
        return self


class CloningVisitor(ClauseVisitor):
    """Base class for visitor objects which can traverse using
    the cloned_traverse() function.

    """

    def copy_and_process(self, list_):
        """Apply cloned traversal to the given list of elements, and return
        the new list.

        """
        return [self.traverse(x) for x in list_]

    def traverse(self, obj):
        """traverse and visit the given expression structure."""

        return cloned_traverse(
            obj, self.__traverse_options__, self._visitor_dict)


class ReplacingCloningVisitor(CloningVisitor):
    """Base class for visitor objects which can traverse using
    the replacement_traverse() function.

    """

    def replace(self, elem):
        """receive pre-copied elements during a cloning traversal.

        If the method returns a new element, the element is used
        instead of creating a simple copy of the element.  Traversal
        will halt on the newly returned element if it is re-encountered.
        """
        return None

    def traverse(self, obj):
        """traverse and visit the given expression structure."""

        def replace(elem):
            for v in self._visitor_iterator:
                e = v.replace(elem)
                if e is not None:
                    return e
        return replacement_traverse(obj, self.__traverse_options__, replace)


def iterate(obj, opts):
    """traverse the given expression structure, returning an iterator.

    traversal is configured to be breadth-first.

    """
    stack = deque([obj])
    while stack:
        t = stack.popleft()
        yield t
        for c in t.get_children(**opts):
            stack.append(c)


def iterate_depthfirst(obj, opts):
    """traverse the given expression structure, returning an iterator.

    traversal is configured to be depth-first.

    """
    stack = deque([obj])
    traversal = deque()
    while stack:
        t = stack.pop()
        traversal.appendleft(t)
        for c in t.get_children(**opts):
            stack.append(c)
    return iter(traversal)


def traverse_using(iterator, obj, visitors):
    """visit the given expression structure using the given iterator of
    objects.

    """
    for target in iterator:
        meth = visitors.get(target.__visit_name__, None)
        if meth:
            meth(target)
    return obj


def traverse(obj, opts, visitors):
    """traverse and visit the given expression structure using the default
     iterator.

    """
    return traverse_using(iterate(obj, opts), obj, visitors)


def traverse_depthfirst(obj, opts, visitors):
    """traverse and visit the given expression structure using the
    depth-first iterator.

    """
    return traverse_using(iterate_depthfirst(obj, opts), obj, visitors)


def cloned_traverse(obj, opts, visitors):
    """clone the given expression structure, allowing
    modifications by visitors."""

    cloned = {}
    stop_on = set(opts.get('stop_on', []))

    def clone(elem):
        if elem in stop_on:
            return elem
        else:
            if id(elem) not in cloned:
                cloned[id(elem)] = newelem = elem._clone()
                newelem._copy_internals(clone=clone)
                meth = visitors.get(newelem.__visit_name__, None)
                if meth:
                    meth(newelem)
            return cloned[id(elem)]

    if obj is not None:
        obj = clone(obj)
    return obj


def replacement_traverse(obj, opts, replace):
    """clone the given expression structure, allowing element
    replacement by a given replacement function."""

    cloned = {}
    stop_on = set([id(x) for x in opts.get('stop_on', [])])

    def clone(elem, **kw):
        if id(elem) in stop_on or \
            'no_replacement_traverse' in elem._annotations:
            return elem
        else:
            newelem = replace(elem)
            if newelem is not None:
                stop_on.add(id(newelem))
                return newelem
            else:
                if elem not in cloned:
                    cloned[elem] = newelem = elem._clone()
                    newelem._copy_internals(clone=clone, **kw)
                return cloned[elem]

    if obj is not None:
        obj = clone(obj, **opts)
    return obj