# sql/visitors.py # Copyright (C) 2005-2021 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 the purposes of applying transformations to expressions. Examples of how the visit system is used can be seen in the source code of for example the ``sqlalchemy.sql.util`` and the ``sqlalchemy.sql.compiler`` modules. Some background on clause adaption is also at http://techspot.zzzeek.org/2008/01/23/expression-transformations/ . """ from collections import deque import operator from .. import exc from .. import util __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, util.compat.string_types): # 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 as err: util.raise_( exc.UnsupportedCompilationError(visitor, cls), replace_context=err, ) 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 as err: util.raise_( exc.UnsupportedCompilationError(visitor, cls), replace_context=err, ) 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 :class:`.visitors.VisitableType` metaclass. The :class:`.Visitable` class is essentially at the base of the :class:`_expression.ClauseElement` hierarchy. """ class ClauseVisitor(object): """Base class for visitor objects which can traverse using the :func:`.visitors.traverse` function. Direct usage of the :func:`.visitors.traverse` function is usually preferred. """ __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 :func:`.visitors.cloned_traverse` function. Direct usage of the :func:`.visitors.cloned_traverse` function is usually preferred. """ 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 :func:`.visitors.replacement_traverse` function. Direct usage of the :func:`.visitors.replacement_traverse` function is usually preferred. """ 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): r"""Traverse the given expression structure, returning an iterator. Traversal is configured to be breadth-first. The central API feature used by the :func:`.visitors.iterate` and :func:`.visitors.iterate_depthfirst` functions is the :meth:`_expression.ClauseElement.get_children` method of :class:`_expression.ClauseElement` objects. This method should return all the :class:`_expression.ClauseElement` objects which are associated with a particular :class:`_expression.ClauseElement` object. For example, a :class:`.Case` structure will refer to a series of :class:`_expression.ColumnElement` objects within its "whens" and "else\_" member variables. :param obj: :class:`_expression.ClauseElement` structure to be traversed :param opts: dictionary of iteration options. This dictionary is usually empty in modern usage. """ # fasttrack for atomic elements like columns children = obj.get_children(**opts) if not children: return [obj] traversal = deque() stack = deque([obj]) while stack: t = stack.popleft() traversal.append(t) for c in t.get_children(**opts): stack.append(c) return iter(traversal) def iterate_depthfirst(obj, opts): """Traverse the given expression structure, returning an iterator. Traversal is configured to be depth-first. :param obj: :class:`_expression.ClauseElement` structure to be traversed :param opts: dictionary of iteration options. This dictionary is usually empty in modern usage. .. seealso:: :func:`.visitors.iterate` - includes a general overview of iteration. """ # fasttrack for atomic elements like columns children = obj.get_children(**opts) if not children: return [obj] 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. :func:`.visitors.traverse_using` is usually called internally as the result of the :func:`.visitors.traverse` or :func:`.visitors.traverse_depthfirst` functions. :param iterator: an iterable or sequence which will yield :class:`_expression.ClauseElement` structures; the iterator is assumed to be the product of the :func:`.visitors.iterate` or :func:`.visitors.iterate_depthfirst` functions. :param obj: the :class:`_expression.ClauseElement` that was used as the target of the :func:`.iterate` or :func:`.iterate_depthfirst` function. :param visitors: dictionary of visit functions. See :func:`.traverse` for details on this dictionary. .. seealso:: :func:`.traverse` :func:`.traverse_depthfirst` """ 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. e.g.:: from sqlalchemy.sql import visitors stmt = select([some_table]).where(some_table.c.foo == 'bar') def visit_bindparam(bind_param): print("found bound value: %s" % bind_param.value) visitors.traverse(stmt, {}, {"bindparam": visit_bindparam}) The iteration of objects uses the :func:`.visitors.iterate` function, which does a breadth-first traversal using a stack. :param obj: :class:`_expression.ClauseElement` structure to be traversed :param opts: dictionary of iteration options. This dictionary is usually empty in modern usage. :param visitors: dictionary of visit functions. The dictionary should have strings as keys, each of which would correspond to the ``__visit_name__`` of a particular kind of SQL expression object, and callable functions as values, each of which represents a visitor function for that kind of object. """ 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. The iteration of objects uses the :func:`.visitors.iterate_depthfirst` function, which does a depth-first traversal using a stack. Usage is the same as that of :func:`.visitors.traverse` function. """ return traverse_using(iterate_depthfirst(obj, opts), obj, visitors) def cloned_traverse(obj, opts, visitors): """Clone the given expression structure, allowing modifications by visitors. Traversal usage is the same as that of :func:`.visitors.traverse`. The visitor functions present in the ``visitors`` dictionary may also modify the internals of the given structure as the traversal proceeds. The central API feature used by the :func:`.visitors.cloned_traverse` and :func:`.visitors.replacement_traverse` functions, in addition to the :meth:`_expression.ClauseElement.get_children` function that is used to achieve the iteration, is the :meth:`_expression.ClauseElement._copy_internals` method. For a :class:`_expression.ClauseElement` structure to support cloning and replacement traversals correctly, it needs to be able to pass a cloning function into its internal members in order to make copies of them. .. seealso:: :func:`.visitors.traverse` :func:`.visitors.replacement_traverse` """ cloned = {} stop_on = set(opts.get("stop_on", [])) def clone(elem, **kw): 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, **kw) meth = visitors.get(newelem.__visit_name__, None) if meth: meth(newelem) return cloned[id(elem)] if obj is not None: obj = clone(obj) clone = None # remove gc cycles return obj def replacement_traverse(obj, opts, replace): """Clone the given expression structure, allowing element replacement by a given replacement function. This function is very similar to the :func:`.visitors.cloned_traverse` function, except instead of being passed a dictionary of visitors, all elements are unconditionally passed into the given replace function. The replace function then has the option to return an entirely new object which will replace the one given. If it returns ``None``, then the object is kept in place. The difference in usage between :func:`.visitors.cloned_traverse` and :func:`.visitors.replacement_traverse` is that in the former case, an already-cloned object is passed to the visitor function, and the visitor function can then manipulate the internal state of the object. In the case of the latter, the visitor function should only return an entirely different object, or do nothing. The use case for :func:`.visitors.replacement_traverse` is that of replacing a FROM clause inside of a SQL structure with a different one, as is a common use case within the ORM. """ cloned = {} stop_on = {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) clone = None # remove gc cycles return obj