"""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/?p=19 .
"""
from collections import deque
import re
from sqlalchemy import util
import operator
__all__ = ['VisitableType', 'Visitable', 'ClauseVisitor',
'CloningVisitor', 'ReplacingCloningVisitor', 'iterate',
'iterate_depthfirst', 'traverse_using', 'traverse',
'cloned_traverse', 'replacement_traverse']
class VisitableType(type):
"""Metaclass which checks for a `__visit_name__` attribute and
applies `_compiler_dispatch` method to classes.
"""
def __init__(cls, clsname, bases, clsdict):
if cls.__name__ == 'Visitable' or not hasattr(cls, '__visit_name__'):
super(VisitableType, cls).__init__(clsname, bases, clsdict)
return
# set up an optimized visit dispatch function
# for use by the compiler
if '__visit_name__' in cls.__dict__:
visit_name = cls.__visit_name__
if isinstance(visit_name, str):
getter = operator.attrgetter("visit_%s" % visit_name)
def _compiler_dispatch(self, visitor, **kw):
return getter(visitor)(self, **kw)
else:
def _compiler_dispatch(self, visitor, **kw):
return getattr(visitor, 'visit_%s' % self.__visit_name__)(self, **kw)
cls._compiler_dispatch = _compiler_dispatch
super(VisitableType, cls).__init__(clsname, bases, clsdict)
class Visitable(object):
"""Base class for visitable objects, applies the
``VisitableType`` metaclass.
"""
__metaclass__ = VisitableType
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 = util.column_dict()
def clone(element):
if element not in cloned:
cloned[element] = element._clone()
return cloned[element]
obj = clone(obj)
stack = [obj]
while stack:
t = stack.pop()
if t in cloned:
continue
t._copy_internals(clone=clone)
meth = visitors.get(t.__visit_name__, None)
if meth:
meth(t)
for c in t.get_children(**opts):
stack.append(c)
return obj
def replacement_traverse(obj, opts, replace):
"""clone the given expression structure, allowing element replacement by a given replacement function."""
cloned = util.column_dict()
stop_on = util.column_set(opts.get('stop_on', []))
def clone(element):
newelem = replace(element)
if newelem is not None:
stop_on.add(newelem)
return newelem
if element not in cloned:
cloned[element] = element._clone()
return cloned[element]
obj = clone(obj)
stack = [obj]
while stack:
t = stack.pop()
if t in stop_on:
continue
t._copy_internals(clone=clone)
for c in t.get_children(**opts):
stack.append(c)
return obj
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