Visitor and Traversal Utilities

The sqlalchemy.sql.visitors module consists of classes and functions that serve the purpose of generically traversing a Core SQL expression structure. This is not unlike the Python ast module in that is presents a system by which a program can operate upon each component of a SQL expression. Common purposes this serves are locating various kinds of elements such as Table or BindParameter objects, as well as altering the state of the structure such as replacing certain FROM clauses with others.

Note

the sqlalchemy.sql.visitors module is an internal API and is not fully public. It is subject to change and may additionally not function as expected for use patterns that aren’t considered within SQLAlchemy’s own internals.

The sqlalchemy.sql.visitors module is part of the internals of SQLAlchemy and it is not usually used by calling application code. It is however used in certain edge cases such as when constructing caching routines as well as when building out custom SQL expressions using the Custom SQL Constructs and Compilation Extension.

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/ .

Object Name	Description
ClauseVisitor	Base class for visitor objects which can traverse using the traverse() function.
cloned_traverse(obj, opts, visitors)	Clone the given expression structure, allowing modifications by visitors.
CloningVisitor	Base class for visitor objects which can traverse using the cloned_traverse() function.
iterate(obj, opts)	Traverse the given expression structure, returning an iterator.
iterate_depthfirst(obj, opts)	Traverse the given expression structure, returning an iterator.
replacement_traverse(obj, opts, replace)	Clone the given expression structure, allowing element replacement by a given replacement function.
ReplacingCloningVisitor	Base class for visitor objects which can traverse using the replacement_traverse() function.
traverse(obj, opts, visitors)	Traverse and visit the given expression structure using the default iterator.
traverse_depthfirst(obj, opts, visitors)	traverse and visit the given expression structure using the depth-first iterator.
traverse_using(iterator, obj, visitors)	Visit the given expression structure using the given iterator of objects.
Visitable	Base class for visitable objects, applies the VisitableType metaclass.
VisitableType	Metaclass which assigns a _compiler_dispatch method to classes having a __visit_name__ attribute.

class sqlalchemy.sql.visitors.ClauseVisitor

Base class for visitor objects which can traverse using the traverse() function.

Members

chain(), iterate(), traverse(), visitor_iterator

Direct usage of the traverse() function is usually preferred.

method sqlalchemy.sql.visitors.ClauseVisitor.chain(visitor)

‘Chain’ an additional ClauseVisitor onto this ClauseVisitor.

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

method sqlalchemy.sql.visitors.ClauseVisitor.iterate(obj)

Traverse the given expression structure, returning an iterator of all elements.

method sqlalchemy.sql.visitors.ClauseVisitor.traverse(obj)

Traverse and visit the given expression structure.

attribute sqlalchemy.sql.visitors.ClauseVisitor.visitor_iterator

Iterate through this visitor and each ‘chained’ visitor.

class sqlalchemy.sql.visitors.CloningVisitor

Base class for visitor objects which can traverse using the cloned_traverse() function.

Direct usage of the cloned_traverse() function is usually preferred.

Members

copy_and_process(), traverse()

Class signature

class sqlalchemy.sql.visitors.CloningVisitor (sqlalchemy.sql.visitors.ClauseVisitor)

method sqlalchemy.sql.visitors.CloningVisitor.copy_and_process(list_)

Apply cloned traversal to the given list of elements, and return the new list.

method sqlalchemy.sql.visitors.CloningVisitor.traverse(obj)

Traverse and visit the given expression structure.

class sqlalchemy.sql.visitors.ReplacingCloningVisitor

Base class for visitor objects which can traverse using the replacement_traverse() function.

Direct usage of the replacement_traverse() function is usually preferred.

Members

replace(), traverse()

Class signature

class sqlalchemy.sql.visitors.ReplacingCloningVisitor (sqlalchemy.sql.visitors.CloningVisitor)

method sqlalchemy.sql.visitors.ReplacingCloningVisitor.replace(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.

method sqlalchemy.sql.visitors.ReplacingCloningVisitor.traverse(obj)

Traverse and visit the given expression structure.

class sqlalchemy.sql.visitors.Visitable

Base class for visitable objects, applies the VisitableType metaclass.

The Visitable class is essentially at the base of the ClauseElement hierarchy.

class sqlalchemy.sql.visitors.VisitableType(clsname, bases, clsdict)

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.

Class signature

class sqlalchemy.sql.visitors.VisitableType (builtins.type)

function sqlalchemy.sql.visitors.cloned_traverse(obj, opts, visitors)

Clone the given expression structure, allowing modifications by visitors.

Traversal usage is the same as that of 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 cloned_traverse() and replacement_traverse() functions, in addition to the ClauseElement.get_children() function that is used to achieve the iteration, is the ClauseElement._copy_internals() method. For a 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.

See also

traverse()

replacement_traverse()

function sqlalchemy.sql.visitors.iterate(obj, opts)

Traverse the given expression structure, returning an iterator.

Traversal is configured to be breadth-first.

The central API feature used by the iterate() and iterate_depthfirst() functions is the ClauseElement.get_children() method of ClauseElement objects. This method should return all the ClauseElement objects which are associated with a particular ClauseElement object. For example, a Case structure will refer to a series of ColumnElement objects within its “whens” and “else_” member variables.

Parameters:

• obj – ClauseElement structure to be traversed

• opts – dictionary of iteration options. This dictionary is usually empty in modern usage.

function sqlalchemy.sql.visitors.iterate_depthfirst(obj, opts)

Traverse the given expression structure, returning an iterator.

Traversal is configured to be depth-first.

Parameters:

• obj – ClauseElement structure to be traversed

• opts – dictionary of iteration options. This dictionary is usually empty in modern usage.

See also

iterate() - includes a general overview of iteration.

function sqlalchemy.sql.visitors.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 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 cloned_traverse() and 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 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.

function sqlalchemy.sql.visitors.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 iterate() function, which does a breadth-first traversal using a stack.

Parameters:

• obj – ClauseElement structure to be traversed

• opts – dictionary of iteration options. This dictionary is usually empty in modern usage.

• 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.

function sqlalchemy.sql.visitors.traverse_depthfirst(obj, opts, visitors)

traverse and visit the given expression structure using the depth-first iterator.

The iteration of objects uses the iterate_depthfirst() function, which does a depth-first traversal using a stack.

Usage is the same as that of traverse() function.

function sqlalchemy.sql.visitors.traverse_using(iterator, obj, visitors)

Visit the given expression structure using the given iterator of objects.

traverse_using() is usually called internally as the result of the traverse() or traverse_depthfirst() functions.

Parameters:

• iterator – an iterable or sequence which will yield ClauseElement structures; the iterator is assumed to be the product of the iterate() or iterate_depthfirst() functions.

• obj – the ClauseElement that was used as the target of the iterate() or iterate_depthfirst() function.

• visitors – dictionary of visit functions. See traverse() for details on this dictionary.

See also

traverse()

traverse_depthfirst()