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 |
---|---|
Base class for visitor objects which can traverse using
the |
|
|
Clone the given expression structure, allowing modifications by visitors. |
Base class for visitor objects which can traverse using
the |
|
|
Traverse the given expression structure, returning an iterator. |
|
Traverse the given expression structure, returning an iterator. |
|
Clone the given expression structure, allowing element replacement by a given replacement function. |
Base class for visitor objects which can traverse using
the |
|
|
Traverse and visit the given expression structure using the default iterator. |
|
traverse and visit the given expression structure using the depth-first iterator. |
|
Visit the given expression structure using the given iterator of objects. |
Base class for visitable objects, applies the
|
|
Metaclass which assigns a |
sqlalchemy.sql.visitors.
ClauseVisitor
¶Base class for visitor objects which can traverse using
the traverse()
function.
Direct usage of the traverse()
function is usually
preferred.
sqlalchemy.sql.visitors.ClauseVisitor.
chain
(visitor)¶‘Chain’ an additional ClauseVisitor onto this ClauseVisitor.
The chained visitor will receive all visit events after this one.
sqlalchemy.sql.visitors.ClauseVisitor.
iterate
(obj)¶Traverse the given expression structure, returning an iterator of all elements.
sqlalchemy.sql.visitors.ClauseVisitor.
traverse
(obj)¶Traverse and visit the given expression structure.
sqlalchemy.sql.visitors.ClauseVisitor.
visitor_iterator
¶Iterate through this visitor and each ‘chained’ visitor.
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.
Class signature
class sqlalchemy.sql.visitors.CloningVisitor
(sqlalchemy.sql.visitors.ClauseVisitor
)
sqlalchemy.sql.visitors.CloningVisitor.
copy_and_process
(list_)¶Apply cloned traversal to the given list of elements, and return the new list.
sqlalchemy.sql.visitors.CloningVisitor.
traverse
(obj)¶Traverse and visit the given expression structure.
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.
Class signature
class sqlalchemy.sql.visitors.ReplacingCloningVisitor
(sqlalchemy.sql.visitors.CloningVisitor
)
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.
sqlalchemy.sql.visitors.ReplacingCloningVisitor.
traverse
(obj)¶Traverse and visit the given expression structure.
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.
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
)
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.
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.
obj¶ – ClauseElement
structure to be traversed
opts¶ – dictionary of iteration options. This dictionary is usually empty in modern usage.
sqlalchemy.sql.visitors.
iterate_depthfirst
(obj, opts)¶Traverse the given expression structure, returning an iterator.
Traversal is configured to be depth-first.
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.
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.
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.
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.
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.
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.
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.
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