SQLAlchemy 1.4 Documentation
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/ .
-
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
visitors.iterate()
andvisitors.iterate_depthfirst()
functions is theClauseElement.get_children()
method ofClauseElement
objects. This method should return all theClauseElement
objects which are associated with a particularClauseElement
object. For example, aCase
structure will refer to a series ofColumnElement
objects within its “whens” and “else_” member variables.- Parameters
obj¶ –
ClauseElement
structure to be traversedopts¶ – 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.
- Parameters
obj¶ –
ClauseElement
structure to be traversedopts¶ – dictionary of iteration options. This dictionary is usually empty in modern usage.
See also
visitors.iterate()
- includes a general overview of iteration.
-
sqlalchemy.sql.visitors.
traverse_using
(iterator, obj, visitors)¶ visit the given expression structure using the given iterator of objects.
visitors.traverse_using()
is usually called internally as the result of thevisitors.traverse()
orvisitors.traverse_depthfirst()
functions.- Parameters
iterator¶ – an iterable or sequence which will yield
ClauseElement
structures; the iterator is assumed to be the product of thevisitors.iterate()
orvisitors.iterate_depthfirst()
functions.obj¶ – the
ClauseElement
that was used as the target of theiterate()
oriterate_depthfirst()
function.visitors¶ – dictionary of visit functions. See
traverse()
for details on this dictionary.
-
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
visitors.iterate()
function, which does a breadth-first traversal using a stack.- Parameters
obj¶ –
ClauseElement
structure to be traversedopts¶ – 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
visitors.iterate_depthfirst()
function, which does a depth-first traversal using a stack.Usage is the same as that of
visitors.traverse()
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
visitors.traverse()
. The visitor functions present in thevisitors
dictionary may also modify the internals of the given structure as the traversal proceeds.The central API feature used by the
visitors.cloned_traverse()
andvisitors.replacement_traverse()
functions, in addition to theClauseElement.get_children()
function that is used to achieve the iteration, is theClauseElement._copy_internals()
method. For aClauseElement
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.
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
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 returnsNone
, then the object is kept in place.The difference in usage between
visitors.cloned_traverse()
andvisitors.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
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.
-
class
sqlalchemy.sql.visitors.
Traversible
¶ Base class for visitable objects, applies the
visitors.TraversibleType
metaclass.
-
class
sqlalchemy.sql.visitors.
TraversibleType
(clsname, bases, clsdict)¶ Bases:
builtins.type
Metaclass which assigns dispatch attributes to various kinds of “visitable” classes.
Attributes include:
The
_compiler_dispatch
method, corresponding to__visit_name__
. This is called “external traversal” because the caller of each visit() method is responsible for sub-traversing the inner elements of each object. This is appropriate for string compilers and other traversals that need to call upon the inner elements in a specific pattern.internal traversal collections
_children_traversal
,_cache_key_traversal
,_copy_internals_traversal
, generated from an optional_traverse_internals
collection of symbols which comes from theInternalTraversal
list of symbols. This is called “internal traversal” MARKMARK
-
class
sqlalchemy.sql.visitors.
ExternalTraversal
¶ Base class for visitor objects which can traverse externally using the
visitors.traverse()
function.Direct usage of the
visitors.traverse()
function is usually preferred.-
chain
(visitor)¶ ‘chain’ an additional ClauseVisitor onto this ClauseVisitor.
the chained visitor will receive all visit events after this one.
-
iterate
(obj)¶ traverse the given expression structure, returning an iterator of all elements.
-
traverse
(obj)¶ traverse and visit the given expression structure.
-
visitor_iterator
¶ iterate through this visitor and each ‘chained’ visitor.
-
-
class
sqlalchemy.sql.visitors.
InternalTraversal
¶ Defines visitor symbols used for internal traversal.
The
InternalTraversal
class is used in two ways. One is that it can serve as the superclass for an object that implements the various visit methods of the class. The other is that the symbols themselves ofInternalTraversal
are used within the_traverse_internals
collection. Such as, theCase
object defines_travserse_internals
as_traverse_internals = [ ("value", InternalTraversal.dp_clauseelement), ("whens", InternalTraversal.dp_clauseelement_tuples), ("else_", InternalTraversal.dp_clauseelement), ]
Above, the
Case
class indicates its internal state as the attribtues namedvalue
,whens
, andelse\_
. They each link to anInternalTraversal
method which indicates the type of datastructure referred towards.Using the
_traverse_internals
structure, objects of typeInternalTraversible
will have the following methods automatically implemented:Traversible.get_children()
Traversible._copy_internals()
Traversible._gen_cache_key()
Subclasses can also implement these methods directly, particularly for the
Traversible._copy_internals()
method, when special steps are needed.New in version 1.4.
-
dispatch
(visit_symbol)¶ Given a method from
InternalTraversal
, return the corresponding method on a subclass.
-
dp_annotations_state
= symbol('A')¶ Visit the state of the
Annotatated
version of an object.
-
dp_anon_name
= symbol('AN')¶ Visit a potentially “anonymized” string value.
The string value is considered to be significant for cache key generation.
-
dp_boolean
= symbol('B')¶ Visit a boolean value.
The boolean value is considered to be significant for cache key generation.
-
dp_clauseelement
= symbol('CE')¶ Visit a
ClauseElement
object.
-
dp_clauseelement_list
= symbol('CL')¶ Visit a list of
ClauseElement
objects.
-
dp_clauseelement_tuples
= symbol('CT')¶ Visit a list of tuples which contain
ClauseElement
objects.
-
dp_clauseelement_unordered_set
= symbol('CU')¶ Visit an unordered set of
ClauseElement
objects.
-
dp_fromclause_canonical_column_collection
= symbol('FC')¶ Visit a
FromClause
object in the context of thecolumns
attribute.The column collection is “canonical”, meaning it is the originally defined location of the
ColumnClause
objects. Right now this means that the object being visited is aTableClause
orTable
object only.
-
dp_fromclause_ordered_set
= symbol('CO')¶ Visit an ordered set of
FromClause
objects.
-
dp_has_cache_key
= symbol('HC')¶ Visit a
HasCacheKey
object.
-
dp_named_ddl_element
= symbol('DD')¶ Visit a simple named DDL element.
The current object used by this method is the
Sequence
.The object is only considered to be important for cache key generation as far as its name, but not any other aspects of it.
-
dp_operator
= symbol('O')¶ Visit an operator.
The operator is a function from the
sqlalchemy.sql.operators
module.The operator value is considered to be significant for cache key generation.
-
dp_plain_dict
= symbol('PD')¶ Visit a dictionary with string keys.
The keys of the dictionary should be strings, the values should be immutable and hashable. The dictionary is considered to be significant for cache key generation.
-
dp_plain_obj
= symbol('PO')¶ Visit a plain python object.
The value should be immutable and hashable, such as an integer. The value is considered to be significant for cache key generation.
-
dp_prefix_sequence
= symbol('PS')¶ Visit the sequence represented by
HasPrefixes
orHasSuffixes
.
-
dp_string
= symbol('S')¶ Visit a plain string value.
Examples include table and column names, bound parameter keys, special keywords such as “UNION”, “UNION ALL”.
The string value is considered to be significant for cache key generation.
-
dp_string_clauseelement_dict
= symbol('CD')¶ Visit a dictionary of string keys to
ClauseElement
objects.
-
dp_string_multi_dict
= symbol('MD')¶ Visit a dictionary of string keys to values which may either be plain immutable/hashable or
HasCacheKey
objects.
-
dp_type
= symbol('T')¶ Visit a
TypeEngine
objectThe type object is considered to be significant for cache key generation.
-
dp_unknown_structure
= symbol('UK')¶ Visit an unknown structure.