SQLAlchemy 1.4 Documentation
SQLAlchemy Core
- SQL Expression Language Tutorial (1.x API)
- SQL Statements and Expressions API
- Column Elements and Expressions
- Operator Reference
- Selectables, Tables, FROM objects
- Insert, Updates, Deletes
- SQL and Generic Functions
- Custom SQL Constructs and Compilation Extension
- Expression Serializer Extension
- SQL Expression Language Foundational Constructs
- Visitor and Traversal Utilities¶
ExternalTraversal
InternalTraversal
InternalTraversal.dispatch()
InternalTraversal.dp_annotations_key
InternalTraversal.dp_anon_name
InternalTraversal.dp_boolean
InternalTraversal.dp_clauseelement
InternalTraversal.dp_clauseelement_list
InternalTraversal.dp_clauseelement_tuple
InternalTraversal.dp_clauseelement_tuples
InternalTraversal.dp_dialect_options
InternalTraversal.dp_dml_multi_values
InternalTraversal.dp_dml_ordered_values
InternalTraversal.dp_dml_values
InternalTraversal.dp_fromclause_canonical_column_collection
InternalTraversal.dp_fromclause_ordered_set
InternalTraversal.dp_has_cache_key
InternalTraversal.dp_has_cache_key_list
InternalTraversal.dp_named_ddl_element
InternalTraversal.dp_operator
InternalTraversal.dp_plain_dict
InternalTraversal.dp_plain_obj
InternalTraversal.dp_prefix_sequence
InternalTraversal.dp_propagate_attrs
InternalTraversal.dp_statement_hint_list
InternalTraversal.dp_string
InternalTraversal.dp_string_clauseelement_dict
InternalTraversal.dp_string_list
InternalTraversal.dp_string_multi_dict
InternalTraversal.dp_table_hint_list
InternalTraversal.dp_type
InternalTraversal.dp_unknown_structure
Traversible
TraversibleType
cloned_traverse()
iterate()
replacement_traverse()
traverse()
traverse_using()
- Schema Definition Language
- SQL Datatype Objects
- Engine and Connection Use
- Core API Basics
- SQLAlchemy 2.0 Future (Core)
Project Versions
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- Visitor and Traversal Utilities
ExternalTraversal
InternalTraversal
InternalTraversal.dispatch()
InternalTraversal.dp_annotations_key
InternalTraversal.dp_anon_name
InternalTraversal.dp_boolean
InternalTraversal.dp_clauseelement
InternalTraversal.dp_clauseelement_list
InternalTraversal.dp_clauseelement_tuple
InternalTraversal.dp_clauseelement_tuples
InternalTraversal.dp_dialect_options
InternalTraversal.dp_dml_multi_values
InternalTraversal.dp_dml_ordered_values
InternalTraversal.dp_dml_values
InternalTraversal.dp_fromclause_canonical_column_collection
InternalTraversal.dp_fromclause_ordered_set
InternalTraversal.dp_has_cache_key
InternalTraversal.dp_has_cache_key_list
InternalTraversal.dp_named_ddl_element
InternalTraversal.dp_operator
InternalTraversal.dp_plain_dict
InternalTraversal.dp_plain_obj
InternalTraversal.dp_prefix_sequence
InternalTraversal.dp_propagate_attrs
InternalTraversal.dp_statement_hint_list
InternalTraversal.dp_string
InternalTraversal.dp_string_clauseelement_dict
InternalTraversal.dp_string_list
InternalTraversal.dp_string_multi_dict
InternalTraversal.dp_table_hint_list
InternalTraversal.dp_type
InternalTraversal.dp_unknown_structure
Traversible
TraversibleType
cloned_traverse()
iterate()
replacement_traverse()
traverse()
traverse_using()
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
https://techspot.zzzeek.org/2008/01/23/expression-transformations/ .
Object Name | Description |
---|---|
cloned_traverse(obj, opts, visitors) |
Clone the given expression structure, allowing modifications by visitors. |
Base class for visitor objects which can traverse externally using
the |
|
Defines visitor symbols used for internal traversal. |
|
iterate(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. |
traverse(obj, opts, visitors) |
Traverse and visit the given expression structure using the default iterator. |
traverse_using(iterator, obj, visitors) |
Visit the given expression structure using the given iterator of objects. |
Base class for visitable objects, applies the
|
|
Metaclass which assigns dispatch attributes to various kinds of “visitable” classes. |
- class sqlalchemy.sql.visitors.ExternalTraversal¶
Base class for visitor objects which can traverse externally using the
traverse()
function.Members
Direct usage of the
traverse()
function is usually preferred.-
method
sqlalchemy.sql.visitors.ExternalTraversal.
chain(visitor)¶ ‘Chain’ an additional ClauseVisitor onto this ClauseVisitor.
The chained visitor will receive all visit events after this one.
-
method
sqlalchemy.sql.visitors.ExternalTraversal.
iterate(obj)¶ Traverse the given expression structure, returning an iterator of all elements.
-
method
sqlalchemy.sql.visitors.ExternalTraversal.
traverse(obj)¶ Traverse and visit the given expression structure.
-
attribute
sqlalchemy.sql.visitors.ExternalTraversal.
visitor_iterator¶ Iterate through this visitor and each ‘chained’ visitor.
-
method
- 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_traverse_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 attributes 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._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.Members
dispatch(), dp_annotations_key, dp_anon_name, dp_boolean, dp_clauseelement, dp_clauseelement_list, dp_clauseelement_tuple, dp_clauseelement_tuples, dp_dialect_options, dp_dml_multi_values, dp_dml_ordered_values, dp_dml_values, dp_fromclause_canonical_column_collection, dp_fromclause_ordered_set, dp_has_cache_key, dp_has_cache_key_list, dp_named_ddl_element, dp_operator, dp_plain_dict, dp_plain_obj, dp_prefix_sequence, dp_propagate_attrs, dp_statement_hint_list, dp_string, dp_string_clauseelement_dict, dp_string_list, dp_string_multi_dict, dp_table_hint_list, dp_type, dp_unknown_structure
New in version 1.4.
-
method
sqlalchemy.sql.visitors.InternalTraversal.
dispatch(visit_symbol)¶ Given a method from
InternalTraversal
, return the corresponding method on a subclass.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_annotations_key = symbol('AK')¶ Visit the _annotations_cache_key element.
This is a dictionary of additional information about a ClauseElement that modifies its role. It should be included when comparing or caching objects, however generating this key is relatively expensive. Visitors should check the “_annotations” dict for non-None first before creating this key.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_anon_name = symbol('AN')¶ Visit a potentially “anonymized” string value.
The string value is considered to be significant for cache key generation.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_boolean = symbol('B')¶ Visit a boolean value.
The boolean value is considered to be significant for cache key generation.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_clauseelement = symbol('CE')¶ Visit a
ClauseElement
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_clauseelement_list = symbol('CL')¶ Visit a list of
ClauseElement
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_clauseelement_tuple = symbol('CT')¶ Visit a tuple of
ClauseElement
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_clauseelement_tuples = symbol('CTS')¶ Visit a list of tuples which contain
ClauseElement
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_dialect_options = symbol('DO')¶ Visit a dialect options structure.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_dml_multi_values = symbol('DML_MV')¶ Visit the values() multi-valued list of dictionaries of an
Insert
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_dml_ordered_values = symbol('DML_OV')¶ Visit the values() ordered tuple list of an
Update
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_dml_values = symbol('DML_V')¶ Visit the values() dictionary of a
ValuesBase
(e.g. Insert or Update) object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_fromclause_ordered_set = symbol('CO')¶ Visit an ordered set of
FromClause
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_has_cache_key = symbol('HC')¶ Visit a
HasCacheKey
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_has_cache_key_list = symbol('HL')¶ Visit a list of
HasCacheKey
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_prefix_sequence = symbol('PS')¶ Visit the sequence represented by
HasPrefixes
orHasSuffixes
.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_propagate_attrs = symbol('PA')¶ Visit the propagate attrs dict. This hardcodes to the particular elements we care about right now.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_statement_hint_list = symbol('SH')¶ Visit the
_statement_hints
collection of aSelect
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
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.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_string_clauseelement_dict = symbol('CD')¶ Visit a dictionary of string keys to
ClauseElement
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_string_list = symbol('SL')¶ Visit a list of strings.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_string_multi_dict = symbol('MD')¶ Visit a dictionary of string keys to values which may either be plain immutable/hashable or
HasCacheKey
objects.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_table_hint_list = symbol('TH')¶ Visit the
_hints
collection of aSelect
object.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_type = symbol('T')¶ Visit a
TypeEngine
objectThe type object is considered to be significant for cache key generation.
-
attribute
sqlalchemy.sql.visitors.InternalTraversal.
dp_unknown_structure = symbol('UK')¶ Visit an unknown structure.
- class sqlalchemy.sql.visitors.Traversible¶
Members
Base class for visitable objects, applies the
TraversibleType
metaclass.-
method
sqlalchemy.sql.visitors.Traversible.
get_children(omit_attrs=(), **kw)¶ Return immediate child
Traversible
elements of thisTraversible
.This is used for visit traversal.
**kw may contain flags that change the collection that is returned, for example to return a subset of items in order to cut down on larger traversals, or to return child items from a different context (such as schema-level collections instead of clause-level).
-
method
- class sqlalchemy.sql.visitors.TraversibleType(clsname, bases, clsdict)¶
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 signature
class
sqlalchemy.sql.visitors.TraversibleType
(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 thevisitors
dictionary may also modify the internals of the given structure as the traversal proceeds.The central API feature used by the
cloned_traverse()
andreplacement_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.
- 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()
function 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.
- 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 returnsNone
, then the object is kept in place.The difference in usage between
cloned_traverse()
andreplacement_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 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.
- 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 thetraverse()
function.- Parameters:
iterator¶ – an iterable or sequence which will yield
ClauseElement
structures; the iterator is assumed to be the product of theiterate()
function.obj¶ – the
ClauseElement
that was used as the target of theiterate()
function.visitors¶ – dictionary of visit functions. See
traverse()
for details on this dictionary.
See also
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