SQL Expression Language Foundational Constructs

Base classes and mixins that are used to compose SQL Expression Language elements.

Object Name Description

CacheKey

The key used to identify a SQL statement construct in the SQL compilation cache.

ClauseElement

Base class for elements of a programmatically constructed SQL expression.

DialectKWArgs

Establish the ability for a class to have dialect-specific arguments with defaults and constructor validation.

HasCacheKey

Mixin for objects which can produce a cache key.

LambdaElement

A SQL construct where the state is stored as an un-invoked lambda.

StatementLambdaElement

Represent a composable SQL statement as a LambdaElement.

class sqlalchemy.sql.expression.CacheKey(key, bindparams)

The key used to identify a SQL statement construct in the SQL compilation cache.

method sqlalchemy.sql.expression.CacheKey.to_offline_string(statement_cache, statement, parameters)

Generate an “offline string” form of this CacheKey

The “offline string” is basically the string SQL for the statement plus a repr of the bound parameter values in series. Whereas the CacheKey object is dependent on in-memory identities in order to work as a cache key, the “offline” version is suitable for a cache that will work for other processes as well.

The given statement_cache is a dictionary-like object where the string form of the statement itself will be cached. This dictionary should be in a longer lived scope in order to reduce the time spent stringifying statements.

class sqlalchemy.sql.expression.ClauseElement

Base class for elements of a programmatically constructed SQL expression.

Class signature

class sqlalchemy.sql.expression.ClauseElement (sqlalchemy.sql.roles.SQLRole, sqlalchemy.sql.annotation.SupportsWrappingAnnotations, sqlalchemy.sql.traversals.MemoizedHasCacheKey, sqlalchemy.sql.traversals.HasCopyInternals, sqlalchemy.sql.visitors.Traversible)

method sqlalchemy.sql.expression.ClauseElement.compare(other, **kw)

Compare this ClauseElement to the given ClauseElement.

Subclasses should override the default behavior, which is a straight identity comparison.

**kw are arguments consumed by subclass compare() methods and may be used to modify the criteria for comparison (see ColumnElement).

method sqlalchemy.sql.expression.ClauseElement.compile(bind=None, dialect=None, **kw)

Compile this SQL expression.

The return value is a Compiled object. Calling str() or unicode() on the returned value will yield a string representation of the result. The Compiled object also can return a dictionary of bind parameter names and values using the params accessor.

Parameters
  • bind – An Engine or Connection from which a Compiled will be acquired. This argument takes precedence over this ClauseElement’s bound engine, if any.

  • column_keys – Used for INSERT and UPDATE statements, a list of column names which should be present in the VALUES clause of the compiled statement. If None, all columns from the target table object are rendered.

  • dialect – A Dialect instance from which a Compiled will be acquired. This argument takes precedence over the bind argument as well as this ClauseElement ‘s bound engine, if any.

  • compile_kwargs

    optional dictionary of additional parameters that will be passed through to the compiler within all “visit” methods. This allows any custom flag to be passed through to a custom compilation construct, for example. It is also used for the case of passing the literal_binds flag through:

    from sqlalchemy.sql import table, column, select
    
    t = table('t', column('x'))
    
    s = select(t).where(t.c.x == 5)
    
    print(s.compile(compile_kwargs={"literal_binds": True}))

    New in version 0.9.0.

method sqlalchemy.sql.expression.ClauseElement.get_children(omit_attrs=(), **kw)

inherited from the Traversible.get_children() method of Traversible

Return immediate child Traversible elements of this Traversible.

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).

class memoized_attribute(fget, doc=None)

A read-only @property that is only evaluated once.

method sqlalchemy.sql.expression.ClauseElement.classmethod memoized_instancemethod(fn)

inherited from the HasMemoized.memoized_instancemethod() method of HasMemoized

Decorate a method memoize its return value.

method sqlalchemy.sql.expression.ClauseElement.params(*optionaldict, **kwargs)

Return a copy with bindparam() elements replaced.

Returns a copy of this ClauseElement with bindparam() elements replaced with values taken from the given dictionary:

>>> clause = column('x') + bindparam('foo')
>>> print(clause.compile().params)
{'foo':None}
>>> print(clause.params({'foo':7}).compile().params)
{'foo':7}
method sqlalchemy.sql.expression.ClauseElement.self_group(against=None)

Apply a ‘grouping’ to this ClauseElement.

This method is overridden by subclasses to return a “grouping” construct, i.e. parenthesis. In particular it’s used by “binary” expressions to provide a grouping around themselves when placed into a larger expression, as well as by select() constructs when placed into the FROM clause of another select(). (Note that subqueries should be normally created using the Select.alias() method, as many platforms require nested SELECT statements to be named).

As expressions are composed together, the application of self_group() is automatic - end-user code should never need to use this method directly. Note that SQLAlchemy’s clause constructs take operator precedence into account - so parenthesis might not be needed, for example, in an expression like x OR (y AND z) - AND takes precedence over OR.

The base self_group() method of ClauseElement just returns self.

method sqlalchemy.sql.expression.ClauseElement.unique_params(*optionaldict, **kwargs)

Return a copy with bindparam() elements replaced.

Same functionality as ClauseElement.params(), except adds unique=True to affected bind parameters so that multiple statements can be used.

class sqlalchemy.sql.base.DialectKWArgs

Establish the ability for a class to have dialect-specific arguments with defaults and constructor validation.

The DialectKWArgs interacts with the DefaultDialect.construct_arguments present on a dialect.

method sqlalchemy.sql.base.DialectKWArgs.classmethod argument_for(dialect_name, argument_name, default)

Add a new kind of dialect-specific keyword argument for this class.

E.g.:

Index.argument_for("mydialect", "length", None)

some_index = Index('a', 'b', mydialect_length=5)

The DialectKWArgs.argument_for() method is a per-argument way adding extra arguments to the DefaultDialect.construct_arguments dictionary. This dictionary provides a list of argument names accepted by various schema-level constructs on behalf of a dialect.

New dialects should typically specify this dictionary all at once as a data member of the dialect class. The use case for ad-hoc addition of argument names is typically for end-user code that is also using a custom compilation scheme which consumes the additional arguments.

Parameters
  • dialect_name – name of a dialect. The dialect must be locatable, else a NoSuchModuleError is raised. The dialect must also include an existing DefaultDialect.construct_arguments collection, indicating that it participates in the keyword-argument validation and default system, else ArgumentError is raised. If the dialect does not include this collection, then any keyword argument can be specified on behalf of this dialect already. All dialects packaged within SQLAlchemy include this collection, however for third party dialects, support may vary.

  • argument_name – name of the parameter.

  • default – default value of the parameter.

New in version 0.9.4.

attribute sqlalchemy.sql.base.DialectKWArgs.dialect_kwargs

A collection of keyword arguments specified as dialect-specific options to this construct.

The arguments are present here in their original <dialect>_<kwarg> format. Only arguments that were actually passed are included; unlike the DialectKWArgs.dialect_options collection, which contains all options known by this dialect including defaults.

The collection is also writable; keys are accepted of the form <dialect>_<kwarg> where the value will be assembled into the list of options.

New in version 0.9.2.

Changed in version 0.9.4: The DialectKWArgs.dialect_kwargs collection is now writable.

See also

DialectKWArgs.dialect_options - nested dictionary form

attribute sqlalchemy.sql.base.DialectKWArgs.dialect_options

A collection of keyword arguments specified as dialect-specific options to this construct.

This is a two-level nested registry, keyed to <dialect_name> and <argument_name>. For example, the postgresql_where argument would be locatable as:

arg = my_object.dialect_options['postgresql']['where']

New in version 0.9.2.

See also

DialectKWArgs.dialect_kwargs - flat dictionary form

attribute sqlalchemy.sql.base.DialectKWArgs.kwargs

A synonym for DialectKWArgs.dialect_kwargs.

class sqlalchemy.sql.traversals.HasCacheKey

Mixin for objects which can produce a cache key.

attribute sqlalchemy.sql.traversals.HasCacheKey.inherit_cache = None

Indicate if this HasCacheKey instance should make use of the cache key generation scheme used by its immediate superclass.

The attribute defaults to None, which indicates that a construct has not yet taken into account whether or not its appropriate for it to participate in caching; this is functionally equivalent to setting the value to False, except that a warning is also emitted.

This flag can be set to True on a particular class, if the SQL that corresponds to the object does not change based on attributes which are local to this class, and not its superclass.

See also

Enabling Caching Support for Custom Constructs - General guideslines for setting the HasCacheKey.inherit_cache attribute for third-party or user defined SQL constructs.

class sqlalchemy.sql.expression.LambdaElement(fn, role, opts=<class 'sqlalchemy.sql.lambdas.LambdaOptions'>, apply_propagate_attrs=None)

A SQL construct where the state is stored as an un-invoked lambda.

The LambdaElement is produced transparently whenever passing lambda expressions into SQL constructs, such as:

stmt = select(table).where(lambda: table.c.col == parameter)

The LambdaElement is the base of the StatementLambdaElement which represents a full statement within a lambda.

New in version 1.4.

class sqlalchemy.sql.expression.StatementLambdaElement(fn, role, opts=<class 'sqlalchemy.sql.lambdas.LambdaOptions'>, apply_propagate_attrs=None)

Represent a composable SQL statement as a LambdaElement.

The StatementLambdaElement is constructed using the lambda_stmt() function:

from sqlalchemy import lambda_stmt

stmt = lambda_stmt(lambda: select(table))

Once constructed, additional criteria can be built onto the statement by adding subsequent lambdas, which accept the existing statement object as a single parameter:

stmt += lambda s: s.where(table.c.col == parameter)

New in version 1.4.

Class signature

class sqlalchemy.sql.expression.StatementLambdaElement (sqlalchemy.sql.roles.AllowsLambdaRole, sqlalchemy.sql.lambdas.LambdaElement)

method sqlalchemy.sql.expression.StatementLambdaElement.add_criteria(other, enable_tracking=True, track_on=None, track_closure_variables=True, track_bound_values=True)

Add new criteria to this StatementLambdaElement.

E.g.:

>>> def my_stmt(parameter):
...     stmt = lambda_stmt(
...         lambda: select(table.c.x, table.c.y),
...     )
...     stmt = stmt.add_criteria(
...         lambda: table.c.x > parameter
...     )
...     return stmt

The StatementLambdaElement.add_criteria() method is equivalent to using the Python addition operator to add a new lambda, except that additional arguments may be added including track_closure_values and track_on:

>>> def my_stmt(self, foo):
...     stmt = lambda_stmt(
...         lambda: select(func.max(foo.x, foo.y)),
...         track_closure_variables=False
...     )
...     stmt = stmt.add_criteria(
...         lambda: self.where_criteria,
...         track_on=[self]
...     )
...     return stmt

See lambda_stmt() for a description of the parameters accepted.

method sqlalchemy.sql.expression.StatementLambdaElement.spoil()

Return a new StatementLambdaElement that will run all lambdas unconditionally each time.