The attributeimpl for a deferred_expression does not
support a scalar loader, add new configurability so that
the impl can have this flag turned off.   Document
that the with_expression() system currently does not
offer any deferred loading.

To eliminate confusion over "deferred", which refers to
lazy loading of column attributes, and "with_expression",
which refers to an attribute that is explicitly at
query time only, rename deferred_expression to query_expression.

Change-Id: I07c4a050ed68c79ccbde9492e9de1630b7470d74

Added new event handler :meth:`.AttributeEvents.modified` which is
triggered when the func:`.attributes.flag_modified` function is
invoked, which is common when using the :mod:`sqlalchemy.ext.mutable`
extension module.

Change-Id: Ic152f1d5c53087d780b24ed7f1f1571527b9e8fc
Fixes: #3303

The :meth:`.Query.update` method can now accommodate both
hybrid attributes as well as composite attributes as a source
of the key to be placed in the SET clause.   For hybrids, an
additional decorator :meth:`.hybrid_property.update_expression`
is supplied for which the user supplies a tuple-returning function.

Change-Id: I15e97b02381d553f30b3301308155e19128d2cfb
Fixes: #3229

The :func:`.attributes.flag_modified` function now raises
:class:`.InvalidRequestError` if the named attribute key is not
present within the object, as this is assumed to be present
in the flush process.  To mark an object "dirty" for a flush
without referring to any specific attribute, the
:func:`.attributes.flag_dirty` function may be used.

Change-Id: I6c64e4d253c239e38632f38c27bb16e68fe8dfbe
Fixes: #3753

Added new attribute event :meth:`.AttributeEvents.bulk_replace`.
This event is triggered when a collection is assigned to a
relationship, before the incoming collection is compared with the
existing one.  This early event allows for conversion of incoming
non-ORM objects as well.  The event is integrated with the
``@validates`` decorator.

The ``@validates`` decorator now allows the decorated method to receive
objects from a "bulk collection set" operation that have not yet
been compared to the existing collection.  This allows incoming values
to be converted to compatible ORM objects as is already allowed
from an "append" event.   Note that this means that the
``@validates`` method is called for **all** values during a collection
assignment, rather than just the ones that are new.

Change-Id: I27f59db008d9e521d31a3e30143d7cd997e4b7b3
Fixes: #3896

Corrects some warnings and adds tox config.  Adds DeprecationWarning
to the error category.   Large sweep for string literals w/ backslashes
as this is common in docstrings

Co-authored-by: Andrii Soldatenko
Fixes: #3886
Change-Id: Ia7c838dfbbe70b262622ed0803d581edc736e085
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/337

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

The scalar object set() method calls upon the lazy loader
to get at the "old" value of the attriute, however doesn't
ensure that the "committed" value of the foreign key attributes
is used.  If the user has manipulated these attributes and they
themselves have pending, non committed changes, we get the
"new" value which these attributes would have set up if they
were flushed.  "old" vs "new" value is always about how the
value has changed since the load, so we always have to use the
DB-persisted values for everything when looking for "old".

Change-Id: I82bdc40ad0cf033c3a98f3361776cf3161542cd6
Fixes: #3708

This allows us to build default-setting recipes such
as one that allows us to actively read column-level
defaults.  An example suite is also added.

Change-Id: I7b022d52cc89526132d5bc4201ac27fea4cf088d
Fixes: #1311