To answer my own question and put it on the ML for future reference,
creating classes for SQL-Alchemy SQL data types that bind a trace
callback does indeed synchronize changes that happen to the widget
onscreen with the SQL-A object.

This is very convenient because my forms are all a single transaction
in the DB, and when the changes are synced my save call is merely
commit.

Thanks.

# ----------------------------------------------------------------------
class stringFieldVar(Variable):
"""Override Tk Variable class to allow sync and validation with SQLA objects"""

def __init__(self, datum, field):

self.datum = datum
self.field = field

super().__init__( value = self._tkConvert(getattr(self.datum, self.field, '')) )
self.trace("w", self.synchronizeWithSQL)


def _tkConvert(self, value):
"""Convert to a string for Tk, only used on initial set from DB"""
if value == None:
result = ''
else:
result = str(value)
return result


def _sqlConvert(self, value):
"""Convert a Tk string back to SQL datatype, happens every widget refresh"""
if value == '':
result = None
elif value:
result = str(value)
return result


def synchronizeWithSQL(self, *args):
"""Convert data for DB type, the widget calls this repeatedly with changes"""

try:
# only update the sql object if the value is valid
logger.debug("Testing {}, is '{}' valid?".format(self.field, self.get()))
sqlValue = self._sqlConvert(self.get())
setattr(self.datum, self.field, sqlValue)
logger.debug("Setting {}, converted '{}', to '{}'".format(self.field, self.get(), sqlValue))
except (AttributeError, TypeError):
pass



class intFieldVar(stringFieldVar):

def _sqlConvert(self, value):
if value == '':
result = None
elif value:
result = int(value)
return result



class numericFieldVar(stringFieldVar):

def _sqlConvert(self, value):
if value == '':
result = None
elif value:
result = D.decimal(value)
return result



class booleanFieldVar(stringFieldVar):

def _tkConvert(self, value):
"""Convert to a string for Tk, only used on initial set from DB"""
if value == None:
result = ''
elif value == True:
result = '1'
else:
result = '0'
return result


def _sqlConvert(self, value):
if value == '':
result = None
elif value == '1':
result = True
else:
result = False
return result



class dateFieldVar(stringFieldVar):

def _tkConvert(self, value):
return '' if value == None else value.strftime("%Y/%m/%d")


def _sqlConvert(self, value):
if value == '':
result = None
elif value == 'now':
result = 'now'
elif value:
result = time.strftime("%Y/%m/%d", value)
return result



class datetimeFieldVar(stringFieldVar):

def _tkConvert(self, value):
return '' if value == None else value.strftime("%Y/%m/%d %H:%M:%S")


def _sqlConvert(self, value):
if value == '':
result = None
elif value == 'now':
result = 'now'
elif value:
result = time.strftime("%Y/%m/%d %H:%M:%S", value)
return result





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