class User(db.Model):
  ...
  email = db.Column(db.String(255)) # don't worry about uniqueness for now

  @validates('email')
  def validate_email(self, key, address):
    assert '@' in address # of course a regex is better, I know
    return address

try:
  user = User('fidel#cuba.cu')
  db.session.add(user)
  db.session.commit()
except Exception as e:
  print e.message # returns an empty string ''

...
except Exception as e:
  # json_error is a custom function of my own
  return json_error(e.message, 400)

class User(db.Model):
  # ... snip ...

  @validates('email')
  def validate_email(self, key, address):
    assert '@' in address, "Invalid email"
    return address