id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28431	default='' (non-bytestring) on BinaryField crashes some migration operations	James	Hasan Ramezani	"== Description ==
Initial migration has a default value {{{''}}} for BinaryField. 
Later, change default value to {{{b''}}} and migrate.
Trying to undo this migration fails. It seems like {{{''}}} is allowed during migration, but not in reverse migration.

== Related issue ==
#22851 Default value for BinaryField

 == Reproduce ==
 Python 3.6.0, Django 1.10.6, Postgres 9.5.4

1. startproject djangoproject
2. startapp firstapp
3. firstapp/models.py:

{{{
class TableOne(models.Model):
    field1 = models.BinaryField(default = '')
}}}

4. makemigrations firstapp
5. migrate firstapp 0001
5. Modify firstapp/models.py

{{{
class TableOne(models.Model):
    field1 = models.BinaryField(default = b'')
}}}

6. migrate firstapp 0002
7. migrate firstapp 0001

Error:  TypeError: can't escape str to binary

{{{
Traceback (most recent call last):
  File ""manage.py"", line 22, in <module>
    execute_from_command_line(sys.argv)
  File ""C:\Py\py3_64\lib\site-packages\django\core\management\__init__.py"", line 367, in execute_from_command_line
    utility.execute()
  File ""C:\Py\py3_64\lib\site-packages\django\core\management\__init__.py"", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""C:\Py\py3_64\lib\site-packages\django\core\management\base.py"", line 294, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""C:\Py\py3_64\lib\site-packages\django\core\management\base.py"", line 345, in execute
    output = self.handle(*args, **options)
  File ""C:\Py\py3_64\lib\site-packages\django\core\management\commands\migrate.py"", line 204, in handle
    fake_initial=fake_initial,
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\executor.py"", line 119, in migrate
    state = self._migrate_all_backwards(plan, full_plan, fake=fake)
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\executor.py"", line 194, in _migrate_all_backwards
    self.unapply_migration(states[migration], migration, fake=fake)
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\executor.py"", line 264, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\migration.py"", line 178, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\operations\fields.py"", line 210, in database_backwards
    self.database_forwards(app_label, schema_editor, from_state, to_state)
  File ""C:\Py\py3_64\lib\site-packages\django\db\migrations\operations\fields.py"", line 205, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\base\schema.py"", line 506, in alter_field
    old_db_params, new_db_params, strict)
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\postgresql\schema.py"", line 118, in _alter_field
    new_db_params, strict,
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\base\schema.py"", line 660, in _alter_field
    params,
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\base\schema.py"", line 112, in execute
    cursor.execute(sql, params)
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\utils.py"", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ""C:\Py\py3_64\lib\site-packages\django\db\backends\utils.py"", line 65, in execute
    return self.cursor.execute(sql, params)
TypeError: can't escape str to binary
}}}


== Notes ==
site-packages\django\db\backends\base\shema.py {{{ def effective_default(self, field): }}}

determines default as an empty <class 'str'>, when  {{{(default = '')}}}

== Possible Fix? ==
site-packages\django\db\backends\base\shema.py ~line 197

{{{
def effective_default(self, field):
        if field.has_default():
            default = field.get_default()
            if field.get_internal_type() == ""BinaryField"" and not default:
                default = six.binary_type()
        elif not field.null and field.blank and field.empty_strings_allowed:
            if field.get_internal_type() == ""BinaryField"":
                default = six.binary_type()
            else:
                default = six.text_type()
        elif getattr(field, 'auto_now', False)
}}}
"	Bug	closed	Core (System checks)	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0