MultiValueDictKeyError exception has large performance penalty

[Margie Roginski]

I am seeing a large potential performance penalty due to a string argument representing the entire POST dict getting created when a key is not found in the POST dict.

In datastructures.py, in class MultiValueDict, __getitem__() has the following code:

        try:
            list_ = super(!MultiValueDict, self).__getitem__(key)
        except !KeyError:
            raise !MultiValueDictKeyError("Key %r not found in %r" % (key, self))

During form validation, this exception is raised for each field/key not found in the POST dict. If the POST dict is very large, the %r associated with self is very large and can have a very significant time penalty. In a case where I had 300 of these exceptions, the time penalty of creating this string was almost 2 seconds.

It is not that unusual to have POST keys missing that are checked in this way. Here are a few of the common cases I've seen:

1. checkboxes that are unchecked
2. url arguments that could be POSTED but which are omitted
3. fields that are dynamic - ie, they may not be there unless some client code creates them.

Do we need to provide a string argument at all in this case, given the potential time penalty and the frequency of getting values from the POST dict?

[Adrian Holovaty]

This is a fair point. I would be in favor of changing the exception to match that of the built-in dictionary type: KeyError: 'key'.

[Tim Graham]

​https://github.com/django/django/pull/1635

[Tim Graham <timograham@…>]

In 893198509e3b821a56a56e4326929e0613aad983:

Fixed #15625 -- Made message in MultiValueDictKeyError less verbose.

Thanks margieroginski for the suggestion.

[Tim Graham <timograham@…>]

In 275497c5709792b47c71538a7e58bb40e8d8d3a9:

[1.6.x] Fixed #15625 -- Made message in MultiValueDictKeyError less verbose.

Thanks margieroginski for the suggestion.

Backport of 893198509e from master