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
15625	MultiValueDictKeyError exception has large performance penalty	Margie Roginski	nobody	"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?"	Cleanup/optimization	closed	HTTP handling	dev	Normal	fixed			Accepted	1	0	0	0	0	0