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deprecate OrderedMultiDict #2968
Description
OrderedMultiDict is not used by Werkzeug. It has a complex implementation, and is documented to be slow. dict maintains key insertion order in all supported versions of Python. MultiDict already maintains the insertion order of values within a key. OrderedMultiDict maintains the insertion order of each item, so [(a, 1), (b, 2), (a, 3)] comes out the same instead of [(a, 1), (a, 3), (b, 2)]. It's not clear there's a use case for this, searching sourcegraph shows some use of OrderedMultiDict but it's not clear if the use is actually required vs using MultiDict. Other libraries such as boltons also provide implementations. Mark this as deprecated and see if anything comes up.
Activity
davidism commentedon Oct 27, 2024
https://sourcegraph.com/search?q=context:global+lang:Python+werkzeug+OrderedMultiDict+-file:venv+-file:site-packages+-file:werkzeug+-repo:pallets/werkzeug+&patternType=keyword&sm=0 shows ~24 repos using
OrderedMultiDict. Browsing through them, most uses do not seem necessary.I did see a couple cases where knowing the exact order that
request.argsorrequest.formwas parsed in was important. But in that case, it's more straightforward to compare the exact data sent rather than the parsed version:request.query_stringis the exact value from the URL that gets parsed intorequest.argsrequest.get_data()called before accessingrequest.formreturns the exact request body bodydavidism commentedon Oct 28, 2024
@ThiefMaster Indico uses it in a few places: https://github.com/search?q=repo%3Aindico%2Findico%20orderedmultidict&type=code, thoughts? It's not clear that it's needed.
ThiefMaster commentedon Oct 28, 2024
I think
parameter_storage_class = ImmutableOrderedMultiDict(on the Request subclass) is the most interesting one, because I added that one due to how crappy the PayPal webhook validation is (they require you to send back data you received in the exact same order).IIRC there was either something in the docs on on some SO post recommending this, so there may be more people using it when they have a paypal integration in their flask app.
FWIW depending on the architecture of an application (especially when plugins are involved) it may be hard to ensure that gets called before anything else accesses the form data.
The other two places where it's used are most likely leftovers from before
dictwas guaranteed to be ordered by insertion order, so if the standard MultiDict already does that now as well it's enough.davidism commentedon Oct 28, 2024
PayPal IPN was one use case I saw, seems like a weird API design. The other use case I found seemed to be trying to check that a URL was canonical based not only on the query args but their exact order in the URL.
I know it's not great to have to change it, but doing something like the following would probably be enough for the PayPal case, if an app can't guarantee that the data is cached first. The query args case could probably just look at
query_stringorparse_qsl(query_string)instead.Then you can access
form.original_itemsif needed. Even seems preferable toOrderedMultiDict, which is slower and uses even more memory.I did see that Werkzeug's documentation for
parameter_storage_classmentionsImmutableOrdereMultiDictas an option, so that's probably where the advice came from.ThiefMaster commentedon Oct 28, 2024
FWIW I don't think paypal has any multi-value keys, so probably a normal MultiDict is enough nowadays...
davidism commentedon Oct 28, 2024
Oh yeah, that level of ordering only matters if there are actually multiple values per key, and they're submitted with keys interleaved. Seems exceedingly unlikely.