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Comment by jgraham | Hacker News Reader

[I was the original author of much of html5lib]

Yes, http://speed.pypy.org/comparison/ has html5lib parsing (an old, static copy of) the HTML5 spec as a benchmark. It's about 3x faster than CPython, which is to say, still very slow.

I agree there's a good argument that writing new parser code in C is not a great idea, but I don't think it's possible — or at least easy — to get reasonable parsing speed in pure Python. My ideal solution here would be bindings to a Rust parser e.g. html5ever, although there is a tradeoff there in terms of ease of distribution.

The main missing feature in html5ever for these purposes is the ability to construct the final parse tree in the rust code (c.f. lxml), without which the performance bottlneck becomes constructing the Python objects to represent the document. Of course a streaming API like SAX can be even faster, but often isn't all that useful.

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[I'm the current mostly absentee maintainer of html5lib]

https://speed.python.org/comparison/?exe=12%2BL%2Bmaster%2C1... has an up-to-date version of html5lib, albeit only on CPython: notably, both 3.6 and the latest 3.7 build are significantly faster than 2.7.

That said, I don't think html5lib is going to become massively quicker: string allocations are just going to become an ever bigger issue (i.e., s[1:10] causes an allocation in Python v. just referencing the subsequence), and even using Cython, at least under CPython, isn't going to help with that.

Couldn't a little toolbox of helpers take care of those allocations? Or would that turn the whole thing into something too complicated?

I thought you could use memoryview over a string to get rid of that allocation even in 2.7

https://docs.python.org/3/library/stdtypes.html#memoryview

Allocations are challenging for HTML parsers, even in C, because of the presence of entity references and case-normalization of attribute & tag names. That means that a lot of the time when you think you ought to be able to just use a slice or memoryview into the original source text, you can't; for example, if any of your text nodes contains < ('<') or &ldquo (smart double quote), you can't use the original source buffer, because you're supposed to have decoded the entity to a unicode character, which will leave the string a different length. This happens stupidly often in real HTML.

I initially had the API for Gumbo use string slices a lot more than the final released API, and then found that I couldn't do it and needed to allocate in order to maintain correctness. I'd done a patch that arena-allocated all memory used in the parse, which gave a fairly significant CPU speedup, but it also bloated max memory usage in ways that some clients found unacceptable, so I never merged it. Small C strings at least are quite lightweight; Python strings have a lot of additional overhead, and much of the PyObject structure itself requires chasing pointers.

Only over bytes objects, not over unicode objects.

If just the tokenizer was in Rust, that might help. Much of the HTML5 error handling involves dealing with malformed tokens. Comments that begin "<-" instead of "<--". All that stuff at the beginning of a file for guessing the character set. None of those require creating elaborate Python structures from non-Python code.

(I run a web crawler written in Python, which means code exposed to arbitrarily bad HTML. I've had to report and fix bugs in BeautifulSoup and html5lib. At least they fail in a well-defined way. C code can fail in arbitrarily bad ways, and C string processing is notorious for being troublesome.)

> My ideal solution here would be bindings to a Rust parser e.g. html5ever

Funny how somebody submitted exactly this on HN a few hours ago : https://news.ycombinator.com/item?id=14591017

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