Small micro optimisations in w3lib.http

[abebus]

This PR includes some small micro-optimizations in w3lib.http. They're not huge changes, but they help improve performance a bit here and there.

I'm really thankful for all the amazing work done by the Scrapy team and contributors — I’ve learned a lot from the ecosystem. This PR is my small way of saying thanks.

I'm currently very into micro-optimizations, but I hope to keep contributing more (and maybe more useful things!) over time.

Also, please let me know: should I add a pytest-benchmark suite, or is a simple synthetic benchmark per PR enough?

Thanks for your time

[abebus]

I used the following script for benchmarking:

import timeit

from w3lib.http import headers_dict_to_raw, headers_raw_to_dict

# Sample headers for testing
headers_raw = (
    b"Content-Type: text/html\r\n"
    b"Accept: gzip\r\n"
    b"Cache-Control: no-cache\r\n"
    b"X-Test: value1\r\n"
    b"X-Test: value2\r\n"
)

headers_dict = {
    b"Content-Type": [b"text/html"],
    b"Accept": [b"gzip"],
    b"Cache-Control": [b"no-cache"],
    b"X-Test": [b"value1", b"value2"],
}

# Benchmark wrappers
def benchmark_headers_raw_to_dict():
    headers_raw_to_dict(headers_raw)

def benchmark_headers_dict_to_raw():
    headers_dict_to_raw(headers_dict)

# Run benchmarks
if __name__ == "__main__":
    raw_to_dict_time = timeit.timeit(benchmark_headers_raw_to_dict, number=1_000_000)
    dict_to_raw_time = timeit.timeit(benchmark_headers_dict_to_raw, number=1_000_000)

    print("Benchmark results (1,000,000 runs):")
    print(f"headers_raw_to_dict: {raw_to_dict_time:.4f} seconds")
    print(f"headers_dict_to_raw: {dict_to_raw_time:.4f} seconds")

On my machine, here are the best results (out of 10 runs):

Before changes:

headers_raw_to_dict: 2.4984 seconds
headers_dict_to_raw: 2.9800 seconds

After changes:

headers_raw_to_dict: 2.2313 seconds
headers_dict_to_raw: 2.1083 seconds

[Gallaecio]

Nice

[abebus]

In my latest commit, headers_raw_to_dict now completes in approximately 2.004 seconds in the benchmark script I provided. I believe the speedup comes from reducing Python function calls by using BytesIO directly, which delegates iteration to the underlying C implementation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.96%. Comparing base (5423e0a) to head (a5d30c0).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   97.92%   97.96%   +0.03%     
==========================================
  Files           9        9              
  Lines         483      491       +8     
  Branches       78       83       +5     
==========================================
+ Hits          473      481       +8     
  Misses          6        6              
  Partials        4        4              

Files with missing lines	Coverage Δ
w3lib/http.py	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[wRAR]

I don't have experience with pytest-benchmark, but if you have any specific ideas you are welcome to implement them, it's fine to do as a separate PR.

[abebus]

Okay, will do, but I think this might require a major refactor of the tests or lead to quite a bit of code duplication, unfortunately... As a reference, I’ll use the aiohttp benchmarking suite; they also use codspeed for generating nice benchmarking reports.

[abebus]

Surprisingly, concatenating plain bytes is noticeably faster than using bytearray. I believe this is thanks to Python’s "new" adaptive interpreter

[abebus]

Well, it's only faster with relatively small headers — see #247. I had incorrectly assumed that bytes would be faster due to adaptive interpreter optimizations. But now I believe the real reason is that strings and bytes don’t always recreate themselves on each concatenation, as explained here

[abebus]

So, summing up, the previous benchmark times were:

headers_raw_to_dict: 2.4984 
headers_dict_to_raw: 2.9800 seconds

And now:

headers_raw_to_dict: 2.0040 seconds
headers_dict_to_raw: 1.7675 seconds

So, headers_raw_to_dict is approximately 20% faster, and headers_dict_to_raw is about 40% faster.

[abebus]

Do I need to fix the codecov report with missing base or will the problem be automatically resolved after the merge?

[wRAR]

codecov has some problems recently, I don't think we can do anything about that.

[wRAR]

Thanks! Please let us know when you think it's ready for merging.

[abebus]

I believe it’s ready to merge. I’ve done my best on these functions and don’t plan any further changes :)