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I checked correctness of the return expression replacement, using test_cases above, with

for s in test_cases:
    (pat(s) is not None) is (s.strip() in ('', '#'))

and all are True.

I checked correctness of the return expression replacement, using test_cases above, with

Ok, thank you, but this test_cases is too small a sample and therefore I have supplemented my script with the following checks:

import string
import random
from itertools import product

# First Checks:
for case in test_cases:
    assert is_line_junk_regex(case) == is_line_junk_no_regex(case), f"Inconsistent behavior for {repr(case)}"

def random_string(length=10):
    chars = string.printable + " " * 5 + "#" * 5
    return "".join(random.choice(chars) for _ in range(length))

# Second Checks:
for _ in range(10000):
    case = random_string(random.randint(0, 20))
    assert is_line_junk_regex(case) == is_line_junk_no_regex(case), f"Inconsistent behavior for {repr(case)}"

# Third Checks:
chars = " #\tabc"
for length in range(6):
    for case in map("".join, product(chars, repeat=length)):
        assert is_line_junk_regex(case) == is_line_junk_no_regex(case), f"Inconsistent behavior for {repr(case)}"

Then I checked the tests in Lib/test/test_difflib.py and everything finished well 👍

I have made the requested changes; please review again

Thanks for making the requested changes!

@terryjreedy, @AA-Turner: please review the changes made to this pull request.

Since we're improving performance, we don't want to recompute line.strip()

Sorry, I suspected this but hoped that there was a cache mechanism here :)

I checked correctness of the return expression replacement, using test_cases above, with

for s in test_cases:
    (pat(s) is not None) is (s.strip() in ('', '#'))

and all are True.

I am going to be neutral an whether the basic change should be merged. If it is, I personally think the 'pat' parameter can be considered to be private and eliminated. But I would defer the back-compatibility to others, include 'encukou'. The only documented use of the function, marked as a constant, is to be passed as an argument. The argument is always passed on to SequenceMatcher, which calls it once with 1 argument, which would be a line.

@serhiy-storchaka

In any case, the relative performance depends on the input -- for a long string with a single space character at one end (e.g. 'x'*10000+' ') the non-regex version is more than 10 times slower than the regex version. For length 100000 the difference is more than 100, and for length 1000000 -- more than 1000.

I tested your cases but didn't get the result you expected (maybe I did something wrong?)
I ran next script (accidentally mixed up the dividend and divisor, but it doesn't matter):

import re
import timeit

def is_line_junk_regex(line, pat=re.compile(r"\s*(?:#\s*)?$").match):
    return pat(line) is not None

def is_line_junk_no_regex(line):
    return line.strip() == "" or line.lstrip().rstrip() == "#"

def benchmark(func, cases, n=1_000):
    return timeit.timeit(lambda: [func(line) for line in cases], number=n)

lengths = [10_000, 100_000, 1_000_000]
for length in lengths:
    test_string = "x" * length + " "
    regex_time = benchmark(is_line_junk_regex, test_string)
    no_regex_time = benchmark(is_line_junk_no_regex, test_string)

    print(f"String length: {length}")
    print(f"Regex version: {regex_time:.6f} seconds")
    print(f"Non-regex version: {no_regex_time:.6f} seconds")
    print(f"Speedup: {no_regex_time / regex_time:.2f}x\n")

and got next results:

$ ./python -B bench.py
String length: 10000
Regex version: 4.220341 seconds
Non-regex version: 1.983180 seconds
Speedup: 0.47x

String length: 100000
Regex version: 42.698992 seconds
Non-regex version: 20.897327 seconds
Speedup: 0.49x

String length: 1000000
Regex version: 516.844790 seconds
Non-regex version: 262.632697 seconds
Speedup: 0.51x

Aren't you iterating over a single character instead of a full line:

[func(line) for line in cases]

You should pass [test_string] instead of just test_string (sorry if I'm wrong, I'm tired)

I was the one who originally reqested restoring pat, but I do find Terry's argument compelling, and I wouldn't oppose removing pat.

I think we should restore this PR to the simpler previous version (the differences micro-benchmarks between string comparisons are so small they will be swallowed by noise). The class-level re import should be resolved before merge, though.

A

@tim-one Do you have a reason to want to keep the regex?

Can't say I much care, but I don't get it. What's the point? It's dubious on the face of it to build an entirely new string object just to do simple lexical analysis. If this is some "micro-optimizztion" thing, instead of

        stripped = line.strip()
        return stripped == '' or stripped == '#'

try

        return line.strip() in '#' # empty or single '#'

?

Can't say I much care, but I don't get it. What's the point? It's dubious on the face of it to build an entirely new string object just to do simple lexical analysis. If this is some "micro-optimizztion" thing, instead of

        stripped = line.strip()

        return stripped == '' or stripped == '#'

try

        return line.strip() in '#' # empty or single '#'

?

that's how it was originally :)

I'll probably return it so as not to overcomplicate things

@rhettinger

@tim-one Do you have a reason to want to keep the regex?

Tim doesn't seem to mind

I don't know who I can ping but it seems like there are more suggestions