I noticed there are a few more common cases that can be sped up in bytes.join().

When:
 - The sequence length and separator length are both 0
 - The separator length is 0 or 1
 - The separator string contains only 1 distinct byte

These could also be applied to other sequence types.

for i in {{0..5}}; do
./python -m timeit -s "f=open('/usr/share/dict/words', 'rb'); L=f.readlines()" \
"b' '.join(L)"; 
done

- BEFORE -
100 loops, best of 3: 3.79 msec per loop
100 loops, best of 3: 3.5 msec per loop
100 loops, best of 3: 3.75 msec per loop
- AFTER -
100 loops, best of 3: 2.81 msec per loop
100 loops, best of 3: 2.81 msec per loop
100 loops, best of 3: 3.03 msec per loop

~20% speedup

Same test on a smaller list (lines from LICENSE file) yields a similar 15-20% speedup.
Same test on L = [b'1', b'2', b'3'] yields 10% speedup

Honestly, I don't think there's much point in these optimizations. The first one ("The sequence length and separator length are both 0") is probably useless.

> Honestly, I don't think there's much point in these optimizations. The first one ("The sequence length and separator length are both 0") is probably useless.

This optimization changes the behavior.

...     def __repr__(self): return 'B(' + super().__repr__() + ')'
... 
>>> B()
B(b'')
>>> B().join([])
b''

With the patch we get B(b'').

Regardless of whether optimization (which is negligible in this case), I don't know whether to consider such side effect desirable or undesirable.

bytes.join need to optimize for the 0- and 1-byte delimiter, but the proposed patch leads to pessimization:

Unpatched:
$ ./python -m timeit -s 'seq=[bytes([i]*100000) for i in range(256)]' 'b"".join(seq)'
10 loops, best of 3: 43.3 msec per loop

Patched:
$ ./python -m timeit -s 'seq=[bytes([i]*100000) for i in range(256)]' 'b" ".join(seq)'
10 loops, best of 3: 70.3 msec per loop

> Regardless of whether optimization (which is negligible in this case), I don't know whether to consider such side effect desirable or undesirable.

After some thought, I realized that this is an erroneous behavior.

Side note: Windows requires that args be quoted with ", not ', to work properly, at least with these args.

Main note: the patched test adds a space to the separator, but that is not enough to account for the difference.

c:\Programs\Python32>python -m timeit -s "seq=[bytes([i]*1000) for i in range(256)]" "b''.join(seq)"
10000 loops, best of 3: 31.7 usec per loop

c:\Programs\Python32>python -m timeit -s "seq=[bytes([i]*1000) for i in range(256)]" "b' '.join(seq)"
10000 loops, best of 3: 34.1 usec per loop

The behavior change is wrong and test_bytes.py seems to need augmentation. It begins with "XXX This is a mess. [...]".

class BaseBytesTest(unittest.TestCase):
    def test_join(self)
tests b''.join([]) but as far as I can tell, not b'something'.join([]), the failing case found by Serhiy.
It end with "        # XXX more...".

> Main note: the patched test adds a space to the separator, but that is not enough to account for the difference.

Sorry, I copied the wrong line.

$ ./python -m timeit -s "seq=[bytes([i]*100000) for i in range(256)]" "b' '.join(seq)"
10 loops, best of 3: 45.6 msec per loop

But the conclusions are not affected, for larger byte strings patched
version will be 1.5-2 times slower in limit.

I adapted a patch to the current code (now it works for bytearrays too), 
deleted the behavior change, left optimization only for 0- and 1-bytes 
separator, deleted tests (bytes.join tests already exists), wrote a benchmark 
and after long experiments has set a condition for this optimization. Now, I 
believe, the patch never leads to regression. The patch has been simplified, 
it's only 13 lines long.

Thank you, John O'Connor.

I don't understand the point of this optimization. It looks rather cryptic and very specific. Can you elaborate?

In a normal loop we copy into the output buffer for each iteration the element and the separator. As you know, the processor can copy large amounts of data several times more efficiently than byte-for-byte. Therefore, filling the buffer by memset more efficient than calling Py_MEMCPY on each iteration if the buffer size is not too large. Experimenting on different machines I found that Py_MEMCPY is more expensive than sequential filling up to 32 bytes (may be even more, but I chose a conservative estimate). And for empty separator we can just omit copying and filling (it saves several checks per iteration).

Ok, but does it really make a difference and in which cases?
In other words, do you have benchmark numbers? :)

> Ok, but does it really make a difference and in which cases?

Up to 40% on Athlon and up to 70% on Atom.

> In other words, do you have benchmark numbers? :)

Attached results for AMD Athlon 64 X2 4600+ and Intel Atom N570 @ 1.66GHz 
under 32-bit Linux. You can run the benchmark on your machine (requires Posix-
compatible shell, sed awk).

Ok, here are the benchmark results here with a 1-byte separator:

  10 x 10       0.244 usec   0.202 usec   +21%
 100 x 10       0.325 usec   0.326 usec    -0%
1000 x 10       0.691 usec   0.689 usec    +0%
  10 x 1000      18.2 usec    14.2 usec   +28%
 100 x 1000      39.8 usec    40.6 usec    -2%
1000 x 1000      94.6 usec      96 usec    -1%

and with an empty separator:

  10 x 10       0.245 usec   0.198 usec   +24%
 100 x 10       0.335 usec   0.286 usec   +17%
1000 x 10       0.637 usec   0.593 usec    +7%
  10 x 1000      18.9 usec    14.1 usec   +34%
 100 x 1000      40.3 usec    36.2 usec   +11%
1000 x 1000      93.7 usec    96.9 usec    -3%

(Core i5 2500K, 64-bit, gcc)

I would say the empty separator case is interesting, because a common use case for bytes.join() is indeed fast concatenation. However, the 1-byte separator case could be dropped, which would simplify the patch and the heuristic.

Indeed, 1-char separator is more common for strings, but I found several b'\n'.join or b','.join even in stdlib. 3 lines of difference are only 2.3% of Objects/stringlib/join.h.

Here is a patch with dropped the 1-byte separator case.

New changeset bfa715f98c0f by Antoine Pitrou in branch 'default':
Issue #12805: Make bytes.join and bytearray.join faster when the separator is empty.
http://hg.python.org/cpython/rev/bfa715f98c0f

I have committed the simple optimization patch, thank you!

I think that's a good compromise and much better than what I had originally.

- John