[issue8271] str.decode('utf8', 'replace') -- conformance with Unicode 5.2.0

[Ezio Melotti]

Ezio Melotti <ezio.melotti at gmail.com> added the comment:

Here are some benchmarks:
Commands:
# half of the bytes are invalid
./python -m timeit -s 'b = bytes(range(256)); b_dec = b.decode' 'b_dec("utf-8", "surrogateescape")'
./python -m timeit -s 'b = bytes(range(256)); b_dec = b.decode' 'b_dec("utf-8", "replace")'
./python -m timeit -s 'b = bytes(range(256)); b_dec = b.decode' 'b_dec("utf-8", "ignore")'

With patch:
1000 loops, best of 3: 854 usec per loop
1000 loops, best of 3: 509 usec per loop
1000 loops, best of 3: 415 usec per loop

Without patch:
1000 loops, best of 3: 670 usec per loop
1000 loops, best of 3: 470 usec per loop
1000 loops, best of 3: 382 usec per loop

Commands (from the interactive interpreter):
# all valid codepoints
import timeit
b = "".join(chr(c) for c in range(0x110000) if c not in range(0xD800, 0xE000)).encode("utf-8")
b_dec = b.decode
timeit.Timer('b_dec("utf-8")', 'from __main__ import b_dec').timeit(100)/100
timeit.Timer('b_dec("utf-8", "surrogateescape")', 'from __main__ import b_dec').timeit(100)/100
timeit.Timer('b_dec("utf-8", "replace")', 'from __main__ import b_dec').timeit(100)/100
timeit.Timer('b_dec("utf-8", "ignore")', 'from __main__ import b_dec').timeit(100)/100

With patch:
0.03830226898193359
0.03849360942840576
0.03835036039352417
0.03821949005126953

Without patch:
0.03750091791152954
0.037977190017700196
0.04067679166793823
0.038579678535461424

Commands:
# near-worst case scenario, 1 byte dropped every 5 from a valid utf-8 string
b2 = bytes(c for k,c in enumerate(b) if k%5)
b2_dec = b2.decode
timeit.Timer('b2_dec("utf-8", "surrogateescape")', 'from __main__ import b2_dec').timeit(10)/10
timeit.Timer('b2_dec("utf-8", "replace")', 'from __main__ import b2_dec').timeit(10)/10
timeit.Timer('b2_dec("utf-8", "ignore")', 'from __main__ import b2_dec').timeit(10)/10

With patch:
9.645482301712036
6.602735090255737
5.338080596923828

Without patch:
8.124328684806823
5.804249691963196
4.851014900207519

All tests done on wide 3.2.

Since the changes are about errors, decoding of valid utf-8 strings is not affected.  Decoding with non-strict error handlers and invalid strings are slower, but I don't think the difference is significant.
If the patch is fine I will commit it.

----------

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue8271>
_______________________________________