Why are some unicode error handlers "encode only"?

[Steven D'Aprano]

At least two standard error handlers are documented as working for
encoding only:

xmlcharrefreplace
backslashreplace

See http://docs.python.org/library/codecs.html#codec-base-classes

and http://docs.python.org/py3k/library/codecs.html

Why is this? I don't see why they shouldn't work for decoding as well.
Consider this example using Python 3.2:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'cp932' codec can't decode bytes in position 9-10:
illegal multibyte sequence

The two bytes b'\xe9!' is an illegal multibyte sequence for CP-932 (also
known as MS-KANJI or SHIFT-JIS). Is there some reason why this shouldn't
or can't be supported?

# This doesn't actually work.
b"aaa--\xe9z--\xe9!--bbb".decode("cp932", "backslashreplace")
=> r'aaa--騷--\xe9\x21--bbb'

and similarly for xmlcharrefreplace.

 

[Walter Dörwald]

At least two standard error handlers are documented as working for
encoding only:

xmlcharrefreplace
backslashreplace

See http://docs.python.org/library/codecs.html#codec-base-classes

and http://docs.python.org/py3k/library/codecs.html

Why is this? I don't see why they shouldn't work for decoding as well.

Because xmlcharrefreplace and backslashreplace are *error* handlers.
However the bytes sequence b'〹' does *not* contain any bytes that
are not decodable for e.g. the ASCII codec. So there are no errors to
handle.

Consider this example using Python 3.2:

Traceback (most recent call last):
File "<stdin>", line 1, in<module>
UnicodeDecodeError: 'cp932' codec can't decode bytes in position 9-10:
illegal multibyte sequence

The two bytes b'\xe9!' is an illegal multibyte sequence for CP-932 (also
known as MS-KANJI or SHIFT-JIS). Is there some reason why this shouldn't
or can't be supported?

The byte sequence b'\xe9!' however is not something that would have been
produced by the backslashreplace error handler. b'\\xe9!' (a sequence
containing 5 bytes) would have been (and this probably would decode
without any problems with the cp932 codec).

# This doesn't actually work.
b"aaa--\xe9z--\xe9!--bbb".decode("cp932", "backslashreplace")
=> r'aaa--騷--\xe9\x21--bbb'

and similarly for xmlcharrefreplace.

This would require a postprocess step *after* the bytes have been
decoded. This is IMHO out of scope for Python's codec machinery.

Servus,
Walter

 

[Terry Reedy]

At least two standard error handlers are documented as working for
encoding only:

xmlcharrefreplace
backslashreplace

See http://docs.python.org/library/codecs.html#codec-base-classes

and http://docs.python.org/py3k/library/codecs.html

Why is this?

I presume the purpose of both is to facilitate transmission of unicode
text via byte transmission by extending incomplete byte encodings by
replacing unicode chars that do not fit in the given encoding by a ascii
byte sequence that will fit.

I don't see why they shouldn't work for decoding as well.
Consider this example using Python 3.2:

Traceback (most recent call last):
File "<stdin>", line 1, in<module>
UnicodeDecodeError: 'cp932' codec can't decode bytes in position 9-10:
illegal multibyte sequence

The two bytes b'\xe9!' is an illegal multibyte sequence for CP-932 (also
known as MS-KANJI or SHIFT-JIS). Is there some reason why this shouldn't
or can't be supported?

# This doesn't actually work.
b"aaa--\xe9z--\xe9!--bbb".decode("cp932", "backslashreplace")
=> r'aaa--騷--\xe9\x21--bbb'

This output does not round-trip and would be a bit of a fib since it
somewhat misrepresents what the encoded bytes were:
'aaa--騷--\\xe9\\x21--bbb'

Python 3 added surrogateescape error handling to solve this problem.

and similarly for xmlcharrefreplace.

Since xml character references are representations of unicode chars, and
not bytes, I do not see how that would work. By analogy, perhaps you
mean to have '' in your output instead of '\xe9\x21', but
those would not properly be xml numeric character references.