CJK encode/decode functions only have three error-handler fast-paths:
    replace
    ignore
    strict  
See the code: [1][2]

If use other built-in error-handlers, need to get the error-handler object, and call it with an Unicode Exception argument. See the code: [3]

But the error-handler object is not cached, it needs to be looked up from a dict every time, which is very inefficient.


Another possible optimization is to write fast-path for common error-handlers, Python has these built-in error-handlers:

    strict
    replace
    ignore
    backslashreplace
    xmlcharrefreplace
    namereplace
    surrogateescape
    surrogatepass (only for utf-8/utf-16/utf-32 family)

For example, maybe `xmlcharrefreplace` is heavily used in Web application, it can be implemented as a fast-path, so that no need to call the error-handler object every time.
Just like the `xmlcharrefreplace` fast-path in `PyUnicode_EncodeCharmap` [4].

[1] encode function:
https://github.com/python/cpython/blob/v3.9.0b4/Modules/cjkcodecs/multibytecodec.c#L192

[2] decode function:
https://github.com/python/cpython/blob/v3.9.0b4/Modules/cjkcodecs/multibytecodec.c#L347

[3] `call_error_callback` function:
https://github.com/python/cpython/blob/v3.9.0b4/Modules/cjkcodecs/multibytecodec.c#L82

[4] `xmlcharrefreplace` fast-path in `PyUnicode_EncodeCharmap`:
https://github.com/python/cpython/blob/v3.9.0b4/Objects/unicodeobject.c#L8662

I am not even sure it was worth to add fast path for "xmlcharrefreplace". "surrogateescape" and "surrogatepass" are most likely used in performance critical cases. It is also easy to add support of "ignore" and "replace". "strict" raises an exception in any case, and "backslashreplace", "xmlcharrefreplace" and "namereplace" are too complex and used in cases when coding time is not dominant (error reporting, debugging, formatting complex documents).

IMO "xmlcharrefreplace" is useful for Web application.

For example, the page's charset is "gbk", then this statement can generate the bytes content easily & safely:

    s.encode('gbk', 'xmlcharrefreplace')

Maybe some HTML-related frameworks use this way to escape characters, such as Sphinx [1].


Attached file `error_handers_fast_paths.txt` summarized all current error-handler fast-paths.

[1] Sphinx use 'xmlcharrefreplace' to escape
https://github.com/sphinx-doc/sphinx/blob/e65021fb9b0286f373f01dc19a5777e5eed49576/sphinx/builders/html/__init__.py#L1029

But how many new Python web application use CJK codec instead of UTF-8?

> But how many new Python web application use CJK codec instead of UTF-8?

A CJK character usually takes 2-bytes in CJK encodings, but takes 3-bytes in UTF-8.

I tested a Chinese book:
in GBK:     853,025 bytes
in UTF-8: 1,267,523 bytes

For CJK content, UTF-8 is wasteful, maybe CJK encodings will not be eliminated.

In the Web application you need first to generate data (this may involve some network requests, IO operations, and some data transformations), then format the page, then encode it, and finally send it to client. I suppose that the encoding part is minor in comparison with others.

Also, as Inada-san noted, UTF-8 is more popular encoding in modern applications. It is also fast, so you may prefer UTF-8 if the performance of encoding is important to you.

I am also +1 on Serhiy's opinion.

As I am Korean, (I don't know Japan or China environment)
I know that there still exist old Korean websites that use EUC-KR encoding.
But at least 2010s modern Korea website/application.
Most of the applications are built on UTF-8.

At least fix this bug:

    the error-handler object is not cached, it needs to be
    looked up from a dict every time, which is very inefficient.

The code:
https://github.com/python/cpython/blob/v3.9.0b4/Modules/cjkcodecs/multibytecodec.c#L81-L98

I will submit a PR at some point.

Since CJK codecs have been implemented, unicodeobject.c got multiple optimizations:

* _PyUnicodeWriter for decoder: API designed with efficiency and PEP 393 (compact string) in mind
* _PyBytesWriter for encoders: in short, API to overallocate a buffer
* _Py_error_handler enum and "_Py_error_handler _Py_GetErrorHandler(const char *errors)" function to pass an error handler as an integer rather than a string

But rewriting CJK codecs with these is a lot of effort, I'm not sure that it's worth it.

I'm working on issue41265.
If nothing happens, I also would like to write a zstd module for stdlib before the end of the year, but I dare not promise this.

If anyone wants to work on this issue, very grateful.

(off topic)

> If nothing happens, I also would like to write a zstd module for stdlib before the end of the year, but I dare not promise this.

I suggest you to publish it on PyPI. Once it will be mature, you can propose it on python-ideas. Last time someone proposed a new compression algorithm to the stdlib, it was rejected if I recall correctly. I forgot which one was proposed. Maybe search for "compresslib" on python-ideas.