[Python-Dev] Improving the reading part of REPL

Adam Bartoš drekin at gmail.com
Thu Nov 19 16:50:10 EST 2015

It seems that there will be some refactoring of the tokenizer code.
Regarding this, I'd like to recall my proposal on readline hooks. It would
be nice if char* based PyOS_Readline API was replaced by a Python str based
hook customizable by Python code. I propose to add function
sys.readlinehook accepting optional prompt and returning a line read
interactively from a user. There would also be sys.__readlinehook__
containing the original value of sys.readlinehook (similarly to
sys.(__)displayhook(__), sys.(__)excepthook(__) and

Currently, the input is read from C stdin even if sys.stdin is changed (see
http://bugs.python.org/issue17620). This complicates fixing
http://bugs.python.org/issue1602 – the standard sys.std* streams are not
capable of communicating in Unicode with Windows console, and replacing the
streams with custom ones is not enough – one has also to install a custom
readline hook, which is currently complicated. And even after installing a
custom readine hook one finds out that Python tokenizer cannot handle
UTF-16, so he has to wrap the custom stream objects just to let their
encoding attribute have a different value, because readlinehook currently
returns char* rather than a Python string. For more details see the
documentation of my package: https://github.com/Drekin/win-unicode-console.

The pyreadline package also sets up a custom readline so it would benefit
if doing so would be easier. Moreover, the two consumers of PyOS_Readline
API – the input function and the tokenizer – assume a different encoding of
the bytes returned by the readlinehook. Effectively, one assumes
sys.stdout.encoding and the other sys.stdin.encoding, so if these two are
different, there is no way to implement a correct readline hook.

If sys.readlinehook was added, the builting input function would be just a
thin wrapper over sys.readlinehook removing the newline character and
turning no input into EOFError. I thing that the best default value for
sys.readlinehook on Windows would be stdio_readline – just write the prompt
to sys.stdout and read a line from sys.stdin. On Linux, the default
implementation would call GNU readline if it is available and sys.stdin and
sys.stdout are standard TTYs (the check present in the current
implementation of the input function), and it would call stdio_readline

Regards, Adam Bartoš
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