io class name are hardcoded in reprs

[serhiy-storchaka]

BPO	21861
Nosy	@rhettinger, @pitrou, @benjaminp, @PCManticore, @hynek, @serhiy-storchaka, @MojoVampire, @aisk, @Windsooon
PRs	bpo-21861: Improve _io.FileIO.__repr__ #14774 gh-66060: Use actual class name in some _io type's __repr__ #30824

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2014-06-24.17:36:58.710>
labels = ['extension-modules', 'easy', 'type-bug', 'expert-IO']
title = 'io class name are hardcoded in reprs'
updated_at = <Date 2022-01-23.10:01:52.325>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2022-01-23.10:01:52.325>
actor = 'asaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'IO']
creation = <Date 2014-06-24.17:36:58.710>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 21861
keywords = ['patch', 'easy']
message_count = 6.0
messages = ['221476', '221990', '221993', '222011', '222939', '342684']
nosy_count = 12.0
nosy_names = ['rhettinger', 'pitrou', 'benjamin.peterson', 'stutzbach', 'Claudiu.Popa', 'hynek', 'piotr.dobrogost', 'serhiy.storchaka', 'josh.r', 'asaka', 'Windson Yang', 'Philip Dye']
pr_nums = ['14774', '30824']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue21861'
versions = ['Python 2.7', 'Python 3.3', 'Python 3.4']

[serhiy-storchaka]

>>> import io
>>> class F(io.FileIO): pass
... 
>>> f = F('/dev/null', 'r')
>>> f
<_io.FileIO name='/dev/null' mode='rb'>
>>> class B(io.BufferedReader): pass
... 
>>> b = B(f)
>>> b
<B name='/dev/null'>
>>> class T(io.TextIOBufferedReader): pass
io.TextIOBase(     io.TextIOWrapper(  
>>> class T(io.TextIOWrapper): pass
... 
>>> t = T(b)
>>> t
<_io.TextIOWrapper name='/dev/null' encoding='UTF-8'>

Expected results are "<main.F name='/dev/null' mode='rb'>", "<main.B name='/dev/null'>" and "<main.T name='/dev/null' encoding='UTF-8'>". Usually reprs of subclass instances substitute actual module and class names.

[PCManticore]

The same should be done for _pyio?

[MojoVampire]

Is it common for C implementations to introspect to figure out their "real" name? I do this manually for reprs of my user defined classes, but I haven't noticed many built-ins that consider extensibility for the repr. Maybe I'm just not using the classes that do it or I'm overriding the repr without checking?

I just tested, and it looks like frozenset has a subclass friendly repr, while bytearray does not as of 3.4.0. Seems like it might make sense to first determine if introspection should be the default; it would mean a little code bloat every time it's done.

[rhettinger]

Usually reprs of subclass instances substitute actual module
and class names.

This is a sensible idea that makes life easier for people writing subclasses.

+1 for your suggestion.

[serhiy-storchaka]

I now see that this issue is not so easy from C side.

And more, it is very uncommon for either Python or C implementations (especial C implementations) to introspect to figure out their "real" name. Existing cases are rather exceptions. And in many case this is only a side effect of sharing an implementation between several classes (set and frozenset, three buffered file classes in io, etc). In many cases repr's flexibility is limited: hardcoded or omitted module name, used class's __name__ instead of __qualname__, etc.

This should be discussed on Python-Dev maillist.

[Windsooon]

IIUC, in the c code we just hardcode the name "_io.FileIO" for "PyFileIO_Type" in https://github.com/python/cpython/blob/master/Modules/_io/fileio.c#L1180. If we want to get a dynamic name, we should replace the hardcode name with the module name (like "__main__"). I found this also happens on other modules like collections.