Clarify chaining exceptions in tutorial/errors.rst

[greatvovan]

BPO	42179
Nosy	@rhettinger, @merwok, @methane, @maxking, @willingc, @miss-islington, @greatvovan
PRs	bpo-42179: Clarify exception chaining #23160 bpo-42179: Doc/tutorial: Remove mention of __cause__ #23162 [3.9] bpo-42179: Doc/tutorial: Remove mention of __cause__ (GH-23162) #23173

^{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 = <Date 2021-01-06.23:37:55.313>
created_at = <Date 2020-10-28.08:31:31.868>
labels = ['3.9', '3.10', 'docs']
title = 'Clarify chaining exceptions in tutorial/errors.rst'
updated_at = <Date 2021-01-06.23:37:55.311>
user = 'https://github.com/greatvovan'

bugs.python.org fields:

activity = <Date 2021-01-06.23:37:55.311>
actor = 'Vladimir Ryabtsev'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-01-06.23:37:55.313>
closer = 'Vladimir Ryabtsev'
components = ['Documentation']
creation = <Date 2020-10-28.08:31:31.868>
creator = 'Vladimir Ryabtsev'
dependencies = []
files = []
hgrepos = []
issue_num = 42179
keywords = ['patch']
message_count = 15.0
messages = ['379823', '380394', '380396', '380397', '380425', '380432', '380434', '380435', '380436', '380440', '380441', '380570', '380607', '383114', '384542']
nosy_count = 8.0
nosy_names = ['rhettinger', 'eric.araujo', 'methane', 'docs@python', 'maxking', 'willingc', 'miss-islington', 'Vladimir Ryabtsev']
pr_nums = ['23160', '23162', '23173']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42179'
versions = ['Python 3.9', 'Python 3.10']

[greatvovan]

A new section has been added to the page as a result of https://bugs.python.org/issue37826. The change: dcfe111

The wording it uses (in the beginning of section 8.5), defines chaining as setting __cause__ attribute in an exception, and later states that "Exception chaining happens automatically when an exception is raised inside an exception handler or finally section". This may lead the reader to a wrong idea that re-raising an exception without "from" in "except" and "finally" automatically sets __cause__. In reality it sets only __context__ attribute, which is similar concept to __cause__, but work a bit differently, as explained in library/exceptions.rst. I suggest to mention that difference and provide a link to the main article.

[methane]

Please note that tutorial is a tutorial. It is document to help new user who are learning Python.
Do you believe special attributes like __cause__ and __contexts__ are really worth to teach for tutorial readers?

Generally speaking, I think we should *reduce* some details from tutorial.

[greatvovan]

1. Such understanding of a tutorial is debatable. Tutorial is just a material for learning written with some system in mind, which is more interesting to read than dry reference material. A tutorial, generally dpeaking, may be both for beginners and for professionals.

2. The question about exception chaining is popular on Stackoverflow in people who came to Python with Java or C# background (see “python inner exception”).

3. Whatever material is given, it should not cause confusion, but now it does. Since this section has been added recently, it is better to fix it rather than remove entirely, aren’t you agree?

[methane]

1. Such understanding of a tutorial is debatable. Tutorial is just a material for learning written with some system in mind, which is more interesting to read than dry reference material. A tutorial, generally dpeaking, may be both for beginners and for professionals.

OK, I will send this topic to python-dev first.

2. The question about exception chaining is popular on Stackoverflow in people who came to Python with Java or C# background (see “python inner exception”).
3. Whatever material is given, it should not cause confusion, but now it does.

I searched it but I can not find confusion caused by this tutorial section. Please write a concrete URL caused by current tutorial?

Since this section has been added recently, it is better to fix it rather than remove entirely, aren’t you agree?

I prefer removing mention to __cause__, instead of adding mention to __context__.

No need to remove entire section. We can introduce high level overview of context chaining. Describing the default behavior and "from None" is enough for new users.

[greatvovan]

I can not find confusion caused by this tutorial section

Inada, have you read the very first message in this ticket? It explains why this wording may cause confusion (and it did in me), and describes the problem part. A link for your convenience: https://docs.python.org/3/tutorial/errors.html#exception-chaining

Describing the default behavior and "from None" is enough for new users

Strange that you think that "from None" is more useful for beginners than these special attributes.

Without understanding of __cause__ and __context__, stack traceback message looks like magic. Say you want to handle an exception and retrieve its cause (context) in runtime (this is what exception chaining for) – this section makes no clues about how to do that.

[merwok]

I prefer the patch by Inada-san!

> Describing the default behavior and "from None" is enough for new users
Strange that you think that "from None" is more useful for beginners than these special attributes.

Doesn’t feel strange to me: raise Exc from exc or from None shows how to use the mecanism, whereas the special attributes are about the implementation. A tutorial can show how to use language features like for loops or with statements, but shouldn’t explain how to implement the protocols, that’s too much detail if you’re just starting to learn programming.

[greatvovan]

We have automatic chaining, so you don't need to use "from X" unless you want to have some control on the traceback message. Even without knowing of this syntax (and without using "from exc"), a user will get a traceback message similar to what is shown in the example. What is the purpose of the entire section then?

As I see it, the purpose might be providing some details about how exactly chaining works, so a user: a) could make an informed decision whether they need "from X" or not, b) would know how to retrieve the linked exception programmatically.

I generally feel that we don't want to deprive a user from special attributes, in Python they are everywhere, you cannot even construct a class instance without __init__().