[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Roundup Robot]

Roundup Robot added the comment:

New changeset 2d69d0419879 by Martin Panter in branch 'default':
Issue #22797: Synchronize urlopen() doc string with RST documentation
https://hg.python.org/cpython/rev/2d69d0419879

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[R. David Murray]

R. David Murray added the comment:

Thanks, Alexander.

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resolution:  -> fixed
stage:  -> resolved
status: open -> closed
versions: +Python 3.5

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Roundup Robot]

Roundup Robot added the comment:

New changeset b4df20312b78 by R David Murray in branch 'default':
Clean up urlopen doc string.
https://hg.python.org/cpython/rev/b4df20312b78

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8b6b6add8e47 by R David Murray in branch 'default':
psuedo merge: #22797: clarify when URLErrors are raised by urlopen.
https://hg.python.org/cpython/rev/8b6b6add8e47

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Roundup Robot]

Roundup Robot added the comment:

New changeset aed4b9981fca by R David Murray in branch '3.5':
#22797: clarify when URLErrors are raised by urlopen.
https://hg.python.org/cpython/rev/aed4b9981fca

New changeset d085b4f779af by R David Murray in branch '3.5':
Merge: #22797: clarify when URLErrors are raised by urlopen.
https://hg.python.org/cpython/rev/d085b4f779af

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Alexander Liu]

Alexander Liu added the comment:

Fixed the docs to specifically note that only protocol related errors raise the 
UrlError exception.

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Added file: http://bugs.python.org/file43131/urlopen_doc.patch

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[R. David Murray]

R. David Murray added the comment:

(Looking at your new patch...thanks for giving it a try even though I wasn't 
clear).

There are lots of other errors it can raise, too.  I was thinking more along 
the lines of "raises URLError on http protocol errors".  The problem is that's 
not completely accurate, either, but I don't remember exactly when it is that 
HTTPError can leak through.

Just for reference, in my code that calls urlopen and needs to keep running it 
if can't access the URL no matter what the (network-derived) reason, I catch 
ConnectionError, HTTPError, URLError, and IncompleteRead.  

We do not document all possible exceptions.  We document those that are 
specific to the module in question (which is URLError in this case) or that are 
part of the documented API (such as mentioning TimeoutError as what happens if 
the timeout specified by a timeout keyword is exceeded).

ValueErrors are a general class of errors that, when encountered, usually mean 
you need to fix your code (or, in the case of the SSL one you mention, check 
for SSL support at startup, assuming I understood correctly), rather than 
something you would catch around the urlopen call in a typical program.  There 
are, of course, occasions when you *do* catch ValueErrors in specific bits of 
code, but there is no practical way we can document all of the reasons 
ValueError might get raised, so we don't try.

All of that said, it would be lovely to have a reference somewhere (maybe a 
tutorial?) that went over all the possible exceptions one might get while using 
various network libraries, and why they might arise.  It is an issue that comes 
from the fact that the libraries are built on top of each other and the general 
python technique is that lower level errors are allowed to bubble up.  It would 
be a beast to keep up to date, though, which is probably one reason we don't 
have one.  But even that kind of guide wouldn't include ValueErrors.

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Joshua Chin]

Changes by Joshua Chin :


Added file: http://bugs.python.org/file37131/urlopen_doc.patch

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[R. David Murray]

R. David Murray added the comment:

This is a general principle in Python.  A module may raise specific errors, but 
there are always other errors that may be raised.  The wording could be 
clarified, but it should not be removed.

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[issue22797] urllib.request.urlopen documentation falsely guarantees that a URLError will be raised on errors

[Joshua Chin]

New submission from Joshua Chin:

The documentation for urlopen states that it "Raises URLError on errors." 
However, urlopen can raise a ValueError. In fact, test_urllib. 
urlopen_FileTests.test_relativelocalfile  specifically checks if urlopen raises 
a ValueError. I suggest removing the guarantee from the documentation.

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assignee: docs@python
components: Documentation
files: urlopen_doc.patch
keywords: patch
messages: 230640
nosy: Joshua.Chin, docs@python, orsenthil
priority: normal
severity: normal
status: open
title: urllib.request.urlopen documentation falsely guarantees that a URLError 
will be raised on errors
type: behavior
versions: Python 3.4
Added file: http://bugs.python.org/file37129/urlopen_doc.patch

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