[Python-bugs-list] [ python-Bugs-532467 ] 6.9 The raise statement is confusing

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Bugs item #532467, was opened at 2002-03-20 08:37
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Category: Documentation
Group: Python 2.2
>Status: Closed
>Resolution: Fixed
Priority: 7
Submitted By: Patrick K. O'Brien (pobrien)
Assigned to: Fred L. Drake, Jr. (fdrake)
Summary: 6.9 The raise statement is confusing

Initial Comment:
The description supplied for "6.9 The raise statement" 
is confusing. In particular, it starts by explaining 
the intent of each "expresssion" and midway switches 
to "object" instead of expression. In particular, I 
can't make heads or tails out of the following passage:

"If the first object is a string, it then raises the 
exception identified by the first object, with the 
second one (or None) as its parameter. If the first 
object is a class or instance, it raises the exception 
identified by the class of the instance determined in 
the previous step, with the instance as its parameter."

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>Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2002-06-20 17:00

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I've checked in a different patch as Doc/ref/ref4.tex 1.32
and Doc/ref/ref6.tex 1.54.

Please review the changes; if not sufficient, please re-open
this report with comments on what else is needed (or file a
new report).

http://www.python.org/dev/doc/devel/ref/raise.html

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Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2002-06-20 00:09

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Raymond is still waiting, so bumping to 7 so I notice it in
the morning (when I'm not really asleep).

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Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2002-05-29 15:45

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Bumped priority since Raymond is waiting for my review.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2002-05-18 21:25

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Thanks Patrick.

Since I'm the new guy, I'll defer additional changes to 
Fred.  My patch style is to make the least invasive change 
that improves clarity and accuracy.  

Also, this may be a case where additional wording, even if 
informative and accurate, may detract from a strong 
understanding of how raise works.  

This section's cyclomatic complexity (nested if/elif/else 
paths) already impinges its understandability.

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Comment By: Patrick K. O'Brien (pobrien)
Date: 2002-05-16 10:34

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Raymond, I really appreciate your patches. The big patch is 
terrific and gets my vote. I do have one more request for 
clarification in what you wrote. You say:

"If the first expression is a class object, it raises the 
exception identified by the class of an instance determined 
by the second expression.  If the second expression is an 
instance, that instance is used."

So we're talking about the form: raise class, instance

What I'm picturing is a situation where the second 
expression is an instance of a class other than the class 
object used as the first expression. Is this allowed, 
disallowed, allowed but nonsensical or allowed because it 
offers some practical benefit that I'm just not grasping?

I found and read the essay on "Standard Exception Classes 
in Python 1.5" 
(http://www.python.org/doc/essays/stdexceptions.html) and I 
can see that much was done to maintain backward 
compatibility. and apparently the "raise class, instance" 
form goes back quite a ways. Is this just a wart to be 
avoided? I'm probably expecting too much to expect the 
language reference to distinguish between what is strictly 
allowed vs. why it is allowed and whether it should even be 
used at all. It's just that the more I dug into this the 
more confused I became. I think I'm understanding the 
situation now.

In any case, make of this what you will. The bottom line is 
that your batch is definitely an improvement and I greatly 
appreciate your time and effort on this. Thanks.


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Comment By: Raymond Hettinger (rhettinger)
Date: 2002-05-16 00:10

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Attached two alternative patches.

The first makes minimal changes to achieve a clear and 
consistent use of "objects" versus "expressions".

The second alternative re-paragraphs the text and uses 
parallel sentence contruction to provide (IMHO) 
greater clarity without getting a headache reading it.

If accepted, please re-assign to Raymond for a commit to 
2.3 and to release22-maint.

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