[issue46910] Expect IndentationError, get SyntaxError: 'break' outside loop

[Jonathan Fine]

Jonathan Fine  added the comment:

My main concern is that the door not be closed on improving the user experience 
relating to this behaviour of the compiler.

This issue was raised as a bug for the compiler (which is C-coded). I'd be very 
happy for this issue to be closed as 'not a bug' for the compiler, provided the 
door is left open for Python-coded improvements for the user experience. 

I suggest that the issue title be changed to: The two-pass compile(bad_src, 
...) sometimes does not report first error in bad_src

These two changes to the details of closure would be sufficient to meet my 
concern. I hope they can be accepted.

By the way, I see these improvements being done as a third-party pure-Python 
module outside Python's Standard Library, at least until they've reached a wide 
measure of community acceptance.

--

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[issue46910] Expect IndentationError, get SyntaxError: 'break' outside loop

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Unfortunately that's not how the parser works. I insist, for the compiler to 
raise the propper error the parser needs to succeed first. That would require 
us to modify the input string in arbitrary ways (many times we don't know 
exactly what went wrong) and recompile an arbitrary number of times.

I'm very sorry, but I will leave this issue closed as "won't fix" as the 
complexity of this solution is too much for the given benefit.

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[issue46910] Expect IndentationError, get SyntaxError: 'break' outside loop

[Jonathan Fine]

Jonathan Fine  added the comment:

Many thanks Pablo for the clear explanation. I'd prefer that the issue remain 
open, as there's an important user experience issue here. I suspect there are 
other similar examples of how the compiler error messages could be improved.

Here's a change that doesn't seem to be too hard, that could fix the problem at 
hand.

The IndentationError occurred at a known location in the input string. So as 
part of error reporting truncate the input string and try to compile that. In 
other words, make a good faith attempt to find an earlier error.

I've attached a funny_break_error_fix.py which is a first draft implementation 
of this idea. 

Here's the output:
===
$ python3 funny_break_error_fix.py funny_break_error.py
unexpected indent (funny_break_error.py, line 6)
Traceback (most recent call last):
  File "funny_break_error_fix.py", line 3, in compile_fix
compile(source, filename, 'exec')
  File "funny_break_error.py", line 6
else:
^
IndentationError: unexpected indent

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "funny_break_error_fix.py", line 18, in 
compile_fix(src.read(), filename, 'exec')
  File "funny_break_error_fix.py", line 9, in compile_fix
compile(new_source, filename, 'exec')
  File "funny_break_error.py", line 5
break
^
SyntaxError: 'break' outside loop
===

And in this case we've got hold of the first error (at the cost of compiling 
part of the source file twice). Many thanks again for the clear explanation, 
which I found most helpful when formulating the above fix.

--
Added file: https://bugs.python.org/file50656/funny_break_error_fix.py

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[issue46910] Expect IndentationError, get SyntaxError: 'break' outside loop

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Unfortunately, we cannot do much here. The reason is that the parser allows 
break and continue outside loops as they count as statements. As these are 
associated with control flow, is the compiler the one that will show the Syntax 
warning once it tries to make sense of the abstract syntax tree that the parser 
generates.

The error you are getting happens because for the parser, the unindented else 
*is* a syntax error so it fails much sooner and prevents the compiler to 
complain about the break.

This means that in the presence of two syntax errors, one being a parser error 
and the other a compiler error, the parser will always be first, no matter if 
the other one appears before in the code.

Given this, I am afraid we need to close this issue as "won't fix" :(

Bing said that, if someone devises some easy way to do this without major 
changes everywhere, I am happy to reopen it

--
resolution:  -> wont fix
stage:  -> resolved
status: open -> closed

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[issue46910] Expect IndentationError, get SyntaxError: 'break' outside loop

[Jonathan Fine]

New submission from Jonathan Fine :

This arises from a request for help made by Nguyễn Ngọc Tiến to the visually 
impaired programmers lists, see 
https://www.freelists.org/post/program-l/python,48. Please keep this in mind.

Nguyễn asked for help with the syntax error created by
===
count = 0
while count < 1:
 count = count + 1
 print(count)
break
 else:
 print("no break")
===

When I saved this to a file and ran it I got:
===
$ python3.8 funny_break_error.py 
  File "funny_break_error.py", line 6
else:
^
IndentationError: unexpected indent
===

However, remove the last two lines and you get the more helpful error
===
$ python3.8 funny_break_error.py 
  File "funny_break_error.py", line 5
break
^
SyntaxError: 'break' outside loop
===

Python3.6 and 3.7 also behave as above.

Note. I've heard that blind Python programmers prefer a single space to denote 
indent. I think this is because they hear the leading spaces via a screen 
reader, rather than see the indent with their eyes.

--
components: Parser
files: funny_break_error.py
messages: 414424
nosy: jfine2358, lys.nikolaou, pablogsal
priority: normal
severity: normal
status: open
title: Expect IndentationError, get SyntaxError: 'break' outside loop
type: behavior
versions: Python 3.7, Python 3.8
Added file: https://bugs.python.org/file50655/funny_break_error.py

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