Show expected input for right shift operator usage in custom "print" error message

[CuriousLearner]

BPO	30721
Nosy	@terryjreedy, @pfmoore, @ncoghlan, @tjguk, @zware, @serhiy-storchaka, @zooba, @CuriousLearner
PRs	bpo-30721: Show correct syntax hint in Py3 when using Py2 redirection syntax #2345 bpo-30721: Addresses minor fixes of adding ? to redirection syntax #3131 [3.6] bpo-31232: Backport custom print rshift message #3155

^{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 2017-08-18.12:19:05.924>
created_at = <Date 2017-06-21.05:13:56.910>
labels = ['type-feature', '3.7']
title = 'Show expected input for right shift operator usage in custom "print" error message'
updated_at = <Date 2017-08-19.06:59:41.955>
user = 'https://github.com/CuriousLearner'

bugs.python.org fields:

activity = <Date 2017-08-19.06:59:41.955>
actor = 'ncoghlan'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-18.12:19:05.924>
closer = 'ncoghlan'
components = []
creation = <Date 2017-06-21.05:13:56.910>
creator = 'CuriousLearner'
dependencies = []
files = []
hgrepos = []
issue_num = 30721
keywords = []
message_count = 12.0
messages = ['296518', '296525', '299128', '299139', '299238', '300484', '300485', '300487', '300488', '300492', '300493', '300569']
nosy_count = 8.0
nosy_names = ['terry.reedy', 'paul.moore', 'ncoghlan', 'tim.golden', 'zach.ware', 'serhiy.storchaka', 'steve.dower', 'CuriousLearner']
pr_nums = ['2345', '3131', '3155']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30721'
versions = ['Python 3.7']

[CuriousLearner]

While working on issue: http://bugs.python.org/issue30597 to enhance the custom error message by showing expected output in Python3 syntax when someone uses Python2 syntax, a PR was raised: #2009, where we just handled the case for print with soft-space and excessive white-space.

In the implementation discussion, an issue was raised to handle the case with right shift operator by Nick as in here:
http://bugs.python.org/issue30597#msg295484

Nick suggested here about the possible patch: #2009 (comment)

[ncoghlan]

The specific error in question here is the one where Python 3 reads the old Python 2 stream redirection syntax as a combination of the right shift operator and tuple creation:

>>> print >> sys.stderr, "message"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for >>: 'builtin_function_or_method' and '_io.TextIOWrapper'

Searching for that error message indicates that people are hitting it reasonably frequently, so we may be able to save them a trip to Google by adding a custom 'Did you mean "print(<output>, file={:100!r})'.format(rhs)"? message when the right-shift operand dispatch is about to report the default "no implementation" type error, and the LHS is the print builtin.

[terryjreedy]

The test added in the PR passes on linux (Travis) and Mac (op's machine), fails on Windows (Appveyor). The patch itself modifies abstract.c. Could some Windows expert take a look?

test_string_with_stream_redirection (test.test_print.TestPy2MigrationHint) ... FAIL
FAIL: test_string_with_stream_redirection (test.test_print.TestPy2MigrationHint)

Traceback (most recent call last):
File "C:\projects\cpython\lib\test\test_print.py", line 165, in test_string_with_stream_redirection
'file=<output_stream>)', str(context.exception))
AssertionError: 'Did you mean "print(, file=<output_stream>)' not found in "unsupported operand type(s) for >>: 'builtin_function_or_method' and '_io.StringIO'"

[zooba]

The "op_slot == 96" looks suspicious - how is this value actually supposed to be calculated?

[ncoghlan]

Checking how we do it elsewhere, NB_SLOT(nb_rshift) looks like the right replacement.

That's a compiler-dependent struct field offset calculation, so a discrepancy there could easily be the cause of a Windows-only failure.

[serhiy-storchaka]

New changeset 5e2eb35 by Serhiy Storchaka (Sanyam Khurana) in branch 'master':
bpo-30721: Show correct syntax hint in Py3 when using Py2 redirection syntax (bpo-2345)
5e2eb35

[ncoghlan]

Checking the merged implementation locally, I belatedly noticed that we forgot the trailing question mark on the question:

>>> import sys
>>> print >> sys.stderr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for >>: 'builtin_function_or_method' and '_io.TextIOWrapper'. Did you mean "print(<message>, file=<output_stream>)"

So putting this back to "needs patch" (no NEWS entry needed, just the missing question marks in the patch and the tests).

[CuriousLearner]

Ah, sorry for that. I'll just fix that right away in a few mins :)

[ncoghlan]

Note that I filed a separate issue to ask Ned about potentially backporting this to 3.6: https://bugs.python.org/issue31232

[ncoghlan]

New changeset a7c449b by Nick Coghlan (Sanyam Khurana) in branch 'master':
bpo-30721: Add missing '?' to new error message (GH-3131)
a7c449b

[ncoghlan]

And done - thanks folks!

[ncoghlan]

New changeset 1a05e87 by Nick Coghlan in branch '3.6':
[3.6] bpo-31232: Backport custom print rshift message (GH-3155)
1a05e87