string.Formatter accepts empty fields but displays wrong when nested

[AvdN]

BPO	25034
Nosy	@AvdN, @ericvsmith, @jayvdb
Files	25034.patch

^{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 = 'https://github.com/ericvsmith'
closed_at = <Date 2015-09-29.14:34:36.217>
created_at = <Date 2015-09-08.19:23:13.767>
labels = ['type-bug', 'library']
title = 'string.Formatter accepts empty fields but displays wrong when nested'
updated_at = <Date 2015-12-16.22:16:03.112>
user = 'https://github.com/AvdN'

bugs.python.org fields:

activity = <Date 2015-12-16.22:16:03.112>
actor = 'jayvdb'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2015-09-29.14:34:36.217>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2015-09-08.19:23:13.767>
creator = 'anthon'
dependencies = []
files = ['40419']
hgrepos = []
issue_num = 25034
keywords = ['patch']
message_count = 6.0
messages = ['250253', '250254', '250291', '251860', '251861', '255258']
nosy_count = 5.0
nosy_names = ['anthon', 'eric.smith', 'python-dev', 'jayvdb', 'Bill Tutt']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25034'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6']

[AvdN]

Since 3.4.1, string.Formatter() accepts empty keys {}. If these are nested they give different results from explicitly numbered, where the same arguments applied "".format() give the expected results:

    from string import Formatter

    f = Formatter()

    fmt0 = "X {:<{}} {} X"
    fmt1 = "X {0:<{1}} {2} X"

    for fmt in [fmt0, fmt1]:
        x = f.format(fmt, 'ab', 5, 1)
        y = fmt.format(   'ab', 5, 1)
        print(x)
        print(y)
gives:

X ab    5 X
X ab    1 X
X ab    1 X
X ab    1 X

of which the first line is incorrect.

[AvdN]

Here is a patch for Python-3.5.0rc3/Lib/test/test_string.py unittests fail:

--- /opt/python/3.5/lib/python3.5/test/test_string.py	2015-09-08 17:06:07.099197904 +0200
+++ test_string.py	2015-09-08 21:47:01.471634309 +0200
@@ -58,6 +58,8 @@
                          'foo{1}{num}{1}'.format(None, 'bar', num=6))
         self.assertEqual(fmt.format('{:^{}}', 'bar', 6),
                          '{:^{}}'.format('bar', 6))
+        self.assertEqual(fmt.format('{:^{}} {}', 'bar', 6, 'X'),
+                         '{:^{}} {}'.format('bar', 6, 'X'))
         self.assertEqual(fmt.format('{:^{pad}}{}', 'foo', 'bar', pad=6),
                          '{:^{pad}}{}'.format('foo', 'bar', pad=6))

[AvdN]

The problem lies in the recursive call to _vformat which might consume fields without name ({}) and increment auto_arg_index, but that
incremented value was not returned to the parent.

Since the line became longer than PEP-8 allowed I wrapped all of the method call arguments to the next line, hope that that's ok.

The patch is against the mercurial repository and works for 3.4.1 upwards.

[python-dev]

New changeset 9eae18e8af66 by Eric V. Smith in branch '3.4':
Fixed issue bpo-25034: Fix string.Formatter problem with auto-numbering
https://hg.python.org/cpython/rev/9eae18e8af66

New changeset 65d7b4fd0332 by Eric V. Smith in branch '3.5':
Issue bpo-25034: Merge from 3.4.
https://hg.python.org/cpython/rev/65d7b4fd0332

New changeset aef6365294c8 by Eric V. Smith in branch 'default':
Issue bpo-25034: Merge from 3.5.
https://hg.python.org/cpython/rev/aef6365294c8

[ericvsmith]

Fixed in 3.4, 3.5, and 3.6. Thanks for the bug report and patch! I added you to the Misc/ACKS file.

[BillTutt]

I don't suppose this change could make it into 2.7.11 as well?

Thanks,
Bill