Failing test case: "  \tboo\n \tghost"

expected:          " \tboo\n\tghost"
returns:           "  \tboo\n \tghost"

Confirmed that the test case fails on 2.7, 3.4, and 3.5

This one isn't hard.  Would you like to make a patch?  If not, I get to it this evening.

Hi Raymond- Rob and I have a patch ready. We are figuring out how to upload/submit it. New to this... :)

YJ and I are adding a patch and an additional test.

Raymond confirmed that the issue exists on 2.7 and 3.4, so we'll keep those in the version list.  Whoever makes the commit will take care of backporting the patch, though.

Tested and it works fine on CentOS 6.4 in 2.7, 3.4 and 3.5

I've got it from here.

Not surprising also occurs on Python 3.2, 3.3, & 3.6.
I'm not sure whether fixes like this are suitable to be merged into 3.2/3.3.(is there a document describing this?)

LGTM. But the implementation can be more efficient. Here is optimized patch.

34e88a05562f.diff:
$ ./python -m timeit -s 'import textwrap; s = "{0} x\n{0}\tx\n".format(" "*100)' -- 'textwrap.dedent(s)'
10000 loops, best of 3: 87.5 usec per loop
$ ./python -m timeit -s 'import textwrap; s = "{0} x\n{0}\tx\n".format(" "*1000)' -- 'textwrap.dedent(s)'
1000 loops, best of 3: 780 usec per loop

issue21827.patch:
$ ./python -m timeit -s 'import textwrap; s = "{0} x\n{0}\tx\n".format(" "*100)' -- 'textwrap.dedent(s)'
10000 loops, best of 3: 51 usec per loop
$ ./python -m timeit -s 'import textwrap; s = "{0} x\n{0}\tx\n".format(" "*1000)' -- 'textwrap.dedent(s)'
1000 loops, best of 3: 395 usec per loop

It's one of the PEPs (I forget which one).  We only apply non-security fixes to the most recent non-development branch...except that for now we also apply bug fixes to 2.7.  So, 2.7, 3.5, and 3.6 are the targets for this issue.

Serhiy, your way looks best.  Go ahead with it.

I thought that 3.4 will switch to "security fixes only" mode after releasing first release since releasing 3.5.0. It was so with 3.2 and 3.3.

You are right, I'd forgotten we hadn't done 3.4 final yet.

New changeset faeeb8dbe432 by Serhiy Storchaka in branch '3.4':
Issue #21827: Fixed textwrap.dedent() for the case when largest common
https://hg.python.org/cpython/rev/faeeb8dbe432

New changeset c307ae7b3d9f by Serhiy Storchaka in branch '2.7':
Issue #21827: Fixed textwrap.dedent() for the case when largest common
https://hg.python.org/cpython/rev/c307ae7b3d9f

New changeset 3f29be82c944 by Serhiy Storchaka in branch '3.5':
Issue #21827: Fixed textwrap.dedent() for the case when largest common
https://hg.python.org/cpython/rev/3f29be82c944

New changeset 8d3932671e48 by Serhiy Storchaka in branch 'default':
Issue #21827: Fixed textwrap.dedent() for the case when largest common
https://hg.python.org/cpython/rev/8d3932671e48

Thank you Robert for your report and patch.