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Consider this:
def defaultKeepsIdentity(arg = "str_value"):
 print(arg is "str_value")
defaultKeepsIdentity()
This has been outputing "True" on every Python release I have seen so far, but not so on 3.6.0a4. Normally string values come from a "co_const" and will be "is" identical if used as literals in a module, but no longer in this case.
This seems wasteful at best, needlessly increasing the number of strings in usage. 
Yours,
Kay

Can confirm the expected behaviour (printing True) in Python 2.4 through 2.7, 3.3, Jython 2.5, and even venerable old Python 1.5 (where it prints 1).
But *not* IronPython 2.6, where it prints False.
In 3.6, the difference seems to be here:
py> f = defaultKeepsIdentity
py> f.__defaults__[0] is f.__code__.co_consts[1]
False
py> f.__defaults__[0] == f.__code__.co_consts[1]
True
This behaviour is not specified by the language. Caching and re-use of strings has always been subject to change. Nevertheless, perhaps it is time for this to be make a language feature: inside a function, any use of the same string literal should use the same object?

This likely is an interference of issue27095 with issue26148. I consider this as a regression. This causes not only a waste of memory, but can affect a performance, since comparing non-identical strings is slower than comparing identical strings.

Same with 3.6b1, still present.

It would be nice to get this fixed.

Proposed patch interns string constants recursively in tuples and frozensets. This fixes issue26148 and issue25981.

New changeset 78bea78d9335 by Serhiy Storchaka in branch '3.5':
Issue #27942: String constants now interned recursively in tuples and frozensets.
https://hg.python.org/cpython/rev/78bea78d9335
New changeset 0ce63a7651b9 by Serhiy Storchaka in branch '3.6':
Issue #27942: String constants now interned recursively in tuples and frozensets.
https://hg.python.org/cpython/rev/0ce63a7651b9
New changeset 44af6bd21b94 by Serhiy Storchaka in branch 'default':
Issue #27942: String constants now interned recursively in tuples and frozensets.
https://hg.python.org/cpython/rev/44af6bd21b94
New changeset 7cea3bf44acb by Serhiy Storchaka in branch '2.7':
Issue #27942: String constants now interned recursively in tuples and frozensets.
https://hg.python.org/cpython/rev/7cea3bf44acb 

Thanks Naoki and Josh for reviews.
Backported to 2.7 and 3.5 since the patch also fixes other issues.
Note that the result of ``arg is "str_value"`` is implementation detail. Other Python implementations may not intern string constants. Even CPython interns only strings constants consisting of ASCII alphanumerical characters.

The patch is causing refleaks:
 test_ast leaked [98, 98, 98] references, sum=294
 test_code leaked [2, 2, 2] references, sum=6
Please review the attached patch fixing that. Please review.

Looks like the attached patch also fixes
 test_trace leaked [12, 12, 12] references, sum=36

Good catch! The patch LGTM, thanks Yury.
Interesting, what tests in test_ast leak? I expected that this branch is executed in rare circumstances.

> Interesting, what tests in test_ast leak? I expected that this branch is executed in rare circumstances.
The test that tests it all :) test_stdlib_validates

New changeset 41613bb27f80 by Yury Selivanov in branch '2.7':
Issue #27942: Fix memory leak in codeobject.c
https://hg.python.org/cpython/rev/41613bb27f80
New changeset 2c6825c9ecfd by Yury Selivanov in branch '3.5':
ssue #27942: Fix memory leak in codeobject.c
https://hg.python.org/cpython/rev/2c6825c9ecfd
New changeset b671ac7ae620 by Yury Selivanov in branch '3.6':
Merge 3.5 (issue #27942)
https://hg.python.org/cpython/rev/b671ac7ae620
New changeset c27269c0d619 by Yury Selivanov in branch 'default':
Merge 3.6 (issue #27942)
https://hg.python.org/cpython/rev/c27269c0d619 

Thanks for the review, Serhiy!

> The test that tests it all :) test_stdlib_validates
:)