One of my students found this bug. For ascii characters it works as you expect, but for greek alphabet it works unexpectedly.
The program works correctly for Python 3.2.x but for 3.4.x and 3.5 it gives erroneous result.

I'm afraid I'm unable to replicate this bug report in Python 3.4.

If you are able to replicate it, can you tell us the exact version number of Python you are using? Also, which operating system are you using?

Confirmed on IDLE.

>>> s = ''
>>> for i in range(5):
	s += '\xe0'
	print(s)

	
à
àà
àà
àà
àà
>>> s = ''
>>> for i in range(5):
	s += chr(0xe0)
	print(s)

	
à
àà
àà
àà
àà
>>> s = ''
>>> for i in range(5):
	s += '\u0107'
	print(s)

	
ć
ćć
ćć
ćć
ćć
>>> s = ''
>>> for i in range(5):
	s += chr(0x107)
	print(s)

	
ć
ć
ć
ć
ć

This issue can be related to details of IDLE's standard streams and RPC.

Here is reproducer without IDLE. Looks as pickle is a culprit.

>>> import pickle
>>> s = ''
>>> for i in range(5):
...         s += chr(0xe0)
...         print(len(s), s, s.encode(), repr(s))
...         print('   ', pickle.dumps(s))
... 
1 à b'\xc3\xa0' 'à'
    b'\x80\x03X\x02\x00\x00\x00\xc3\xa0q\x00.'
2 àà b'\xc3\xa0\xc3\xa0' 'àà'
    b'\x80\x03X\x04\x00\x00\x00\xc3\xa0\xc3\xa0q\x00.'
3 àà b'\xc3\xa0\xc3\xa0' 'ààà'
    b'\x80\x03X\x04\x00\x00\x00\xc3\xa0\xc3\xa0q\x00.'
4 àà b'\xc3\xa0\xc3\xa0' 'àààà'
    b'\x80\x03X\x04\x00\x00\x00\xc3\xa0\xc3\xa0q\x00.'
5 àà b'\xc3\xa0\xc3\xa0' 'ààààà'
    b'\x80\x03X\x04\x00\x00\x00\xc3\xa0\xc3\xa0q\x00.'

I've looked at the raw bytes [through a ctypes pointer to id(s)] of a string affected by the issue, and decoded enough to be able to tell that the bad string has an incorrect UTF-8 length and data, which pickle presumably relies on.

HEAD............length..hash....flgs....wstr....u8len...u8ptr...wstrl...data....
010000003094ac6403000000ffffffffa40000000000000004000000d814480000000000e0e0e000
010000003094ac6403000000e5d0030ca400006500000000060000008814480000000000e0e0e000

I've omitted the UTF-8 data, but both were null-terminated after four and six bytes respectively.

unicode_modifiable in Objects/unicodeobject.c should return 0 if there's cached PyUnicode_UTF8 data. In this case PyUnicode_Append won't operate in place but instead concatenate a new string.

I can't reproduce without pickle. I did some further digging, though, and it *looks like*...

1. Pickle causes the built-in UTF-8 representation of a string to be populated, whereas encode('utf-8') does not. Can anyone think of any other operations that do this?
2. After the UTF-8 representation of the 2-character string is populated, concatenating a new character to it does not update or clear it.
3. However, it worked just fine with the 1-character string - concatenating it caused the UTF-8 representation to be cleared.

The actual operation that creates an inconsistent string is the concatenate operation, but it only happens with a string that has been "primed" by having its UTF-8 representation materialized.

Yes, I have came to the same as random832. String objects have "fast path" for concatenating, and in this path cached UTF8 representation is not cleaned. Pickle is one of simplest ways to reproduce this issue. May be it can be reproduced with compile() or type(), but these ways looks too heavyweighted to me.

Here is a patch that fixes strings concatenation.

It would be good to get this in 3.4.4.

Added test without using pickle.

Serhiy, when does sharing UTF-8 data occur in a compact object? It has to be ASCII since non-ASCII UTF-8 isn't sharable, but PyASCIIObject doesn't have the utf8 field. So it has to be a PyCompactUnicodeObject. But isn't ASCII always allocated as a PyASCIIObject? I need a bit of help getting from point A to point B. Thanks.

I reviewed issue25709_2.patch.

> It would be good to get this in 3.4.4.

Since it's a major bug in the Unicode implementation, it may be worth to fix it in Python 3.3. The bug was introduced in Python 3.3 by the PEP 393.

3.3 is presumably in security mode. Anyone using it would have had to live with the bug for a long time already.

> unicode_modifiable in Objects/unicodeobject.c should return 0 if there's cached PyUnicode_UTF8 data. In this case PyUnicode_Append won't operate in place but instead concatenate a new string.

Shouldn't it still operate in place but clear it? Operating in place is only an option if the old string has no references and will be discarded, right?

I read some comments here and on the patches.  Serhiy's patch adds some code and Victor says you can't call that macro on this object and wow this is badly broken.  Can someone explain in simpler terms what's so broken, exactly?

" and wow this is badly broken "

I mean the currently code is badly broken.

The bug is that sometimes, when a string is resized (which doesn't make sense, strings are immutable, right? :-D), the cached UTF-8 string can become corrupted (old pointer not updated).

It occurs if

* the string is resized (ex: "s += s2")
* the string has a cached UTF-8 byte string (ex: int(s) was called before the resize)
* the resize moves the memory block to a new address

Ok, it's probably unlikely to get these 3 conditions, but from my point of view, it's a major bug because it's in a Python fundamental type (str), it's not a bug in user code and it cannot be worked around (easily).

In updated patch fixed a bug found by Victor and addressed other his comments. Many thanks Victor!

On Mon, Nov 23, 2015 at 09:48:46PM +0000, STINNER Victor wrote:

> * the string has a cached UTF-8 byte string (ex: int(s) was called before the resize)

Why do strings cache their UTF-8 encoding?

I presume that some of Python's internals rely on the UTF-8 encoding 
rather than the internal Latin-1/UCS-2/UTF-32 representation (PEP 393). 
E.g. I infer from the above that int(s) parses the UTF-8 representation 
of s rather than the internal representation. Is that right?

Nevertheless, I wonder why the UTF-8 representation is cached. Is it 
that expensive to generate that it can't be done on the fly, as needed? 
As it stands now, non-ASCII strings may be up to twice as big as they 
need be, once you include the UTF-8 cache. And, as this bug painfully 
shows, the problem with caches is that you run the risk of the cache 
being out of date.

Steven D'Aprano added the comment:
> the problem with caches is that you run the risk of the cache being out
of date.

Since strings are immutable, it's not a big deal. We control where strings
are modified (unicodeobject.c).

> Why do strings cache their UTF-8 encoding?

Strings also cache the wide-string representation. For example:

    from ctypes import *
    s = '\241\242\243'
    pythonapi.PyUnicode_AsUnicodeAndSize(py_object(s), None)
    pythonapi.PyUnicode_AsUTF8AndSize(py_object(s), None)

    >>> hex(id(s))
    '0x7ffff69f8e98'

    (gdb) p *(PyCompactUnicodeObject *)0x7ffff69f8e98
    $1 = {_base = {ob_base = {_ob_next = 0x7ffff697f890,
                              _ob_prev = 0x7ffff6a04d40,
                              ob_refcnt = 1, 
                              ob_type = 0x89d860 <PyUnicode_Type>},
                   length = 3,
                   hash = -5238559198920514942,
                   state = {interned = 0,
                            kind = 1,
                            compact = 1,
                            ascii = 0,
                            ready = 1},
                   wstr = 0x7ffff69690a0 L"¡¢£"},
          utf8_length = 6,
          utf8 = 0x7ffff696b7e8 "¡¢£",
          wstr_length = 3}

    (gdb) p (char *)((PyCompactUnicodeObject *)0x7ffff69f8e98 + 1)
    $2 = 0x7ffff69f8ef0 "\241\242\243"

This object uses 4 bytes for the null-terminated Latin-1 string, which directly follows the PyCompactUnicodeObject struct. It uses 7 bytes for the UTF-8 string. It uses 16 bytes for the wchar_t string (4 bytes per wchar_t).

Fixed yet one bug (thanks Victor again). Test is improved, now it doesn't rely on implementation detail of particular builtin.

On 24.11.2015 02:30, Steven D'Aprano wrote:
> 
> Steven D'Aprano added the comment:
> 
> On Mon, Nov 23, 2015 at 09:48:46PM +0000, STINNER Victor wrote:
> 
>> * the string has a cached UTF-8 byte string (ex: int(s) was called before the resize)
> 
> Why do strings cache their UTF-8 encoding?
> 
> I presume that some of Python's internals rely on the UTF-8 encoding 
> rather than the internal Latin-1/UCS-2/UTF-32 representation (PEP 393). 
> E.g. I infer from the above that int(s) parses the UTF-8 representation 
> of s rather than the internal representation. Is that right?
> 
> Nevertheless, I wonder why the UTF-8 representation is cached. Is it 
> that expensive to generate that it can't be done on the fly, as needed? 
> As it stands now, non-ASCII strings may be up to twice as big as they 
> need be, once you include the UTF-8 cache. And, as this bug painfully 
> shows, the problem with caches is that you run the risk of the cache 
> being out of date.

The cache is needed because it's the only way to get a direct
C char* to the object's UTF-8 representation without having to
worry about memory management on the caller's side. Not having
access to this would break a lot of code using the Python
C API, since the cache is there per design. The speedup aspect
is secondary.

Unicode objects are normally immutable, but there are a few
corner cases during the initialization of the objects where
they are in fact mutable for a short while, e.g. when
creating an empty object which is then filled with data and
resized to the final length before passing it back to
Python.

> Why do strings cache their UTF-8 encoding?

Mainly for compatibility with existing C API. Common way to parse function arguments in implemented in C function is to use special argument parsing API: PyArg_ParseTuple, PyArg_ParseTupleAndKeywords, or PyArg_Parse. Most format codes for Unicode strings returned a C pointer to char array. For that encoded Unicode strings should be kept somewhere at least for the time of executing C function. As well as PyArg_Parse* functions doesn't allow user to specify a storage for encoded string, it should be saved in Unicode object. That is not new to PEP 393 or Python 3, in Python 2 the Unicode objects also keep cached encoded version.

issue25709_4.patch now looks good to me, but I added some minor comments on the review.

Georg, I ask for applying this fix to 3.3.

Is this going in soon?  I want to cherry-pick this for 3.5.1, which I tag in about 80 hours.

I wait only Greg's approving for 3.3. If I'll not get it in a day, I'll commit the patch for 3.4+.

Please commit right now to 3.4+. Backport to 3.3 can be done later.

New changeset 67718032badb by Serhiy Storchaka in branch '3.4':
Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
https://hg.python.org/cpython/rev/67718032badb

New changeset a0e2376768dc by Serhiy Storchaka in branch '3.5':
Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
https://hg.python.org/cpython/rev/a0e2376768dc

New changeset 9e800b2aeeac by Serhiy Storchaka in branch 'default':
Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
https://hg.python.org/cpython/rev/9e800b2aeeac

> New changeset 67718032badb by Serhiy Storchaka in branch '3.4':

Thanks.

I cherry-picked this for 3.5.1.

New changeset 376b100107ba by Serhiy Storchaka in branch '3.5':
Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
https://hg.python.org/cpython/rev/376b100107ba

New changeset b9c8f1c80f47 by Serhiy Storchaka in branch '3.3':
Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
https://hg.python.org/cpython/rev/b9c8f1c80f47

Backpicked to 3.3. Sorry for the wait.

2016-02-11 18:23 GMT+01:00 Georg Brandl <report@bugs.python.org>:
>
> Georg Brandl added the comment:
>
> Backpicked to 3.3. Sorry for the wait.

Good, this bugfix is useful :-)

> I wait only Greg's approving for 3.3. If I'll not get it in a day, I'll commit the patch for 3.4+.

Maybe it was my fault. I made a mistake in Georg's name.

Actually I prefer Greg to Gerg, so it's only half bad. :D

b9c8f1c80f47 added a new head. Should we merge 3.3 -> 3.4 -> 3.5 -> default?

Don't bother. I can do that once 3.3.7 is released.