Documentation: _imaging incompatible with ucs4

[macfreek]

It seems that the _imaging submodule in Pillow (and PIL?) does not work when Python is compiled with the --with-wide-unicode option (causing all internal string representations to be stored as 4-byte Unicode UCS4 instead of 2-byte Unicode UCS2).

Everything seems to work fine:

PIL SETUP SUMMARY
--------------------------------------------------------------------
version      Pillow 2.1.0
platform     darwin 3.2.5 (default, Jul 22 2013, 21:29:27)
             [GCC 4.2.1 Compatible Apple Clang 3.0 (tags/Apple/clang-211.10.1)]
--------------------------------------------------------------------
*** TKINTER support not available
--- JPEG support available
--- ZLIB (PNG/ZIP) support available
--- TIFF G3/G4 (experimental) support available
--- FREETYPE2 support available
--- LITTLECMS support available
--- WEBP support available
--------------------------------------------------------------------

But importing PIL.Image does not work:

>>> from PIL import Image
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/PIL/Image.py", line 149, in <module>
    if hasattr(core, 'DEFAULT_STRATEGY'):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/PIL/Image.py", line 39, in __getattr__
    raise ImportError("The _imaging C module is not installed")
ImportError: The _imaging C module is not installed

The underlying cause seems that --with-wide-unicode, the _PyUnicodeUCS2_FromString function is no longer available:

>>> from PIL import _imaging
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: dlopen(/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/PIL/_imaging.so, 2): Symbol not found: _PyUnicodeUCS2_FromString
  Referenced from: /opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/PIL/_imaging.so
  Expected in: flat namespace
 in /opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/PIL/_imaging.so
>>> 

I suspect this problem no longer occurs with Python 3.3 and up, since that has implemented PEP 393 (Flexible string representations, as either UCS2 or UCS4).

I don't think code changes to Pillow are in order, but I certainly would appreciate a better error as to the problem. The message "The _imaging C module is not installed" was wrong and put me on the wrong foot. If this error can be improved, that would be of great help.

Thanks!

[aclark4life]

Thanks for the report, are you able to send a pull request containing a better handling of the error? I think we have a lot of room for improvement in this area.

[wiredfool]

It appears that in Image.py, 63:74, we're trapping a couple of import errors that we're expecting and warning about them, but silently doing nothing for unexpected errors like this one. I'd say at the very least that every import error should be at least a warning. I don't know if anyone intentionally runs Pillow without the compiled extensions, but I think erroring out sooner rather than later with an actual exception rather than a canned "not installed" one would be reasonable and pythonic. It really doesn't make sense to silently catch an import error and then fail at the first access to the imaging core.

[aclark4life]

+1

[macfreek]

A small correction to the original error report. The problem only occurs when the _imaging extension is build with a different UCS setting than the Python used at runtime. So the error Symbol not found: _PyUnicodeUCS4_FromString can also occur.

[aclark4life]

OK I guess we need someone to send a pull request

[macfreek]

I think this has been fixed in #298 and #299. I'll close it.
I'll test this weekend and if this error error message resurfaces, I'll reopen.

[aclark4life]

Great, thanks