Rearranged WebPImagePlugin _accept for speed

[radarhere]

Minor rearranging for speed - if the first condition fails, the other conditions will not be checked.

[hugovk]

This looks a bit like premature optimisation and the old code is self-documenting. What speed is gained here? I'd guess slicing three strings isn't particularly slow, but have you measured it and found this to be a problem?

[radarhere]

I just thought it was a minor improvement. Feel free to close.

[homm]

I'm for it because this code is executed for every file, not only WebP images.

[wiredfool]

I've just done some stepping through this area of the code, and I think this is of minimal use.

The order that image plugins are tried is:

['BMP', 'GIF', 'TIFF', 'JPEG', 'PPM', 'PNG', 'BUFR', 'CUR', 'PCX', 'DCX', 'DDS', 'EPS', 'FITS', 'FLI', 'FPX', 'FTEX', 'GBR', 'GRIB', 'HDF5', 'JPEG2000', 'ICNS', 'ICO', 'IM', 'IMT', 'IPTC', 'MCIDAS', 'MIC', 'MPEG', 'MSP', 'PCD', 'PIXAR', 'PSD', 'SGI', 'SPIDER', 'SUN', 'TGA', 'WEBP', 'WMF', 'XBM', 'XPM', 'XVTHUMB']

There are several image formats that don't even have _accept functions in the list before webp. Every single one of them makes at least one read to the file pointer, some of them read field by field. That's potentially a disk hit.

 for format in Image.ID: print ("%s, %s" % (format, Image.OPEN[format][1]))
... 
BMP, <function _accept at 0x7f76364f8830>
GIF, <function _accept at 0x7f7635b460e0>
TIFF, <function _accept at 0x7f7635a8fdd0>
JPEG, <function _accept at 0x7f7635b684d0>
PPM, <function _accept at 0x7f7635a58320>
PNG, <function _accept at 0x7f76359fc9e0>
BUFR, <function _accept at 0x7f7635a177a0>
CUR, <function _accept at 0x7f7635a17cb0>
PCX, <function _accept at 0x7f7635a17290>
DCX, <function _accept at 0x7f7635a17560>
DDS, <function _validate at 0x7f76359eae60>
EPS, <function _accept at 0x7f76359e57a0>
FITS, <function _accept at 0x7f76359e5950>
FLI, <function _accept at 0x7f7635ab7440>
FPX, <function _accept at 0x7f7635ab7b00>
FTEX, <function _validate at 0x7f7635a7d5f0>
GBR, <function _accept at 0x7f7635a7d950>
GRIB, <function _accept at 0x7f7635a7def0>
HDF5, <function _accept at 0x7f763575f560>
JPEG2000, <function _accept at 0x7f763572a3b0>
ICNS, <function <lambda> at 0x7f76357497a0>
ICO, <function _accept at 0x7f763574e170>
IM, None
IMT, None
IPTC, None
MCIDAS, <function _accept at 0x7f7635757950>
MIC, <function _accept at 0x7f7635757e60>
MPEG, None
MSP, <function _accept at 0x7f763575c3b0>
PCD, None
PIXAR, <function _accept at 0x7f7634cfb290>
PSD, <function _accept at 0x7f7634cfb8c0>
SGI, <function _accept at 0x7f7634cfbef0>
SPIDER, None
SUN, <function _accept at 0x7f7634d053b0>
TGA, None
WEBP, <function _accept at 0x7f7634d059e0>
WMF, <function _accept at 0x7f7634d0b3b0>
XBM, <function _accept at 0x7f7634d0b680>
XPM, <function _accept at 0x7f7634d0b9e0>
XVTHUMB, <function _accept at 0x7f7634d0bcb0>