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"""

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    The various Exif backends that we have

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"""

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from __future__ import with_statement

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import utils

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class ExifHandler (object) :

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    """

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        Our generic interface for Exif data

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    """

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    def __init__ (self, file) :

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        """

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            Load Exif data from the given File.

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        """

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        self.file = file

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    def image_tag (self, tag, default=None) :

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        """

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            Load and return the human-readable unicode-safe value for the given main image tag.

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        """

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        abstract

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    def image_tags (self, tags) :

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        """

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            Load and return the given tags for the main image as a sequence of (tag, value) tuples , following the same

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            rules as image_tag, except there should mainly be only one call to this function, encompassing all the

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            relevant tags.

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            The returned list should omit those values which were not found.

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        """

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        for tag in tags :

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            # try and fetch value

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            value = self.image_tag(tag)

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            # filter out default not-found

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            if value is not None :

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                yield tag, value

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try :

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    import pyexiv2

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except ImportError :

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    PyExiv2_Handler = None

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else :

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    class PyExiv2_Handler (ExifHandler) :

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        NAME = 'pyexiv2'

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        def __init__ (self, file) :

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            """

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                Load as pyexiv2.Image

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            """

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            # create

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            self.image = pyexiv2.Image(file.path)

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            # load

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            self.image.readMetadata()

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        def image_tag (self, tag, default=None) :

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                return default

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try :

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    from lib import EXIF

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except ImportError :

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    EXIF_Handler = None

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else :

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    class EXIF_Handler (ExifHandler) :

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        NAME = 'EXIFpy'

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        def __init__ (self, file) :

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            """

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                Load using EXIF.process_file

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            """

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            with file.open('rb') as fh :

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                self.tags = EXIF.process_file(fh)

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        def image_tag (self, tag, default=None) :

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# ExifHandler implementations to use, in preference order

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EXIF_HANDLERS = [

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    # reasonably fast, but requires the native library and extension module

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    PyExiv2_Handler,

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    # pure-python, but very, very slow

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    EXIF_Handler,

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]

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# default ExifHandler to use

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EXIF_HANDLER = utils.first(EXIF_HANDLERS)

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# ExifHandler implementations by name

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EXIF_HANDLERS_BY_NAME = dict((h.NAME, h) for h in EXIF_HANDLERS)

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def load (config, file) :

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    """

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        Load Exif data using the configured Exif handler, and return the instance.

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        Returns None if there wasn't any Exif handler available for use.

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    """

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    if config.exif_handler :

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        # explicit

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        exif_handler = config.exif_handler

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    elif config.exif_handler_name :

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        # by name

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        exif_handler = EXIF_HANDLERS_BY_NAME[config.exif_handler_name]

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    else :

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        # default

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        exif_handler = EXIF_HANDLER

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    if exif_handler :

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        # found one, use it

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        return exif_handler(file)

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    else :

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        # nothing :(

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        return None

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