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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate(self, instance, value):
 """Checks if value is an open PNG file, valid filename, or png.Image Returns an open bytestream of the image """

        # Pass if already validated
        if getattr(value, '__valid__', False):
            return value
        # Validate that value is PNG
        if isinstance(value, png.Image):
            pass
        else:
            value = super(ImagePNG, self).validate(instance, value)
            try:
                png.Reader(value).validate_signature()
            except png.FormatError:
                self.error(instance, value, extra='Open file is not PNG.')
            value.seek(0)
        # Write input to new bytestream
        output = BytesIO()
        output.name = self.filename
        output.__valid__ = True
        if isinstance(value, png.Image):
            value.save(output)
        else:
            fid = value
            fid.seek(0)
            output.write(fid.read())
            fid.close()
        output.seek(0)
        return output