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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 get_picture_url(self):
 """ Returns the image containing the picture for the user. The picture can be a uploaded image or a Gravatar. Gravatar functionality will only be used when ``ACCOUNTS_GRAVATAR_PICTURE`` is set to ``True``. :return: ``None`` when Gravatar is not used and no default image is supplied by ``ACCOUNTS_GRAVATAR_DEFAULT``. """

        # First check for a picture and if any return that.
        if self.picture:
            return self.picture.url

        # Use Gravatar if the user wants to.
        if defaults.ACCOUNTS_GRAVATAR_PICTURE:
            return get_gravatar(self.email,
                                defaults.ACCOUNTS_GRAVATAR_SIZE,
                                defaults.ACCOUNTS_GRAVATAR_DEFAULT)

        # Gravatar not used, check for a default image.
        else:
            if defaults.ACCOUNTS_GRAVATAR_DEFAULT not in ['404', 'mm',
                                                                'identicon',
                                                                'monsterid',
                                                                'wavatar']:
                return defaults.ACCOUNTS_GRAVATAR_DEFAULT
            else: return None