<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_layer_description_from_canvas(self, layer, purpose):
 """Obtain the description of a canvas layer selected by user. :param layer: The QGIS layer. :type layer: QgsMapLayer :param purpose: The layer purpose of the layer to get the description. :type purpose: string :returns: description of the selected layer. :rtype: string """

        if not layer:
            return ""

        try:
            keywords = self.keyword_io.read_keywords(layer)
            if 'layer_purpose' not in keywords:
                keywords = None
        except (HashNotFoundError,
                OperationalError,
                NoKeywordsFoundError,
                KeywordNotFoundError,
                InvalidParameterError,
                UnsupportedProviderError):
            keywords = None

        # set the current layer (e.g. for the keyword creation sub-thread)
        self.layer = layer
        if purpose == layer_purpose_hazard['key']:
            self.hazard_layer = layer
        elif purpose == layer_purpose_exposure['key']:
            self.exposure_layer = layer
        else:
            self.aggregation_layer = layer

        # Check if the layer is keywordless
        if keywords and 'keyword_version' in keywords:
            kw_ver = str(keywords['keyword_version'])
            self.is_selected_layer_keywordless = (
                not is_keyword_version_supported(kw_ver))
        else:
            self.is_selected_layer_keywordless = True

        description = layer_description_html(layer, keywords)
        return description