Here's where the plugin hook is called, demonstrating the fallback= argument:

Lines 426 to 472 in 05a312c

    
               def metadata(self, key=None, database=None, table=None, fallback=True): 
        
                   """ 
        
                   Looks up metadata, cascading backwards from specified level. 
        
                   Returns None if metadata value is not found. 
        
                   """ 
        
                   assert not ( 
        
                       database is None and table is not None 
        
                   ), "Cannot call metadata() with table= specified but not database=" 
        
                   metadata = {} 
        
                   for hook_dbs in pm.hook.get_metadata( 
        
                       datasette=self, key=key, database=database, table=table, fallback=fallback 
        
                   ): 
        
                       metadata = self._metadata_recursive_update(metadata, hook_dbs) 
        
                   # security precaution!! don't allow anything in the local config 
        
                   # to be overwritten. this is a temporary measure, not sure if this 
        
                   # is a good idea long term or maybe if it should just be a concern 
        
                   # of the plugin's implemtnation 
        
                   metadata = self._metadata_recursive_update(metadata, self._metadata_local) 
        
                   databases = metadata.get("databases") or {} 
        
                   search_list = [] 
        
                   if database is not None: 
        
                       search_list.append(databases.get(database) or {}) 
        
                   if table is not None: 
        
                       table_metadata = ((databases.get(database) or {}).get("tables") or {}).get( 
        
                           table 
        
                       ) or {} 
        
                       search_list.insert(0, table_metadata) 
        
                   search_list.append(metadata) 
        
                   if not fallback: 
        
                       # No fallback allowed, so just use the first one in the list 
        
                       search_list = search_list[:1] 
        
                   if key is not None: 
        
                       for item in search_list: 
        
                           if key in item: 
        
                               return item[key] 
        
                       return None 
        
                   else: 
        
                       # Return the merged list 
        
                       m = {} 
        
                       for item in search_list: 
        
                           m.update(item) 
        
                       return m

I'm not convinced of the use-case for passing fallback= to the hook here - is there a reason a plugin might care whether fallback is True or False, seeing as the metadata() method already respects that fallback logic on line 459?

	def metadata(self, key=None, database=None, table=None, fallback=True):
	"""
	Looks up metadata, cascading backwards from specified level.
	Returns None if metadata value is not found.
	"""
	assert not (
	database is None and table is not None
	), "Cannot call metadata() with table= specified but not database="
	metadata = {}

	for hook_dbs in pm.hook.get_metadata(
	datasette=self, key=key, database=database, table=table, fallback=fallback
	):
	metadata = self._metadata_recursive_update(metadata, hook_dbs)

	# security precaution!! don't allow anything in the local config
	# to be overwritten. this is a temporary measure, not sure if this
	# is a good idea long term or maybe if it should just be a concern
	# of the plugin's implemtnation
	metadata = self._metadata_recursive_update(metadata, self._metadata_local)

	databases = metadata.get("databases") or {}

	search_list = []
	if database is not None:
	search_list.append(databases.get(database) or {})
	if table is not None:
	table_metadata = ((databases.get(database) or {}).get("tables") or {}).get(
	table
	) or {}
	search_list.insert(0, table_metadata)

	search_list.append(metadata)
	if not fallback:
	# No fallback allowed, so just use the first one in the list
	search_list = search_list[:1]
	if key is not None:
	for item in search_list:
	if key in item:
	return item[key]
	return None
	else:
	# Return the merged list
	m = {}
	for item in search_list:
	m.update(item)
	return m

Uh oh!

Plugin hook for dynamic metadata #1384

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