core: make it possible to specify the api_baseurl init argument in override_config

This is required to be able to make lister classes instanciation easier and more
reliable, especially in the context of cli tools like 'swh lister run', for which
we want to be able to specify any lister init argument as extra parameter of the
command.

swh/lister/core/indexing_lister.py

@@ -245,5 +245,5 @@ class IndexingHttpLister(ListerHttpTransport, IndexingLister):
     """Convenience class for ensuring right lookup and init order
         when combining IndexingLister and ListerHttpTransport."""
     def __init__(self, api_baseurl=None, override_config=None):
-        ListerHttpTransport.__init__(self, api_baseurl=api_baseurl)
         IndexingLister.__init__(self, override_config=override_config)
+        ListerHttpTransport.__init__(self, api_baseurl=api_baseurl)

swh/lister/core/lister_transports.py

@@ -29,7 +29,7 @@ class ListerHttpTransport(abc.ABC):
 
     To be used in conjunction with ListerBase or a subclass of it.
     """
-
+    DEFAULT_URL = None
     PATH_TEMPLATE = AbstractAttribute('string containing a python string'
                                       ' format pattern that produces the API'
                                       ' endpoint path for listing stored'
@@ -143,6 +143,10 @@ class ListerHttpTransport(abc.ABC):
             return False, 0
 
     def __init__(self, api_baseurl=None):
+        if not api_baseurl:
+            api_baseurl = self.config.get('api_baseurl')
+        if not api_baseurl:
+            api_baseurl = self.DEFAULT_URL
         if not api_baseurl:
             raise NameError('HTTP Lister Transport requires api_baseurl.')
         self.api_baseurl = api_baseurl  # eg. 'https://api.github.com'

swh/lister/core/page_by_page_lister.py

@@ -156,5 +156,5 @@ class PageByPageHttpLister(ListerHttpTransport, PageByPageLister):
 
     """
     def __init__(self, api_baseurl=None, override_config=None):
-        ListerHttpTransport.__init__(self, api_baseurl=api_baseurl)
         PageByPageLister.__init__(self, override_config=override_config)
+        ListerHttpTransport.__init__(self, api_baseurl=api_baseurl)