Merge branch '1922-cache-discovery-python'

diff --cc sdk/python/arvados/__init__.py
index b881066bfa1e5766b99d015fcdd3cba6c27e9cb9,18bde85d8acbb55579eb1f23d502e339ac31b3df..2d19259064093d0a898782e06a634c005f12474f
--- 1/sdk/python/arvados/__init__.py
--- 2/sdk/python/arvados/__init__.py
+++ b/sdk/python/arvados/__init__.py
@@@ -128,14 -129,16 +128,22 @@@ def _cast_objects_too(value, schema_typ
          return _cast_orig(value, schema_type)
  apiclient.discovery._cast = _cast_objects_too
  
+ def http_cache(data_type):
+     path = os.environ['HOME'] + '/.cache/arvados/' + data_type
+     try:
+         util.mkdir_dash_p(path)
+     except OSError:
+         path = None
+     return path
+ 
  def api(version=None):
      global services, config
 +
 +    if not config:
 +        config = ArvadosConfig(os.environ['HOME'] + '/.config/arvados')
 +        if 'ARVADOS_DEBUG' in config:
 +            logging.basicConfig(level=logging.DEBUG)
 +
      if not services.get(version):
          apiVersion = version
          if not version: