Fix check connection (#457)

* deprecate check_credentials argument from GoogleCredentials constructor

* remove check_connection argument from guess_credentials function

* add comment explaining why runtime error should never occur

gcsfs/core.py

@@ -203,12 +203,6 @@ class GCSFileSystem(AsyncFileSystem):
         Cache expiration time in seconds for object metadata cache.
         Set cache_timeout <= 0 for no caching, None for no cache expiration.
     secure_serialize: bool (deprecated)
-    check_connection: bool
-        When token=None, gcsfs will attempt various methods of establishing
-        credentials, falling back to anon. It is possible for a method to
-        find credentials in the system that turn out not to be valid. Setting
-        this parameter to True will ensure that an actual operation is
-        attempted before deciding that credentials are valid.
     requester_pays : bool, or str default False
         Whether to use requester-pays requests. This will include your
         project ID `project` in requests as the `userPorject`, and you'll be
@@ -239,7 +233,7 @@ def __init__(
         consistency="none",
         cache_timeout=None,
         secure_serialize=True,
-        check_connection=False,
+        check_connection=None,
         requests_timeout=None,
         requester_pays=False,
         asynchronous=False,
@@ -271,7 +265,13 @@ def __init__(
         self._endpoint = endpoint_url
         self.session_kwargs = session_kwargs or {}
 
-        self.credentials = GoogleCredentials(project, access, token, check_connection)
+        if check_connection:
+            warnings.warn(
+                "The `check_connection` argument is deprecated and will be removed in a future release.",
+                DeprecationWarning,
+            )
+
+        self.credentials = GoogleCredentials(project, access, token)
 
         if not self.asynchronous:
             self._session = sync(

gcsfs/credentials.py

@@ -38,19 +38,24 @@
 
 
 class GoogleCredentials:
-    def __init__(self, project, access, token, check_credentials=False):
+    def __init__(self, project, access, token, check_credentials=None):
         self.scope = "https://www.googleapis.com/auth/devstorage." + access
         self.project = project
         self.access = access
         self.heads = {}
 
-        self.check_credentials = check_credentials
         self.credentials = None
         self.method = None
         self.lock = threading.Lock()
         self.token = token
         self.connect(method=token)
 
+        if check_credentials:
+            warnings.warn(
+                "The `check_credentials` argument is deprecated and will be removed in a future release.",
+                DeprecationWarning,
+            )
+
     @classmethod
     def load_tokens(cls):
         """Get "browser" tokens from disc"""
@@ -213,8 +218,6 @@ def connect(self, method=None):
             for meth in ["google_default", "cache", "cloud", "anon"]:
                 try:
                     self.connect(method=meth)
-                    if self.check_credentials and meth != "anon":
-                        self.ls("anaconda-public-data")
                     logger.debug("Connected with method %s", meth)
                     break
                 except google.auth.exceptions.GoogleAuthError as e:
@@ -223,6 +226,10 @@ def connect(self, method=None):
                     logger.debug(
                         'Connection with method "%s" failed' % meth, exc_info=e
                     )
+            else:
+                # Since the 'anon' connection method should always succeed,
+                # getting here means something has gone terribly wrong.
+                raise RuntimeError("All connection methods have failed!")
         else:
             self.__getattribute__("_connect_" + method)()
             self.method = method