fix: load google credentials properly in GoogleDriveLoader (#12871)

- **Description:** 
- Fix #12870: set scope in `default` func (ref:
https://google-auth.readthedocs.io/en/master/reference/google.auth.html)
- Moved the code to load default credentials to the bottom for clarity
of the logic
	- Add docstring and comment for each credential loading logic
- **Issue:** https://github.com/langchain-ai/langchain/issues/12870
- **Dependencies:** no dependencies change
- **Tag maintainer:** for a quicker response, tag the relevant
maintainer (see below),
- **Twitter handle:** @gymnstcs

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---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/langchain_community/document_loaders/googledrive.py

@@ -7,7 +7,6 @@
 # 4. For service accounts visit
 #   https://cloud.google.com/iam/docs/service-accounts-create
 
-import os
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Sequence, Union
 
@@ -108,7 +107,13 @@ class GoogleDriveLoader(BaseLoader, BaseModel):
         return v
 
     def _load_credentials(self) -> Any:
-        """Load credentials."""
+        """Load credentials.
+        The order of loading credentials:
+        1. Service account key if file exists
+        2. Token path (for OAuth Client) if file exists
+        3. Credentials path (for OAuth Client) if file exists
+        4. Default credentials. if no credentials found, raise DefaultCredentialsError
+        """
         # Adapted from https://developers.google.com/drive/api/v3/quickstart/python
         try:
             from google.auth import default
@@ -126,30 +131,31 @@ class GoogleDriveLoader(BaseLoader, BaseModel):
             )
 
         creds = None
+        # From service account
         if self.service_account_key.exists():
             return service_account.Credentials.from_service_account_file(
                 str(self.service_account_key), scopes=SCOPES
             )
 
+        # From Oauth Client
         if self.token_path.exists():
             creds = Credentials.from_authorized_user_file(str(self.token_path), SCOPES)
 
         if not creds or not creds.valid:
             if creds and creds.expired and creds.refresh_token:
                 creds.refresh(Request())
-            elif "GOOGLE_APPLICATION_CREDENTIALS" not in os.environ:
-                creds, project = default()
-                creds = creds.with_scopes(SCOPES)
-                # no need to write to file
-                if creds:
-                    return creds
-            else:
+            elif self.credentials_path.exists():
                 flow = InstalledAppFlow.from_client_secrets_file(
                     str(self.credentials_path), SCOPES
                 )
                 creds = flow.run_local_server(port=0)
-            with open(self.token_path, "w") as token:
-                token.write(creds.to_json())
+            if creds:
+                with open(self.token_path, "w") as token:
+                    token.write(creds.to_json())
+
+        # From Application Default Credentials
+        if not creds:
+            creds, _ = default(scopes=SCOPES)
 
         return creds