Python unable to Fetch GOOGLE_APPLICATION_CREDENTIALS from Environment Variable (Windows)

I wanted to explore Google Big-Query with Python, and as per this tutorial I have set up a Google Cloud Account (free-tier), and have generated a key. The JSON file is stored in D:\keys\quixotic-folio-318907-64bfdccfb050.json.

The ENVIRONMENT VARIABLES in Windows-10 is also added under GOOGLE_APPLICATION_CREDENTIALS under System Variables:

However, whenever I try to initialize the client, it throws an error - File Not Found:

> from google.cloud import storage > storage.Client(project = "quixotic-folio-318907") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "D:\Anaconda3\lib\site-packages\google\cloud\storage\client.py", line 123, in __init__ super(Client, self).__init__( File "D:\Anaconda3\lib\site-packages\google\cloud\client.py", line 319, in __init__ Client.__init__( File "D:\Anaconda3\lib\site-packages\google\cloud\client.py", line 178, in __init__ credentials, _ = google.auth.default(scopes=scopes) File "D:\Anaconda3\lib\site-packages\google\auth\_default.py", line 454, in default credentials, project_id = checker() File "D:\Anaconda3\lib\site-packages\google\auth\_default.py", line 221, in _get_explicit_environ_credentials credentials, project_id = load_credentials_from_file( File "D:\Anaconda3\lib\site-packages\google\auth\_default.py", line 107, in load_credentials_from_file raise exceptions.DefaultCredentialsError( google.auth.exceptions.DefaultCredentialsError: File D:\keys\quixotic-folio-318907-64bfdccfb050.json; was not found. 

I've tried os method, as suggested here, and it is running perfectly:

> import os > from google.cloud import storage > os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = "D:\keys\quixotic-folio-318907-64bfdccfb050.json" > storage.Client(project = "quixotic-folio-318907") <google.cloud.storage.client.Client object at 0x000002448A4E8AF0> 

I have the following questions:

1. Is this an expected behavior, and why?
2. How do I ensure that I do not have to specifically set os.environ['GOOGLE_APPLICATION_CREDENTIALS'] as it is already defined under System Variables?