How do I properly assert a function with pandas?

Solution for How do I properly assert a function with pandas?
is Given Below:

def get_active_campaign_sheets():
   filePath="//BMG-FPS01/Digital/*****/performance.csv"
   assert os.path.isfile(filePath)
   mapping_df = pd.read_csv(filePath)

So I have this sample code here, but I keep getting an assertion error. I was wondering how would I properly assert this file to use with the pandas module