What is the meaning of Volume in Yahoo Finance (web.DataReader(stock, 'yahoo', start, end))? Is it average daily trading volume or average dollar volume? Is it a number of shares or number of dollars?
According to:
https://biz.yahoo.com/charts/guide20.html
Volume on Yahoo Finance's charts are the physical number of shares traded of that stock (not dollar amount) for your given period of time. This is typically what volume is reported as, so it would be safe to assume their endpoint follows the same protocol.