How can I use price earning ratio instead of price in my strategy in OnData part?

So, instead of 


I would like to have something like 

data[self.symbol].Close / data[self.symbol].CurrentEarnings

I know CurrentEarning are not contained in the data object, but how can I get current earnings in my algo?

I saw examples with universes and fine filtering, but I only need need this for one stock.