Both both fetch_csv() and self-serve forward fill data. Think of the data as 'the last know value' as of a particular date. This is similar to the price field which is forward filled with the last known close price.

So, what if one doesn't want to forward fill and just want's point in time data from their data file. Well, there isn't a switch to flip, however one can write a small pre-processing file for the fetch_csv method. The fetch_csv method provides for two 'hooks' into the processing. One just after the raw text file has been imported, and another once the file has been sorted and re-indexed. One can use the former to add rows to the fetched data. We'll add a row 1 day after each original row but with a weight value of 0. This will effectively turn off the weight so it's only in effect for one day. Something like this

def add_zeros(fetched_df):  
    """  
    add a data row one day after each datapoint to  
    zero out the weight  
    """  
    # Get a copy of our fetched data  
    zero_data = fetched_df.copy()

    # Create a collumn (ie series) which is the string date + 1 day  
    plus_1_day = (pd.to_datetime(fetched_df.date) + pd.DateOffset(1)).dt.strftime('%Y-%m-%d')

    # Set our date to plus_1_day in our zero_data dataframe  
    zero_data.date = plus_1_day  
    # Set the weight to zero  
    zero_data.weight = 0

    # Concatenate the fetched and the new zero dataframes  
    # Put the fetched data last so it will be last in the dataframe  
    # This allows the fetcher data to override the zero data in case of duplicate days  
    fetched_df = pd.concat([zero_data, fetched_df, ])

    return fetched_df

One could also do something similar instead in the post_func. I chose this because it seemed simpler.

@Viridian Hawk . Try this out and see if your algo behaves as you expected.

Thanks for the help!

So for self-serve data, for datasets where forward-filling is incorrect, I'll need to make sure that my script that generates the CSV checks the previous day's values and inserts them as NULL for the current day?

From the docs:

symbol, start date, stock_score  
AA,     2/13/12,     11.7  
WFM,    2/13/12,     15.8  
FDX,    2/14/12,     12.1  
M,      2/16/12,     14.3  
You can backtest the code below during the dates 2/13/2012 - 2/18/2012. When you use this sample file and algorithm, data.fetcher_assets will return, for each day:

2/13/2012: AA, WFM  
2/14/2012: FDX  
2/15/2012: FDX (forward filled because no new data became available)  
2/16/2012: M  

The docs state that Fetcher is only forward-filled on days when there are no entries, but this example makes it explicitly clear that on other dates (such as 2/16/2012) there is not forward-filling, and only the tickers submitted for that date are included in fetcher_assets for that date.

So I guess the docs are wrong. The behavior actually experienced is as follows:

2/13/2012: AA, WFM  
2/14/2012: AA, WFM, FDX  
2/15/2012: AA, WFM, FDX  
2/16/2012: AA, WFM, FDX, M  

I got a solution from Chris Myles, which I'll share here in case anybody else needs to have their self-serve pipeline fields not forward-filled:

days_since_last_update = BusinessDaysSincePreviousEvent(  
                                 inputs=[dataset.asof_date.latest]  
                                 )  
is_stale = days_since_last_update > days_since_last_update.min() # + extra_days if needed

pipe = Pipeline(  
    columns={  
        'my_dataset': dataset.pct_change.latest,  
        'asof_date' : dataset.asof_date.latest,  
        'days_since_last_update' : days_since_last_update  
    },  
    screen=~is_stale &  
           dataset.pct_change.latest.notnull()  
)