Q:

pandas: taking the diff of every second row

I have a dataframe that looks like this
                       price    signal
time                    
2017-10-28 13:54:00   293.18    buy
2017-11-02 17:13:04   285.30    sell
2017-11-02 18:40:04   283.24    buy
2017-11-03 08:40:06   291.23    sell
2017-11-03 12:38:06   294.26    buy
2017-11-05 13:33:06   296.52    sell

I want to create a profit column populated with the difference of the previous buy/sell pair
                       price    signal   profit
time                    
2017-10-28 13:54:00   293.18    buy
2017-11-02 17:13:04   285.30    sell     -7.88
2017-11-02 18:40:04   283.24    buy
2017-11-03 08:40:06   291.23    sell      7.99
2017-11-03 12:38:06   294.26    buy
2017-11-05 13:33:06   296.52    sell      2.29

How might I best go about this?
Alternative or more efficient approaches to calculating profit also welcome

A:

IIUC using cumsum create the group key , then diff 
df.groupby(df.signal.eq('buy').cumsum())['price'].diff()

