How do I average columns in a dataframe based on the last 3 letters of the column name. I'm trying to create a new column that will give me the average of all 'bids' in my dataframe.

My dataframe looks like this:

```
    JPM_bid,   JPM_ask,   Socgen_bid,   Socgen_ask,   UBS_bid,   UBS_ask  ....so forth
0   1.01556    1.01557    1.01552       1.01559       1.01555    1.01558

```

Right now I'm doing calculations like this:

```
df['JPM_spread'] =      df['JPM_ask'].astype(float) - df['JPM_ask'].astype(float)
df['JPM_midpoint'] =    (df['JPM_ask'].astype(float) + df['JPM_ask'].astype(float))/2
df['AVG_bid'] = ""
df['AVG_ask'] = ""
df['JPM_bid_depth'] = ""
...so forth 

```

Thanks for the help!