title: KeyError when using boolean filter on pandas data frame
body: <p>Trying to combine two data frames when a datetime object from one dataframe is within a datetime object range in the other.</p>
  
  <p>Keep getting: KeyError: 'cannot use a single bool to index into setitem' on this line of code in the second chunk I posted.</p>
  
  <pre><code>gametaxidf.loc[arrivemask, 'relevant'] = 1
  </code></pre>
  
  <p>I'm assuming it would happen on the following line with a similar command as well.</p>
  
  <p>This is the part giving me trouble:</p>
  
  <pre><code>with open('/Users/benjaminprice/Desktop/TaxiCombined/Data/combinedtaxifiltered.csv', 'w') as csvfile: 
      fieldnames1 = ['index','pickup_datetime', 'dropoff_datetime', 'pickup_long', 'pickup_lat','dropoff_long','dropoff_lat','passenger_count','trip_distance','fare_amount','tip_amount','total_amount','stadium_code'] 
      writer = csv.DictWriter(csvfile, fieldnames=fieldnames1) 
      writer.writeheader()
  
  for index, row in baseballdf.iterrows(): 
      gametimestart = row['Start.Time'] 
      gametimeend = row['End.Time'] 
      arrivemin = gametimestart - datetime.timedelta(minutes=120) 
      arrivemax = gametimeend - datetime.timedelta(minutes = 30) 
      departmin = gametimeend - datetime.timedelta(minutes = 60) 
      departmax = gametimeend + datetime.timedelta(minutes = 90)
  
      gametaxidf = combineddf[combineddf.DATE==row.DATE]
      gametaxidf['relevant']=0
  
      for index, row in gametaxidf.iterrows():
          arrivemask = (arrivemin &lt; row['dropoff_datetime']) and (row['dropoff_datetime'] &lt; arrivemax)
          departmask = (departmin &lt; row['pickup_datetime']) and (row['pickup_datetime'] &lt; departmax) 
          gametaxidf.loc[arrivemask, 'relevant'] = 1
          gametaxidf.loc[departmask, 'relevant'] = 1
  
          with open('/Users/benjaminprice/Desktop/TaxiCombined/Data/combinedtaxifiltered.csv','a') as combinedtaxi:
              gametaxidf.to_csv(combinedtaxi,header=None)
      print(str(index) + "done")
  </code></pre>
  
  <p>Gametaxidf.head(5):</p>
  
  <pre><code>   index     pickup_datetime    dropoff_datetime  pickup_long  pickup_lat  \
  0    195 2014-04-01 00:08:13 2014-04-01 00:15:32   -73.922218   40.827557   
  1    344 2014-04-01 00:16:30 2014-04-01 00:20:38   -73.846046   40.754566   
  2    558 2014-04-01 00:28:59 2014-04-01 00:36:36   -73.921692   40.831394   
  3    744 2014-04-01 00:42:00 2014-04-01 00:49:46   -73.938080   40.804646   
  4    776 2014-04-01 00:43:54 2014-04-01 00:53:22   -73.952652   40.810577   
  
     dropoff_long  dropoff_lat  passenger_count  trip_distance  fare_amount  \
  0    -73.900620    40.856174                1           2.30          9.0   
  1    -73.890259    40.753246                1           0.56          4.5   
  2    -73.942719    40.823257                1           1.53          7.0   
  3    -73.928490    40.830433                1           2.96         11.0   
  4    -73.924332    40.827320                1           2.28         10.5   
  
     tip_amount  total_amount  stadium_code       DATE  relevant  
  0           0          10.0           1.1 2014-04-01         0  
  1           0           5.5           2.1 2014-04-01         0  
  2           0           8.0           1.1 2014-04-01         0  
  3           0          12.0           1.0 2014-04-01         0  
  4           0          11.5           1.0 2014-04-01         0 
  </code></pre>
  
  <p>Also getting this warning: A value is trying to be set on a copy of a slice from a DataFrame.</p>
  
  <pre><code>Try using .loc[row_indexer,col_indexer] = value instead
  </code></pre>
  
  <p>But it's letting me continue through that... any help would be great.</p>