pandas Series KeyError: -1

前几天写分析方法，遇到的一个错误：

具体我已经在stackoverflow 里面得到了详细的解答，下面我把问题和解决办法总结一下，方便日后的回顾

问题：

sql = "select {}, {} from {} where {};".format(v1, v2, table, where)
        df = pd.read_sql(sql, app.config.get(‘sqlalchemy_engine‘))
        df_dropna = df.dropna()
        dddd = df_dropna[v2]
        print type(dddd)
        print dddd[1]
        # print dddd # Have the answer
        print dddd[-1] # keyerror: -1

错误：

File "/usr/lib/python2.7/dist-packages/flask/app.py", line 2000, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1991, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1567, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python2.7/dist-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/opt/code/my_code/app4wp/app/views.py", line 136, in varianc_get
    print dddd[-1]
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/series.py", line 601, in __getitem__
    result = self.index.get_value(self, key)
  File "/usr/local/lib/python2.7/dist-packages/pandas/indexes/base.py", line 2169, in get_value
    tz=getattr(series.dtype, ‘tz‘, None))
  File "pandas/index.pyx", line 105, in pandas.index.IndexEngine.get_value (pandas/index.c:3567)

  File "pandas/index.pyx", line 113, in pandas.index.IndexEngine.get_value (pandas/index.c:3250)

  File "pandas/index.pyx", line 161, in pandas.index.IndexEngine.get_loc (pandas/index.c:4289)

  File "pandas/src/hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8555)

  File "pandas/src/hashtable_class_helper.pxi", line 410, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8499)

KeyError: -1

为什么呢？

When you use dddd[-1] it is looking for a column with the -1 key, that does not exist. So, to do index slicing you must use .iloc. Pandas dataframes work differently from regular python lists,d ictionaries and etc....

哈哈，就这么解决了！！！

print dddd.iloc[-1]