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遇到的问题：关于df_a_series[-1]为什么要是时间序列索引才能访问,而非时间序列索引,直接报错：
File "pandas/_libs/index.pyx", line 80, in pandas._libs.index.IndexEngine.get_value
File "pandas/_libs/index.pyx", line 88, in pandas._libs.index.IndexEngine.get_value
File "pandas/_libs/index.pyx", line 131, in pandas._libs.index.IndexEngine.get_loc
File "pandas/_libs/hashtable_class_helper.pxi", line 992, in pandas._libs.hashtable.Int64HashTable.get_item
File "pandas/_libs/hashtable_class_helper.pxi", line 998, in pandas._libs.hashtable.Int64HashTable.get_item
KeyError: -1
测试案例【万能用法：df_a_series.iloc[-1]】：
import pandas as pd
se = pd.Series([1, 2, 3, 4])
index = pd.date_range("2019-09-01", "2019-09-02")
df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=["a", "b", "c"])
# print(df)
# 关于Series实例的倒序索引玩法总结
df_a_series = df["a"]
print(df_a_series[-1]) # 直接报错【这种用法经常碰到,测试发现,如果index为时间索引,则可以,否则失败】
print(df_a_series.iloc[-1]) # 这个肯定是成功的【这个是万能的玩法】
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