如何处理 Pandas 中的SettingWithCopyWarning

背景

我刚刚将 Pandas 从 0.11 升级到 0.13.0rc1。现在，该应用程序弹出了许多新的警告。其中之一是这样的：

E:\FinReporter\FM_EXT.py:449: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  quote_df['TVol']   = quote_df['TVol']/TVOL_SCALE

我想知道它到底是什么意思？我需要改变什么吗？

如果我坚持使用，该如何暂停警告quote_df['TVol'] = quote_df['TVol']/TVOL_SCALE?

发出警告的函数

def _decode_stock_quote(list_of_150_stk_str):
    """decode the webpage and return dataframe"""

    from cStringIO import StringIO

    str_of_all = "".join(list_of_150_stk_str)

    quote_df = pd.read_csv(StringIO(str_of_all), sep=',', names=list('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefg')) #dtype={'A': object, 'B': object, 'C': np.float64}
    quote_df.rename(columns={'A':'STK', 'B':'TOpen', 'C':'TPCLOSE', 'D':'TPrice', 'E':'THigh', 'F':'TLow', 'I':'TVol', 'J':'TAmt', 'e':'TDate', 'f':'TTime'}, inplace=True)
    quote_df = quote_df.ix[:,[0,3,2,1,4,5,8,9,30,31]]
    quote_df['TClose'] = quote_df['TPrice']
    quote_df['RT']     = 100 * (quote_df['TPrice']/quote_df['TPCLOSE'] - 1)
    quote_df['TVol']   = quote_df['TVol']/TVOL_SCALE
    quote_df['TAmt']   = quote_df['TAmt']/TAMT_SCALE
    quote_df['STK_ID'] = quote_df['STK'].str.slice(13,19)
    quote_df['STK_Name'] = quote_df['STK'].str.slice(21,30)#.decode('gb2312')
    quote_df['TDate']  = quote_df.TDate.map(lambda x: x[0:4]+x[5:7]+x[8:10])
    
    return quote_df

更多警告信息

E:\FinReporter\FM_EXT.py:449: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  quote_df['TVol']   = quote_df['TVol']/TVOL_SCALE
E:\FinReporter\FM_EXT.py:450: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  quote_df['TAmt']   = quote_df['TAmt']/TAMT_SCALE
E:\FinReporter\FM_EXT.py:453: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  quote_df['TDate']  = quote_df.TDate.map(lambda x: x[0:4]+x[5:7]+x[8:10])

The SettingWithCopyWarning创建它是为了标记可能令人困惑的“链式”分配，例如以下内容，它并不总是按预期工作，特别是当第一个选择返回copy. [see GH5390 https://github.com/pydata/pandas/pull/5390 and GH5597 https://github.com/pydata/pandas/issues/5597用于背景讨论。]

df[df['A'] > 2]['B'] = new_val  # new_val not set in df

该警告提供了重写建议，如下所示：

df.loc[df['A'] > 2, 'B'] = new_val

但是，这不适合您的用法，这相当于：

df = df[df['A'] > 2]
df['B'] = new_val

虽然很明显您并不关心写入使其返回到原始帧（因为您正在覆盖对它的引用），但不幸的是，这种模式无法与第一个链式分配示例区分开来。因此出现（误报）警告。误报的可能性在关于索引的文档 https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy，如果您想进一步阅读。您可以通过以下分配安全地禁用此新警告。

import pandas as pd
pd.options.mode.chained_assignment = None  # default='warn'

其他资源

• pandas 用户指南：索引和选择数据 https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html
• Python 数据科学手册：数据索引和选择 https://jakevdp.github.io/PythonDataScienceHandbook/03.02-data-indexing-and-selection.html
• 真正的Python：Pandas 中的SettingWithCopyWarning：视图与副本 https://realpython.com/pandas-settingwithcopywarning/
• Dataquest：SettingwithCopyWarning：如何修复 Pandas 中的此警告 https://www.dataquest.io/blog/settingwithcopywarning/
• 迈向数据科学：解释 pandas 中的 SettingsWithCopyWarning https://towardsdatascience.com/explaining-the-settingwithcopywarning-in-pandas-ebc19d799d25