如何处理SettingWithCopyWarning在熊猫

背景

我刚刚把我的熊猫从0.11升级到0.13.0rc1。现在，应用程序弹出了许多新的警告。其中一个是这样的:

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

我想知道这到底是什么意思?我需要改变什么吗?

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

给出错误的函数

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

更多错误消息

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])

当前回答

创建SettingWithCopyWarning是为了标记可能令人困惑的“链式”赋值，例如下面的赋值，它并不总是像预期的那样工作，特别是当第一个选择返回一个副本时。[背景讨论见GH5390和GH5597。]

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:

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

然而，这并不符合你的用法，这相当于:

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

虽然很明显，您并不关心写入操作是否使其返回到原始帧(因为您正在覆盖对它的引用)，但不幸的是，此模式无法与第一个链式赋值示例区分开来。因此出现了(假阳性)警告。如果您想进一步阅读，在索引文档中讨论了假阳性的可能性。您可以使用以下赋值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 Python数据科学手册:数据索引和选择真正的Python: SettingWithCopyWarning在Pandas:视图vs副本 Dataquest: SettingwithCopyWarning:如何修复此警告在熊猫走向数据科学:解释熊猫中copywarning的设置

2013-12-17 06:20:23

其他回答

当我执行这部分代码时，我也遇到了同样的警告:

def scaler(self, numericals):
    scaler = MinMaxScaler()
    self.data.loc[:, numericals[0]] = scaler.fit_transform(self.data.loc[:, numericals[0]])
    self.data.loc[:, numericals[1]] = scaler.fit_transform(self.data.loc[:, numericals[1]])

其中标量是一个MinMaxScaler和数字[0]包含三个我的数字列的名字。

当我将代码更改为:

def scaler(self, numericals):
    scaler = MinMaxScaler()
    self.data.loc[:][numericals[0]] = scaler.fit_transform(self.data.loc[:][numericals[0]])
    self.data.loc[:][numericals[1]] = scaler.fit_transform(self.data.loc[:][numericals[1]])

因此，只需将[:，~]改为[:][~]。

2021-07-13 12:19:16

对我来说奏效了:

import pandas as pd
# ...
pd.set_option('mode.chained_assignment', None)

2021-10-16 11:43:25

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:

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

然而，这并不符合你的用法，这相当于:

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

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

其他资源

2013-12-17 06:20:23

这应该可以工作:

quote_df.loc[:,'TVol'] = quote_df['TVol']/TVOL_SCALE

2018-03-09 09:48:49

有些人可能只是想压制这个警告:

class SupressSettingWithCopyWarning:
    def __enter__(self):
        pd.options.mode.chained_assignment = None

    def __exit__(self, *args):
        pd.options.mode.chained_assignment = 'warn'

with SupressSettingWithCopyWarning():
    #code that produces warning

2019-05-17 09:47:34

如何处理SettingWithCopyWarning在熊猫

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