Skip to content
Fix Code Error

How to deal with SettingWithCopyWarning in Pandas

March 13, 2021 by Code Error
Posted By: Anonymous

Background

I just upgraded my Pandas from 0.11 to 0.13.0rc1. Now, the application is popping out many new warnings. One of them like this:

E:FinReporterFM_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

I want to know what exactly it means? Do I need to change something?

How should I suspend the warning if I insist to use quote_df['TVol'] = quote_df['TVol']/TVOL_SCALE?

The function that gives errors

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

More error messages

E:FinReporterFM_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:FinReporterFM_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:FinReporterFM_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])

Solution

The SettingWithCopyWarning was created to flag potentially confusing "chained" assignments, such as the following, which does not always work as expected, particularly when the first selection returns a copy. [see GH5390 and GH5597 for background discussion.]

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

The warning offers a suggestion to rewrite as follows:

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

However, this doesn’t fit your usage, which is equivalent to:

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

While it’s clear that you don’t care about writes making it back to the original frame (since you are overwriting the reference to it), unfortunately this pattern cannot be differentiated from the first chained assignment example. Hence the (false positive) warning. The potential for false positives is addressed in the docs on indexing, if you’d like to read further. You can safely disable this new warning with the following assignment.

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

Other Resources

  • pandas User Guide: Indexing and selecting data
  • Python Data Science Handbook: Data Indexing and Selection
  • Real Python: SettingWithCopyWarning in Pandas: Views vs Copies
  • Dataquest: SettingwithCopyWarning: How to Fix This Warning in Pandas
  • Towards Data Science: Explaining the SettingWithCopyWarning in pandas
Answered By: Anonymous

Related Articles

  • Proper way to assigning new columns to a dataframe
  • Change column type in pandas
  • Vue.Draggable Not Moving Component
  • useEffect Error: Minified React error #321 (GTM…
  • How to take column-slices of dataframe in pandas
  • How are iloc and loc different?
  • Pandas - Get first row value of a given column
  • How do I catch a numpy warning like it's an exception (not…
  • Execute Python script via crontab
  • What is the list of valid @SuppressWarnings warning names in…

Disclaimer: This content is shared under creative common license cc-by-sa 3.0. It is generated from StackExchange Website Network.

Post navigation

Previous Post:

Replace a character at a specific index in a string?

Next Post:

“SyntaxError: Unexpected token < in JSON at position 0"

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

  • Get code errors & solutions at akashmittal.com
© 2022 Fix Code Error