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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:\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 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?
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 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]) 
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If you have ever done data analysis or manipulation with Pandas, it is highly likely that you encounter the SettingWithCopy warning at least once. This warning occurs when we try to do an assignment using chained indexing because chained indexing has inherently unpredictable results.
Remove delimiter using split and str The str. split() function will give us a list of strings. The str[0] will allow us to grab the first element of the list. The assignment operator will allow us to update the existing column.
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' If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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