Python Pandas Plot Warning

Question

When I use pandas plot data in spyder. It will always show a warning message:

C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\indexing.py:494: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_indexer,col_indexer] = value instead. See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy self.obj[item] = s

My scripts are below:

import pandas as pd

loc0 = r'D:\DC_BP00010.T0058_20190804_082758_15_14_PM2_1031.txt'
loc1 = r'D:\DC_BP00010.T0058_20190804_082758_17_16_PM1_1193.txt'

locs = [loc0,loc1]  # Update files in the list

parameters_must_be = ['Time', 'StepNo (Int)'] # Do not change this one
parameters = ["HeadBEPTrend1 (Float)"] # Key in parameters needed
parameters_all = parameters_must_be.append(parameters) 

endpoint_steps = [3] # Update endpoint steps in the list

Titles = 'BP00010' # Titles for plot

Colors = ['black','red','brown','crimson','olive','blue','yellow','darkorange','lime','purple','deeppink','dodgerblue','orange','indigo','darkslateblue','lawngreen','darkslategray','darkgreen','midnightblue','lightseagreen','gold','maroon','navy','teal']

dfs = []
sa = []
ss = []
axs = []
n = len(locs) # Number of datalog files

for i in range(0,n):
    dfs.append(pd.read_csv(locs[i], sep=('\t'), skiprows=6, usecols=parameters_all))
    ss.append(dfs[i][dfs[i]['StepNo (Int)'].isin(endpoint_steps)])
    sa.append(ss[i].loc[:,'Time']-ss[i].loc[:,'Time'].iloc[0])
    ss[i].loc[:,'Time'] = sa[i].loc[:]

ax=ss[0].plot(x='Time', y=parameters, title=Titles, color=Colors[0:len(parameters)], linewidth=1)

if n>1:
    for i in range(1,n):
        axs.append(ss[i].plot(x='Time', y=parameters, color=Colors[len(parameters)*i:len(parameters)*i+len(parameters)], linewidth=1, ax=ax))

ax.set_xlim(0,80)
ax.set_ylim(0,60)
ax.set_ylabel('Endpoint Intensity')
ax.legend(['BP00010#14','BP00010#15','BP00010#16','BP00010#17'], bbox_to_anchor=(1.31, .5), loc=5, borderaxespad=0.)

I guess, this line causes warning dfs[i][dfs[i]['StepNo (Int)'].isin(endpoint_steps)]. Your syntax seems to be weird on this line. What you want to achieve on this line? — vb_rises
– vb_rises, Commented Oct 17, 2019 at 13:01

Trenton McKinney · Accepted Answer · 2019-10-17 19:41:40Z

1

`ss[i].loc[:,'Time'] = sa[i].loc[:]` is causing the error:

You're trying to change a value in another dataframe.
ss is a list of dataframes
ss[i] is a specific dataframe that comes from dfs

`ss` and `sa` are lists of parts of dataframes created from the following:

ss.append(dfs[i][dfs[i]['StepNo (Int)'].isin(endpoint_steps)])
- ss comes from dfs
sa.append(ss[i].loc[:,'Time']-ss[i].loc[:,'Time'].iloc[0])
- sa comes from ss
Creating a dataframe from other dataframes is fine, but doing so makes a reference to the original, not a copy.

Add `.copy()` when creating `ss` and `sa`:

ss.append(dfs[i][dfs[i]['StepNo (Int)'].isin(endpoint_steps)].copy())
sa.append(ss[i].loc[:,'Time']-ss[i].loc[:,'Time'].iloc[0].copy())

edited Oct 17, 2019 at 19:41

answered Oct 17, 2019 at 19:33

Trenton McKinney

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Python Pandas Plot Warning

1 Answer 1

`ss[i].loc[:,'Time'] = sa[i].loc[:]` is causing the error:

`ss` and `sa` are lists of parts of dataframes created from the following:

Add `.copy()` when creating `ss` and `sa`:

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1 Answer 1

ss[i].loc[:,'Time'] = sa[i].loc[:] is causing the error:

ss and sa are lists of parts of dataframes created from the following:

Add .copy() when creating ss and sa:

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`ss[i].loc[:,'Time'] = sa[i].loc[:]` is causing the error:

`ss` and `sa` are lists of parts of dataframes created from the following:

Add `.copy()` when creating `ss` and `sa`: