Python Warning:" SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame"

Question

I've created this new dataframe to store this 2 columns:

#Create a new data frame
new_df = df[period-1:]

new_df['Buy'] = get_signal(new_df)[0]
new_df['Sell'] = get_signal(new_df)[1]

But gives me this error:

[Command: python -u C:\Users\Nicolò\Documents\Git\ProgettoTradingBot\ProgettoTradeBot\GUIprova2.py]
C:\Users\Nicol�\Documents\Git\ProgettoTradingBot\ProgettoTradeBot\GUIprova2.py:80: 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: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  new_df['Buy'] = get_signal(new_df)[0]
C:\Users\Nicol�\Documents\Git\ProgettoTradingBot\ProgettoTradeBot\GUIprova2.py:81: 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: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  new_df['Sell'] = get_signal(new_df)[1]
Traceback (most recent call last):
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\cbook\__init__.py", line 196, in process
    func(*args, **kwargs)
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\animation.py", line 951, in _start
    self._init_draw()
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\animation.py", line 1743, in _init_draw
    self._draw_frame(next(self.new_frame_seq()))
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\animation.py", line 1766, in _draw_frame
    self._drawn_artists = self._func(framedata, *self._args)
  File "C:\Users\Nicol�\Documents\Git\ProgettoTradingBot\ProgettoTradeBot\GUIprova2.py", line 98, in animate
    a.fill_between(x_axis,new_df['Upper'],new_df['Lower'], color = 'grey', alpha= 0.5)
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\__init__.py", line 1565, in inner
    return func(ax, *map(sanitize_sequence, args), **kwargs)
  File "C:\Users\Nicol�\AppData\Local\Programs\Python\Python38\lib\site-packages\matplotlib\axes\_axes.py", line 5158, in fill_between
    where = where & ~functools.reduce(np.logical_or,
ValueError: operands could not be broadcast together with shapes (2208,) (2189,) 
[Finished in 79.149s]

Here's the complete file:

https://github.com/Raccispini/ProgettoTradeBot/blob/master/GUIprova2.py

How can i copy the get_signal(new_df) array into new_df['Buy'] and new_df['Sell'] without get this warning?

Thanks:)

Edit: I've done this piece of program in another project before and it works.I don't understand why here doesent' work. I copy pasted exactly as it was before.

Have you tried using `.loc[row_indexer,col_indexer]` as suggested instead, `new_df.loc[:, 'Buy'] = get_signal(new_df)[0]` ? — Marcus, Oct 16 '20 at 10:04
Done but gives me : `ValueError: cannot set a row with mismatched columns` — Nicolò Raccichini, Oct 16 '20 at 10:07

score 3 · Accepted Answer · answered Oct 16 '20 at 10:25

You have created new_df calling:

new_df = df[period-1:]

so new_df is actually a view of the original DataFrame.

Then you attempt to modify it, invoking new_df['Buy'] = ... and new_df['Sell'] = ..., so this warning occurs.

Maybe you should create new_df as a "independent" DataFrame, with its own data buffer:

new_df = df[period-1:].copy()

Then you are free to modify its content.

Python Warning:" SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame"

1 Answers1