Problem in creating a new column using for loop - pandas

Question

I have to create a new column 'Action' in a dataframe whose values are :
1 if the next day's Close Price is greater than the present day's
-1 if the next day's Close Price is less than the present day's
that is,
Action[i] = 1 if Close Price[i+1]>Close Price[i]
Action[i] = -1 if Close Price[i+1]
I have used the following code:
dt = pd.read_csv("C:\Subhro\ML_Internship\HDFC_Test.csv", sep=',',header=0)
df = pd.DataFrame(dt)
for i in df.index:
if(df['Close Price'][i+1]>df['Close Price'][i]):
df['Action'][i]=1
elif(df['Close Price'][i+1]<df['Close Price'][i]):
df['Action'][i]=-1
print(df)
But I am getting an error :
KeyError: 'Action'
in line:
df['Action'][i]=1
Please help me out

Answer 1

You are getting the key error because you don't have a column called action. Any of the following before the loop will resolve the error:
df['Action'] = 0
or
df['Action'] = np.nan
However, you will get warnings because of the way you are assigning the cell values. (See here)
It is recommended that you instead use e.g.
df.loc[i, "Action"] = 1
Note that with this method, you won't even need to create an empty "Action" column before the loop.