Pandas: if row in column A contains "x", write "y" to row in column B

Question

For pandas, I'm looking for a way to write conditional values to each row in column B, based on substrings for corresponding rows in column A.

So if cell in A contains "BULL", write "Long" to B. Or if cell in A contains "BEAR", write "Short" to B.

Desired output:

A                  B "BULL APPLE X5"    "Long" "BEAR APPLE X5"    "Short" "BULL APPLE X5"    "Long" 

B is initially empty: df = pd.DataFrame([['BULL APPLE X5',''],['BEAR APPLE X5',''],['BULL APPLE X5','']],columns=['A','B'])

Answer 1

Your code would error as you creating the Dataframe incorrectly, just create a single column A then add B based on A:

import pandas as pd df = pd.DataFrame(["BULL","BEAR","BULL"], columns=['A']) df["B"] = ["Long" if ele  == "BULL" else "Short" for ele in df["A"]]  print(df)      A      B 0  BULL   Long 1  BEAR  Short 2  BULL   Long 

Or do you logic with the data before you create the dataframe:

import pandas as pd data = ["BULL","BEAR","BULL"] data2 = ["Long" if ele  == "BULL" else "Short" for ele in data] df = pd.DataFrame(list(zip(data, data2)), columns=['A','B'])  print(df)       A      B  0  BULL   Long  1  BEAR  Short  2  BULL   Long 

For your edit:

df = pd.DataFrame([['BULL APPLE X5',''],['BEAR APPLE X5',''],['BULL APPLE X5','']], columns=['A','B'])  df["B"] = df["A"].map(lambda x: "Long" if "BULL" in x else "Short" if "BEAR" in x else "")  print(df)              A      B 0  BULL APPLE X5   Long 1  BEAR APPLE X5  Short 2  BULL APPLE X5   Long 

Or just add the column after:

df = pd.DataFrame(['BULL APPLE X5','BEAR APPLE X5','BLL APPLE X5'], columns=['A'])  df["B"] = df["A"].map(lambda x: "Long" if "BULL" in x else "Short" if "BEAR" in x else "")  print(df) 

Or using contains:

df = pd.DataFrame([['BULL APPLE X5',''],['BEAR APPLE X5',''],['BULL APPLE X5','']], columns=['A','B'])   df["B"][df['A'].str.contains("BULL")] = "Long" df["B"][df['A'].str.contains("BEAR")] = "Short"  print(df) 0  BULL APPLE X5   Long 1  BEAR APPLE X5  Short 2  BULL APPLE X5   Long