Python：使用函数使用多个Elif语句在数据框中创建新的字符串列

Question

Background: I have a Python dataframe

Goal: I am trying to create a new string column based on values in a series of existing columns. This requires multiple 'elif' statements.

Below is my (sample) code which is erroring out:

def rationale(row):
    if row['Recommendation No. 1'] == 'Category_A':
        result = []
        result.append(row['First_Flag'])
        result.append(row['Second_Flag'])
        result.append(row['Third_Flag'])
        result = ' '.join(result)
        return result
    elif row['Recommendation No. 1'] == 'Category_B':
        result.append(row['Fourth_Flag'])
        result.append(row['Fifth_Flag'])
        result.append(row['Sixth_Flag'])
        result.append(row['Seventh_Flag'])
        result = ' '.join(result)
        return result
    elif row['Recommendation No. 1'] == 'Category_C':
        result.append(row['Eigth_Flag'])
        result.append(row['Ninth_Flag'])
        result.append(row['Tenth_Flag'])
        result.append(row['Eleventh_Flag'])
        result = ' '.join(result)
        return result
    else:
        return np.nan 

df_top3_rationale['Recommendation No. 1 Rationale'] = df_top3_rationale.apply(rationale, axis=1)  

This is the error I get on the 'elif' statements:

UnboundLocalError: ("local variable 'result' referenced before assignment", 'occurred at index 14854')

Any help is greatly appreciated!

Answer 1

The problem is that you define result only inside the first if block, so if the elif statements execute, they do not have access to a variable named result , which is why the error says that the variable result is referenced before assignment (assignment is in this line: result = [] ). What you should do is either put the line result = [] before the conditionals, or include it in every if and elif block.