问如果字符串以特定字符(Pandas)结尾，即将'-‘添加到以'-’结尾的字符串，则如何在字符串中添加前缀

EN

def add_prefix(text):
    # If text is null or empty string the -1 index will result in IndexError
    if text and text[-1] == "-":
        return "-"+text
    return text

df = pd.DataFrame(data={'A':["MAY500", "MAY500-", "", None, np.nan]})
# Change the column to string dtype first
df['A'] = df['A'].astype(str)
df['A'] = df['A'].apply(add_prefix)

0      MAY500
1    -MAY500-
2            
3        None
4         nan
Name: A, dtype: object

#setup
df = pd.DataFrame({'col':['aaaa','bbbb-','cc-','dddddddd-']})

mask = df.col.str.endswith('-'), 'col'
df.loc[mask] = '-' + df.loc[mask]

df
          col
0        aaaa
1      -bbbb-
2        -cc-
3  -dddddddd-

df
  values
0  abcd-
1  a-bcd
2   efg-

df['values'] = np.select([df['values'].str.endswith('-')], ['-' + df['values']], df['values'])

df
   values
0  -abcd-
1   a-bcd
2   -efg-