Add parentheses to column with NaN values

Question

I have the following dataframe:

                 parameter  parameter  parameter
frc_var          -0.01       0.01       0.01
beta               NaN      -0.01      -0.00
Illiquidity        NaN      18.31      17.56

I want it to look like this:

             parameter  parameter  parameter
frc_var         (-0.01)    (0.01)     (0.01)
beta               NaN    (-0.01)    (-0.00)
Illiquidity        NaN    (18.31)    (17.56)

Answer 1

If need add strings () values use DataFrame.mask or DataFrame.where eith test missing or not missing values:

df = ('(' + df.astype(str) + ')').mask(df.isna())
print (df)
            parameter parameter.1 parameter.2
frc_var       (-0.01)      (0.01)      (0.01)
beta              NaN     (-0.01)      (-0.0)
Illiquidity       NaN     (18.31)     (17.56)

Or:

df = df.mask(df.notna(), ('(' + df.astype(str) + ')'))

For tuples is ouput different, solution is:

df = df.applymap(lambda x: (x, ) if pd.notna(x) else x)
print (df)
            parameter parameter.1 parameter.2
frc_var      (-0.01,)     (0.01,)     (0.01,)
beta              NaN    (-0.01,)     (-0.0,)
Illiquidity       NaN    (18.31,)    (17.56,)