Python/Pandas - Cannot append all rows into a dataframe

I have this code

def expander(dataframe, n):
  d = dict(tuple(input_dataframe.groupby(input_dataframe.columns[0])))
  u = input_dataframe['Type'].unique()
  cols = ['Type', 'Variants']
  df2 = pd.DataFrame(columns=cols, index=range(n))

  for i in range(len(u)):
    df2 = d[u[i]].copy()
    df2["Variants"] = np.nan
    df2
  df3 = input_dataframe.append(df2, ignore_index=True)
  print(df3) 

When I run the code with :

expander(input_dataframe=pd.DataFrame({
    'Type':['iPhone', 'iPhone', 'iPhone', 'iPhone', 'Samsung', 'Samsung', 'Samsung', 'Samsung'],
    'Variants':['12 Mini', '12', '12 Pro', '12 Pro Max', 'S21 FE', 'S21', 'S21+', 'S21 Ultra']
}), n = 4)

I was expecting the output to be like this :

But instead, what I got was like this :

How can I solve this?