Why doesn't the second form work?

python string Formatter.py

 # ref https://docs.python.org/2/library/string.html#string-formatting
 from string import Formatter

 class PassThroughFormatter(Formatter):
     pass

 ptf = PassThroughFormatter()

 # works
 s = "I am {name} and I am {age} years old"
 print ptf.format(s, name="roger", age=12)

 #doesn't work
 s = "I am {} and I am {} years old"
 print ptf.format(s, "roger", 12)