python - String Formatting Confusion -

o'reilly's learn python powerful object oriented programming mark lutz teaches different ways format strings.

this following code has me confused. interpreting 'ham' filling format place marker @ index zero, , yet still pops @ index 1 of outputted string. please me understand going on.

here code:

template = '{motto}, {0} , {food}' template.format('ham', motto='spam', food='eggs') 

and here output:

'spam, ham , eggs' 

i expected:

'ham, spam , eggs' 

the thing have understand {0} refers first (zeroeth) unnamed argument sent format(). can see case removing unnamed references , trying use linear fill-in:

>>> "{motto}".format("boom") traceback (most recent call last):   file "<stdin>", line 1, in <module> keyerror: 'motto' 

you expect 'boom' fill in 'motto' if how works. but, instead, format() looks parameter named 'motto'. key hint here keyerror. similarly, if taking sequence of parameters passed format(), wouldn't error, either:

>>> "{0} {1}".format('ham', motto='eggs') traceback (most recent call last):   file "<stdin>", line 1, in <module> indexerror: tuple index out of range 

here, format() looking second unnamed argument in parameter list - doesn't exist gets 'tuple index out of range' error. difference between unnamed (which positionally sensitive) , named arguments passed in python.

see post understand difference between these types arguments, known 'args' , 'kwargs'.