python - "if", and "elif" chain versus a plain "if" chain -

i wondering, why using elif necessary when this?

if true:     ... if false:     ... ... 

you'd use elif when want ensure one branch picked:

foo = 'bar' spam = 'eggs'  if foo == 'bar':     # elif spam == 'eggs':     # won't this. 

compare with:

foo = 'bar' spam = 'eggs'  if foo == 'bar':     # if spam == 'eggs':     # *and* this. 

with if statements, options not exclusive.

this applies when if branch changes program state such elif test might true too:

foo = 'bar'  if foo == 'bar':     #     foo = 'spam' elif foo == 'spam':     # skipped, if foo == 'spam' true     foo = 'ham' 

here foo set 'spam'.

foo = 'bar'  if foo == 'bar':     #     foo = 'spam' if foo == 'spam':     # executed when foo == 'bar' well,      # previous if statement changed 'spam'.     foo = 'ham' 

now foo set 'spam', 'ham'.

technically speaking, elif part of (compound) if statement; python picks first test in series of if / elif branches tests true, or else branch (if present) if none true. using separate if statement starts new selection, independent of previous if compound statement.