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

Question

I was wondering, why is using elif necessary when you could just do this?

if True:
    ...
if False:
    ...
...

Answer 1

You'd use elif when you want to ensure that only one branch is picked:

foo = 'bar'
spam = 'eggs'

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

Compare this with:

foo = 'bar'
spam = 'eggs'

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

With just if statements, the options are not exclusive.

This also applies when the if branch changes the program state such that the elif test might be true too:

foo = 'bar'

if foo == 'bar':
    # do this
    foo = 'spam'
elif foo == 'spam':
    # this is skipped, even if foo == 'spam' is now true
    foo = 'ham'

Here foo will be set to 'spam'.

foo = 'bar'

if foo == 'bar':
    # do this
    foo = 'spam'
if foo == 'spam':
    # this is executed when foo == 'bar' as well, as 
    # the previous if statement changed it to 'spam'.
    foo = 'ham'

Now foo is set to 'spam', then to 'ham'.

Technically speaking, elif is part of the (compound) if statement; Python picks the first test in a series of if / elif branches that tests as true, or the else branch (if present) if none are true. Using a separate if statement starts a new selection, independent of the previous if compound statement.