Fall-through

In C, a case keeps running into the next one unless you write break, and “forgot the break” is a bug old enough to have its own folklore. switchlang turns that around: a case stops after itself unless you ask it to fall through. This page shows how to ask, and what runs when you do.

Opt in per case

Pass fallthrough=True on a case, and when that case matches, the next registered case runs as well, whether or not its own key matches:

from switchlang import switch


def run(value):
    log = []
    with switch(value) as s:
        s.case(1, lambda: log.append('one'))
        s.case(2, lambda: log.append('two'), fallthrough=True)
        s.case(3, lambda: log.append('three'))
        s.case(4, lambda: log.append('four'))
        s.default(lambda: log.append('other'))
    return log


print(run(1), run(2), run(3), run(9))

['one'] ['two', 'three'] ['three'] ['other']

Only the 2 case asked to fall through, so only a value of 2 runs two actions. 3 runs alone; it doesn’t fall into 4.

Chains

A case that was fallen into can itself fall through. The chain continues until it reaches a case without fallthrough=True:

with switch('a') as s:
    s.case('a', lambda: 'a', fallthrough=True)
    s.case('b', lambda: 'b', fallthrough=True)
    s.case('c', lambda: 'c')
    s.case('d', lambda: 'd')
    s.default(lambda: 'default')

print(s.result)

'a' matched and fell through to 'b', which fell through to 'c', which stopped. The 'd' case never ran, and s.result is the return value of the last function that did.

Falling into default

default() is just another registered case, so if the case before it falls through, the default runs too:

with switch('a') as s:
    s.case('a', lambda: 'a', fallthrough=True)
    s.default(lambda: 'default')

print(s.result)

default

This is also a reason to register default() last: it should be the end of any chain, not the middle of one.

With list and range keys

Fall-through on a list or range case applies when any of its items matches. A list or range case that is fallen into runs once, not once per item:

with switch(2) as s:
    s.case([1, 2], lambda: 'one or two', fallthrough=True)
    s.case(range(3, 6), lambda: 'three to five')
    s.default(lambda: 'other')

print(s.result)

three to five

Results under fall-through

Every function in the chain runs, and s.result is the return value of the last one. If you need the intermediate values, collect them yourself, as run() above does with its log list. Results has more.

`fallthrough=None`

The fallthrough parameter accepts True or False. The value None is reserved for switchlang’s own use while it expands list and range keys; don’t pass it.