switchlang
Adds switch blocks to the Python language.
This module adds explicit switch functionality to Python
without changing the language. It builds upon a standard
way to define execution blocks: the with statement.
Example
from switchlang import switch
def main():
num = 7
val = input("Enter a character, a, b, c or any other: ")
with switch(val) as s:
s.case('a', process_a)
s.case('b', lambda: process_with_data(val, num, 'other values still'))
s.default(process_any)
def process_a():
print("Found A!")
def process_any():
print("Found Default!")
def process_with_data(*value):
print("Found with data: {}".format(value))
main()Installation
Simply install via pip:
pip install switchlangFeatures
- More explicit than using dictionaries with functions as values.
- Verifies the signatures of the methods
- Supports default case
- Checks for duplicate keys / cases
- Keys can be anything hashable (numbers, strings, objects, etc.)
- Could be extended for "fall-through" cases (doesn't yet)
- Use range and list for multiple cases mapped to a single action
Multiple cases, one action
You can map ranges and lists of cases to a single action as follows:
# with lists:
value = 4 # matches even number case
with switch(value) as s:
s.case([1, 3, 5, 7], lambda: ...)
s.case([0, 2, 4, 6, 8], lambda: ...)
s.default(lambda: ...)# with ranges:
value = 4 # matches first case
with switch(value) as s:
s.case(range(1, 6), lambda: ...)
s.case(range(6, 10), lambda: ...)
s.default(lambda: ...)Closed vs. Open ranges
Looking at the above code it's a bit weird that 6 appears
at the end of one case, beginning of the next. But range() is
half open/closed.
To handle the inclusive case, I've added closed_range(start, stop).
For example, closed_range(1,5) -> [1,2,3,4,5]
Why not just raw dict?
The biggest push back on this idea is that we already have this problem solved. You write the following code.
switch = {
1: method_on_one,
2: method_on_two,
3: method_three
}
result = switch.get(value, default_method_to_run)()This works but is very low on the functionality level. We have a better solution here I believe. Let's take this example and see how it looks in python-switch vs raw dicts:
# with python-switch:
while True:
action = get_action(action)
with switch(action) as s:
s.case(['c', 'a'], create_account)
s.case('l', log_into_account)
s.case('r', register_cage)
s.case('u', update_availability)
s.case(['v', 'b'], view_bookings)
s.case('x', exit_app)
s.case('', lambda: None)
s.case(range(1,6), lambda: set_level(action))
s.default(unknown_command)
print('Result is {}'.format(s.result))Now compare that to the espoused pythonic way:
# with raw dicts
while True:
action = get_action(action)
switch = {
'c': create_account,
'a': create_account,
'l': log_into_account,
'r': register_cage,
'u': update_availability,
'v': view_bookings,
'b': view_bookings,
'x': exit_app,
1: lambda: set_level(action),
2: lambda: set_level(action),
3: lambda: set_level(action),
4: lambda: set_level(action),
5: lambda: set_level(action),
'': lambda: None,
}
result = switch.get(action, unknown_command)()
print('Result is {}'.format(result))Personally, I much prefer to read and write the one above. That's why I wrote this module. It seems to convey the intent of switch way more than the dict. But either are options.
Why not just if / elif / else?
The another push back on this idea is that we already have this problem solved. Switch statements are really if / elif / else blocks. So you write the following code.
# with if / elif / else
while True:
action = get_action(action)
if action == 'c' or action == 'a':
result = create_account()
elif action == 'l':
result = log_into_account()
elif action == 'r':
result = register_cage()
elif action == 'a':
result = update_availability()
elif action == 'v' or action == 'b':
result = view_bookings()
elif action == 'x':
result = exit_app()
elif action in {1, 2, 3, 4, 5}:
result = set_level(action)
else:
unknown_command()
print('Result is {}'.format(result))I actually believe this is a little better than the raw dict option. But there are still things that are harder.
- How would you deal with fall-through cleanly?
- Did you notice the bug? We forgot to set result in default case (
else) and will result in a runtime error (but only if that case hits). - There is another bug. Update
update_availabilitywill never run because it's command (a) is bound to two cases. This is guarded against in switch and you would receive a duplicate case error the first time it runs at all. - While it's pretty clear, it's much more verbose and less declarative than the switch version.
Again, compare the if / elif / else to what you have with switch. This code is identical except doesn't have the default case bug.
while True:
action = get_action(action)
with switch(action) as s:
s.case(['c', 'a'], create_account)
s.case('l', log_into_account)
s.case('r', register_cage)
s.case('u', update_availability)
s.case(['v', 'b'], view_bookings)
s.case('x', exit_app)
s.case('', lambda: None)
s.case(range(1,6), lambda: set_level(action))
s.default(unknown_command)
print('Result is {}'.format(s.result))