Most elegant way for a switch case as in C

Question

In order to parse some args via cmd line, I've a "switch case" written with if .. elif

if arg == 1:
    ..something...
elif arg >= 2 and arg < 4:
    ..something else ...
elif arg < 6:
    ..something else ...
else:
    ..something else ...

above, elif arg < 6: could be replaced by: elif arg == 4 or arg == 5:

this get's a bit messy when there are more argument values to check.

What could be the most efficient and easy way to read the code?

@jedwards The answers in the dup target just talks about checking if the values are equal. But that is not the case here, right? — thefourtheye, Mar 13 '15 at 01:40
@thefourtheye The question talks about equal values, but there are good references in the answers that handle other situations. — jedwards, Mar 13 '15 at 01:42
@jedwards May be I am not able to find the most appropriate one. Can you please share the answer which can work for this case? — thefourtheye, Mar 13 '15 at 01:43
A switch statement certainly doesn't let you use arbitrary expressions like `arg >= 2`, so I don't understand how switch statements would even apply to your example code. — roippi, Mar 13 '15 at 01:47
@thefourtheye, I missed the random `n<6`, which I assumed dealt with `arg` -- I'll nom for re-opening. — jedwards, Mar 13 '15 at 01:50
in the example `elif arg >= 2 and arg < 4:` just covers 2 conditions [2,3] that would be easily covered in a switch `case 2: case: 3: ...` — Sandt50, Mar 13 '15 at 01:52
@Sandt50 Should `n<6` be `arg<6`? How would you rewrite that as a `switch` -- are you switching on arg or n? — Two-Bit Alchemist, Mar 13 '15 at 01:53
@Sandt50 FWIW `arg >= 2 and arg < 4` could be perhaps more cleanly written `2 <= arg < 4`. You could do some hacky stuff here with lambdas, which can be used as dict keys, and even build up an object that sort of behaves like a switch if you loop over it (one of the ActiveState recipes in the now removed dup target does just this), but at the end of the day, `if..elif..else` is going to be more readable. Unless there is some mathematical relationship you can exploit, you'll have the same number of cases, and even then the simplification could apply to both. — Two-Bit Alchemist, Mar 13 '15 at 02:06
as it stands, `-1` would pass over the `if` and first `elif`, but would satisfy the `elif arg < 6` condition. is this intended? — the_constant, Mar 13 '15 at 03:30

score 2 · Answer 1 · answered Mar 13 '15 at 02:05

You could try something like this:

def case_A():
    .. something ..

def case_B():
    .. something ..

def case_C():
    .. something ..

def case_else():
    .. something ..

{
    1: case_A,
    2: case_B,
    3: case_B,
    4: case_C,
    5: case_C
}.get(arg, case_else)()

score 0 · Answer 2 · answered Mar 13 '15 at 02:25

If your goal really is to parse a command line, use optparse if in python 2.6 or lower, argparse for later python versions. There is also a really nice little package called aaargh that makes for some really concise code. You will need to use decorators with that, so probably save that for a later project.

Use optparse or argparse, you can use things like -d somedir or --dir=somedir. It allows you to handle both optional parameters in any order, as well as required ones.

It also will generate a help message, if the command line does not meet your requirements.

Most elegant way for a switch case as in C

2 Answers2