how can I combine a switch-case and regex in Python

Question

I want to process a string by matching it with a sequence of regular expression. As I'm trying to avoid nested if-then, I'm thinking of switch-case. How can I write the following structure in Python? Thank you

switch str:
   case match(regex1):
       # do something
   case match(regex2):
       # do sth else

I know Perl allows one to do that. Does Python?

Answer 1

First consider why there is no case statement in Python. So reset you brain and forget them.

You can use an object class, function decorators or use function dictionaries to achieve the same or better results.

Here is a quick trivial example:

#!/usr/bin/env python
import re

def hat(found):
    if found: print "found a hat"
    else: print "no hat"

def cat(found):
    if found: print "found a cat"
    else: print "no cat"

def dog(found):    
    if found: print "found a dog"
    else: print "no dog"

st="""
Here is the target string
with a hat and a cat
no d o g 
end
"""

patterns=['hat', 'cat', 'dog']
functions=[hat,cat,dog]

for pattern,case in zip(patterns,functions):
    print "pattern=",pattern
    case(re.search(pattern,st))

C style case / switch statements also "fall through, such as:

   switch(c) {
       case 'a':
       case 'b':
       case 'c':  do_abc();
                  break;
       ... other cases...
   }

Using tuples and lists of callables, you can get the similar behavior:

st="rat kitten snake puppy bug child"

def proc1(st): print "cuddle the %s" % st
def proc2(st): print "kill the %s" % st
def proc3(st): print "pick-up the %s" % st
def proc4(st): print "wear the %s" % st
def proc5(st): print "dispose of the %s" %st
def default(st): print "%s not found" % st

dproc={ ('puppy','kitten','child'): 
            [proc3, proc1], 
        ('hat','gloves'): 
            [proc3, proc4], 
        ('rat','snake','bug'): 
            [proc2, proc3, proc5]}

for patterns,cases in dproc.iteritems():
    for pattern in patterns:
        if re.search(pattern,st):
            for case in cases: case(pattern)
        else: default(pattern)    
        print

This gets the order for the found item correct: 1) pick up child, cuddle the child; 2) kill the rat, pick up the rat... It would be difficult to do the same with a C switch statement in an understandable syntax.

There are many other ways to imitate a C switch statement. Here is one (for integers) using function decorators:

case = {}

def switch_on(*values):
    def case_func(f):
        case.update((v, f) for v in values)
        return f
    return case_func

@switch_on(0, 3, 5)
def case_a(): print "case A"

@switch_on(1,2,4)
def case_b(): print "case B"

def default(): print "default"

for i in (0,2,3,5,22):
    print "Case: %i" % i
    try: 
        case[i]()
    except KeyError:
        default()

To paraphrase Larry Wall, Tom Christiansen, Jon Orwant in Programming Perl regarding understanding context in Perl:

You will be miserable programming Python until you use the idioms that are native to the language...