Recently, I saw some discussions online about how there is no good "switch / case" equivalent in Python. I realize that there are several ways to do something similar - some with lambda, some with dictionaries. There have been other StackOverflow discussions about the alternatives. There were even two PEPs (PEP 0275 and PEP 3103) discussing (and rejecting) the integration of switch / case into the language.
I came up with what I think is an elegant way to do switch / case.
It ends up looking like this:
from switch_case import switch, case # note the import style
x = 42
switch(x) # note the switch statement
if case(1): # note the case statement
print(1)
if case(2):
print(2)
if case(): # note the case with no args
print("Some number besides 1 or 2")
So, my questions are: Is this a worthwhile creation? Do you have any suggestions for making it better?
I put the include file on github, along with extensive examples. (I think the entire include file is about 50 executable lines, but I have 1500 lines of examples and documentation.) Did I over-engineer this thing, and waste a bunch of time, or will someone find this worthwhile?
Edit:
Trying to explain why this is different from other approaches:
1) Multiple paths are possible (executing two or more cases),
which is harder in the dictionary method.
2) can do checking for comparisons other than "equals"
(such as case(less_than(1000)).
3) More readable than the dictionary method, and possibly if/elif method
4) can track how many True cases there were.
5) can limit how many True cases are permitted. (i.e. execute the
first 2 True cases of...)
6) allows for a default case.
Here's a more elaborate example:
from switch_case import switch, case, between
x=12
switch(x, limit=1) # only execute the FIRST True case
if case(between(10,100)): # note the "between" case Function
print ("%d has two digits."%x)
if case(*range(0,100,2)): # note that this is an if, not an elif!
print ("%d is even."%x) # doesn't get executed for 2 digit numbers,
# because limit is 1; previous case was True.
if case():
print ("Nothing interesting to say about %d"%x)
# Running this program produces this output:
12 has two digits.
Here's an example attempting to show how switch_case can be more clear and concise than conventional if/else:
# conventional if/elif/else:
if (status_code == 2 or status_code == 4 or (11 <= status_code < 20)
or status_code==32):
[block of code]
elif status_code == 25 or status_code == 45:
[block of code]
if status_code <= 100:
[block can get executed in addition to above blocks]
# switch_case alternative (assumes import already)
switch(status_code)
if case (2, 4, between(11,20), 32): # significantly shorter!
[block of code]
elif case(25, 45):
[block of code]
if case(le(100)):
[block can get executed in addition to above blocks]
The big savings is in long if statements where the same switch is repeated over and over. Not sure how frequent of a use-case that is, but there seems to be certain cases where this makes sense.
The example file on github has even more examples.
from pyswitch import Switch # pyswitch can be found on PyPI
myswitch = Switch()
@myswitch.case(42)
def case42(value):
print "I got 42!"
@myswitch.case(range(10))
def caseRange10(value):
print "I got a number from 0-9, and it was %d!" % value
@myswitch.caseIn('lo')
def caseLo(value):
print "I got a string with 'lo' in it; it was '%s'" % value
@myswitch.caseRegEx(r'\b([Pp]y\w)\b')
def caseReExPy(matchOb):
print r"I got a string that matched the regex '\b[Pp]y\w\b', and the match was '%s'" % matchOb.group(1)
@myswitch.default
def caseDefault(value):
print "Hey, default handler here, with a value of %r." % value
myswitch(5) # prints: I got a number from 0-9, and it was 5!
myswitch('foobar') # prints: Hey, default handler here, with a value of foobar.
myswitch('The word is Python') # prints: I got a string that matched the regex '\b[Pp]y\w\b', and the match was 'Python'
You get the idea. Why? Yep, dispatch tables are the way to go in Python. I just got tired of writing them over and over, so I wrote a class and some decorators to handle it for me.
So, my questions are: Is this a worthwhile creation?
No.
Do you have any suggestions for making it better?
Yes. Don't bother. What has it saved? Seriously? You have actually made the code more obscure by removing the variable x from each elif condition.. Also, by replacing the obvious elif with if you have created intentional confusion for all Python programmers who will now think that the cases are independent.
This creates confusion.
The big savings is in long if statements where the same switch is repeated over and over. Not sure how frequent of a use-case that is, but there seems to be certain cases where this makes sense.
No. It's very rare, very contrived and very hard to read. Seeing the actual variable(s) involved is essential. Eliding the variable name makes things intentionally confusing. Now I have to go find the owning switch() function to interpret the case.
When there are two or more variables, this completely collapses.
There have been a plethora of discussions that address this issue on Stackoverflow. You can use the search function at the top to look for some other discussions.
However, I fail to see how your solution is better than a basic dictionary:
def switch(x):
return {
1 : 1,
2 : 2,
}[x]
Although, adding a default clause is non-trivial with this method. However, your example seems to replicate a complex if/else statement anyway ? Not sure if I would include an external library for this.
IMHO, the main reason for the switch statement to exist is so it can be translated/compiled into a (very fast) jump table. How would your proposed implementation accomplish that goal? Python's dictionaries do it today, as other posters have shown.
Secondarily, I guess a switch statement might read more clearly than the alternatives in some languages, but in python's case I think if/elif/else wins on clarity.
I have always just used dictionaries, if/elses, or lambdas for my switch like statements. Reading through your code tho =)
docs:
来源：https://stackoverflow.com/questions/5440990/is-there-any-value-to-a-switch-case-implementation-in-python