Are python 'switches' (dict) supposed to call every func inside?

Question

I was trying to code something with the use of a python switch, simulated in a dict as suggested in: Replacements for switch statement in Python? [Second answer, by Nick]

When I tested, for some reason all the functions inside were being called, tried a simple code to see what was wrong and the problem repeated. See:

def switch(f):
    print('Switch got: ', f)
    var = {
        1: func1(),
        2: func2(),
        3: func3()
        }.get(f,False)
    return var

def func3():
    fb = 'Func3 called'
    print(fb)
    return fb

def func1():
    rsp = 'Func1 called'
    print(rsp)
    return rsp

def func2():
    rsp = 'Func2 called'
    print(rsp)
    return rsp

var = switch(1)
print(var)

My expected return was just:

Switch got:  1
Func1 called

Instead i got:

Switch got:  1
Func1 called
Func2 called
Func3 called
Func1 called

What I got from it is that python seems to run every func inside the dict before calling the proper key (1 in the example).

Is this the expected behavior in Python?

Is there a way around, where only the function that corresponds to the key is called?

change to `def switch(f): print('Switch got: ', f) func = { 1: func1, 2: func2, 3: func3 }.get(f) return func() if func is not None else False` — eyllanesc, Apr 04 '19 at 00:01
[The third answer on that page](https://stackoverflow.com/a/60215/102441) looks like exactly what you need — Eric, Apr 04 '19 at 00:08
@Eric Not sure I could use lambda, because the actual code is more complex than the one posted. But I get your point. Thanks — Justcurious, Apr 04 '19 at 00:15
"for some reason all the functions inside were being called" because *you are explicitly calling them* — juanpa.arrivillaga, Apr 04 '19 at 00:33

score 6 · Accepted Answer · answered Apr 04 '19 at 00:00

6

Is this the expected behavior in Python?

Yes.

Is there a way around, where only the function that corresponds to the key is called?

Don't actually call the functions when you create your dispatch dict:

def switch(k):
    print('Switch got: ', k)
    func = {
        1: func1,
        2: func2,
        3: func3,
    }.get(k, bool)
    var = func()
    return var

answered Apr 04 '19 at 00:00

wim

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I think having it return the result of calling type `bool` as the default is at best obscure. – martineau Apr 04 '19 at 00:08

score 1 · Answer 2 · answered Apr 04 '19 at 00:06

The act of initializing that dictionary when the function 'switch' is called also calls the functions. Try something like this,

def switch(f):
    print('Switch got: ', f)
    var = {
        1: func1,
        2: func2,
        3: func3
        }.get(f,False)
    if not var:
      return false
    return var()

Are python 'switches' (dict) supposed to call every func inside?

2 Answers2

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