Beginners' Question -- Assigning variables in a lambda

[Michael Hudson]

digitig at cix.co.uk (Tim Rowe) writes:

> Are beginners' questions ok here? (if not, where?)

Yes.  You might want to try python-help or tutor at python.org, though.

> After I've done my getopt (or maybe instead, if I can get this to work) I 
> want to avoid the if/elif/else from hell by having a dictionary of 
> parameter names and functions, something like:
> 
>    handler = {
>       # Doesn't work!
>       "b": lambda x: eval("bandwidth = get_number(" + x + ")")
>       # , ... more here ...
>       }
>       
> (get_number is essentially string.atof with some special handling)
> then later on do, for example:
>    handler["b"]("0.23")
> 
> and I'd like to call eval("bandwidth = get_number(0.23)") (it seems to get 
> that far) and so assign 0.23 to bandwidth (it won't do that bit!)
> 
> I suspect my problem is that I need to exec("bandwidth = get_number(" + x 
> + ")"), but lambda won't take exec.
> 
> Is there a sensible way to do what I'm tying to do?

Not directly, but there are almost certainly sensible ways of
acheiving your goal.

One attempt might look like this:

option2attr = {
    "b":"bandwidth",
    ...
}

class Settings:
    def __init__(self):
        self.bandwith = 9600
        ...
    def handle_option(self,opt,val)
        setattr(self,option2attr[opt],eval(val,{"__builtins__":{}}))

settings = Settings()

for k,v in get_options():
    settings.handle_option(k,v)

Or similar, but a bit fancier:

class Settings:
    option2meth = {
        "b":"set_bandwidth",
        ...
    }

    def __init__(self):
        self.bandwith = 9600
        ...
    def handle_option(self,opt,val)
        getattr(self,self.option2meth[opt])(eval(val,{"__builtins__":{}}))
    def set_bandwidth(self,val):
        assert 300 < val < 100000 # hmm, you shouldn't assert on 
                                  # user data, but I'm too lazy to
                                  # change it now...
        self.bandwidth = val
    ...

settings = Settings()

for k,v in get_options():
    settings.handle_option(k,v)

HTH,
Michael

-- 
  There are 'infinite' number of developed artifacts and one cannot
  develop appreciation  for them all.  It would be all right to not
  understand something, but it would be imbecilic to put judgements
  on things one don't understand.               -- Xah, comp.lang.lisp