if/elif chain with assignment expressions

Paul Rubin http
Mon Jul 12 20:55:07 CEST 2004

"Larry Bates" <lbates at swamisoft.com> writes:
> What about:
> e_list=[{'condition': 'f(y) < 5', 'call': fred},
>         {'condition': 'f(y) < 7', 'call': ted},
>         {'condition': 'f(y) < 9', 'call': ned}]
> for e in e_list:
>     if eval(e['condition']):
>         e['call'](y)
>         break
> Its easily extendable and would support an unlimited number of
> function calls and conditions by merely extending e_list definition.
> It also already works with current implementation of Python.

It throws away the value from evaluating the condition.  Also, it uses
eval a lot, which adds a lot of overhead.  You wanted something like:

  e_list = [{'expr': lambda: f(x), 'condition': lambda y: y<5, 
               'call': lambda y: fred(y)},
            {'expr': lambda: g(x), 'condition': lambda y: y<7, 
               'call': lambda y: ted(y)},
            {'expr': lambda: h(x), 'condition': lambda y: y<9, 
               'call': lambda y: ned(y)}]
  for e in e_list:
      y = e.expr()
      if e.cond(y):

which is an unbelievably contorted way of replacing a straightforward
if/elif chain.  And of course that wants an assignment expression too:

  for e in e_list:
      if e.cond(y := e.expr()):

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