pep 308: why doing (almost) nothing suffices

[Walter Moreira]

On Sun, Mar 09, 2003 at 09:31:43PM -0700, Steven Taschuk wrote:
> Quoth Edward K. Ream:
>   [...]
> > def choose_eval(cond, a, b): # warning: evals all arguments
> > 
> >     if cond: return eval(a)
> >     else: return eval(b)
> > 
> > Heavy duty stuff by my present bathtub standards.  So to "delay" the
> > evaluation of an argument, just quote it.  Wow--just like lithp.  Example:
> > 
> > choose_eval(a < b,"foo(a)","spam(b)")
> 
> Hm.  What about this little problem?
> 
>     >>> def optional_reverse(s, reverse=0):
>     ...     return choose_eval(reverse, 's.reverse()', 's')
>     ... 
>     >>> optional_reverse('foo', 1)
>     [...]
>     NameError: name 's' is not defined

That problem may be fixed with the following addition to Edward's code:

  import inspect
  def choose_eval(cond, a, b):
      globalns = globals()
      localns = inspect.currentframe(1).f_locals
      if cond: 
          return eval(a, globalns, localns)
      else:
          return eval(b, globalns, localns)

The example works well, then:

  >>> def optional_upper(s, up=0):
  ...     return choose_eval(up, 's.upper()', 's')
  ...
  >>> optional_upper('foo', 1)
  'FOO'
  >>> optional_upper('foo', 0)
  'foo'

But the main point surely is the efficiency if this is used in a loop.
Let's wait for Guido's pronouncement...

Walter

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Walter Moreira   <>  Centro de Matematica  <>  Universidad de la Republica
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