lambda in list comprehension acting funny
ramit.prasad at jpmorgan.com
Fri Jul 13 20:06:31 CEST 2012
> >> VERBOSE = True
> >> def function(arg):
> >> if VERBOSE:
> >> print("calling function with arg %r" % arg)
> >> process(arg)
> >> def caller():
> >> VERBOSE = False
> >> function(1)
> >> ---------------------------------------------
> >> Python semantics: function sees VERBOSE False
> >> Haskell semantics: function sees VERBOSE True
> >>>> def caller():
> > ... VERBOSE = False
> > ... function(1)
> >>>> def function(arg):
> > ... if VERBOSE:
> > ... print("calling function with arg %r" % arg)
> > ...
> >>>> VERBOSE = True
> >>>> caller()
> > calling function with arg 1
> > I might be being OCD, but caller needs `global VERBOSE` for that to
> > work as you explain.
> That would be quite different from what Rusi is after.
> If you add `global VERBOSE` to `caller`, then there is only one
> variable named `VERBOSE` and what `function` does, depends on
> the most recent assignment to that variable.
> If you remove your `global VERBOSE`, then there are two
> variables by that name, one global and one local to `caller`.
> In that case, there is the question of which one `function`
> will use.
But that is not what Rusi writes.
"Python semantics: function sees VERBOSE False" <- function
will not see False without the use of global.
> The function `function` refers to a variable `VERBOSE` that
> isn't local. In some programming langauages, the interpreter
> would then scan the call stack at run-time, looking for a scope
> where that name is defined. It would find the local one in
> `caller`. This is known as "dynamic binding".
> Other interpreters use the `VERBOSE` that was in scope at
> the point in the program text where `function` was defined.
> In this case, that would be the global one. This is called
> "lexical binding".
> Some programming languages allow you to indicate on a per-
> variable basis whether you want dynamic or lexical binding.
> Python is firmly in the lexical camp. Dynamic binding is not
> available in Python, and never will be.
True and a good explanation, but not what I understood
Rusi to mean.
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