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From: "Prasad, Ramit" <ramit.prasad@jpmorgan.com>
To: "python-list@python.org" <python-list@python.org>
Subject: RE: lambda in list comprehension acting funny
Thread-Topic: lambda in list comprehension acting funny
Thread-Index: AQHNYSBhMI5tpoL+PkusODrgV6TbgZcngObg
Date: Fri, 13 Jul 2012 18:06:31 +0000
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> >> VERBOSE =3D True=0D=0A> >>=0D=0A> >> def function(arg):=0D=0A> >>     =
if VERBOSE:=0D=0A> >>        print("calling function with arg %r" % arg)=0D=
=0A> >>     process(arg)=0D=0A> >>=0D=0A> >> def caller():=0D=0A> >>     VE=
RBOSE =3D False=0D=0A> >>     function(1)=0D=0A> >>=0D=0A> >> -------------=
--------------------------------=0D=0A> >> Python semantics: function sees =
VERBOSE False=0D=0A> >> Haskell semantics: function sees VERBOSE True=0D=0A=
> =0D=0A> >>>> def caller():=0D=0A> > =2E=2E=2E     VERBOSE =3D False=0D=0A=
> > =2E=2E=2E     function(1)=0D=0A> >>>> def function(arg):=0D=0A> > =2E=
=2E=2E     if VERBOSE:=0D=0A> > =2E=2E=2E        print("calling function wi=
th arg %r" % arg)=0D=0A> > =2E=2E=2E=0D=0A> >>>> VERBOSE =3D True=0D=0A> >>=
>> caller()=0D=0A> > calling function with arg 1=0D=0A> >=0D=0A> > I might =
be being OCD, but caller needs `global VERBOSE` for that to=0D=0A> > work a=
s you explain=2E=0D=0A> =0D=0A> That would be quite different from what Rus=
i is after=2E=0D=0A> =0D=0A> If you add `global VERBOSE` to `caller`, then =
there is only one=0D=0A> variable named `VERBOSE` and what `function` does,=
 depends on=0D=0A> the most recent assignment to that variable=2E=0D=0A> =
=0D=0A> If you remove your `global VERBOSE`, then there are two=0D=0A> vari=
ables by that name, one global and one local to `caller`=2E=0D=0A> In that =
case, there is the question of which one `function`=0D=0A> will use=2E=0D=
=0A>=0D=0A=0D=0ABut that is not what Rusi writes=2E=0D=0A"Python semantics:=
 function sees VERBOSE False" <- function=0D=0Awill not see False without t=
he use of global=2E =0D=0A=0D=0A> The function `function` refers to a varia=
ble `VERBOSE` that=0D=0A> isn't local=2E  In some programming langauages, t=
he interpreter=0D=0A> would then scan the call stack at run-time, looking f=
or a scope=0D=0A> where that name is defined=2E  It would find the local on=
e in=0D=0A> `caller`=2E  This is known as "dynamic binding"=2E=0D=0A> =0D=
=0A> Other interpreters use the `VERBOSE` that was in scope at=0D=0A> the p=
oint in the program text where `function` was defined=2E=0D=0A> In this cas=
e, that would be the global one=2E  This is called=0D=0A> "lexical binding"=
=2E=0D=0A> =0D=0A> Some programming languages allow you to indicate on a pe=
r-=0D=0A> variable basis whether you want dynamic or lexical binding=2E=0D=
=0A> =0D=0A> Python is firmly in the lexical camp=2E  Dynamic binding is no=
t=0D=0A> available in Python, and never will be=2E=0D=0A=0D=0ATrue and a go=
od explanation, but not what I understood=0D=0ARusi to mean=2E=0D=0A=0D=0AR=
amit=0D=0A=0D=0A=0D=0A=0D=0AThis email is confidential and subject to impor=
tant disclaimers and=0D=0Aconditions including on offers for the purchase o=
r sale of=0D=0Asecurities, accuracy and completeness of information, viruse=
s,=0D=0Aconfidentiality, legal privilege, and legal entity disclaimers,=0D=
=0Aavailable at http://www=2Ejpmorgan=2Ecom/pages/disclosures/email=2E