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From: "Prasad, Ramit" <ramit.prasad@jpmorgan.com>
To: "python-list@python.org" <python-list@python.org>
Subject: RE: len() on mutables vs. immutables
Thread-Topic: len() on mutables vs. immutables
Thread-Index: AQHNrVVpw35MazyeyU+GZFCxCjgf6pe/pWYA///KIsA=
Date: Thu, 18 Oct 2012 19:18:44 +0000
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Xref: csiph.com comp.lang.python:31680

Terry Reedy wrote:=0D=0A> On 10/18/2012 1:23 PM, Demian Brecht wrote:=0D=0A=
> =0D=0A> > When len() is called passing an immutable built-in type (such a=
s a=0D=0A> > string), I'd assume that the overhead in doing so is simply a =
function=0D=0A> > call and there are no on-call calculations done=2E Is tha=
t correct?=0D=0A> =0D=0A> See below=2E=0D=0A> =0D=0A> > I'd also assume tha=
t mutable built-in types (such as a bytearray) would=0D=0A> > cache their s=
ize internally as a side effect of mutation operations=2E Is=0D=0A> =0D=0A>=
 Or the length could be the difference of two pointers -- address of the=0D=
=0A> first empty slot minus address of first item=2E=0D=0A> =0D=0A> > that =
correct? If so, is it safe to assume that at least all built-in=0D=0A> > ty=
pes observe this behavior,=0D=0A> =0D=0A> str, bytes, bytearrays, arrays, s=
ets, frozensets, dicts, dictviews, and=0D=0A> ranges should all return len =
in O(1) time=2E That includes the possibility=0D=0A> of a subtraction as in=
dicated above=2E=0D=0A> =0D=0A=0D=0AWhy does pointer arithmetic work for di=
cts? I would think the position=0D=0Aof a value would be based on the hash =
of the key and thus "random" for=0D=0Athe context of this conversation=2E=
=0D=0A=0D=0ARamit Prasad=0D=0A=0D=0A=0D=0AThis email is confidential and su=
bject to important disclaimers and=0D=0Aconditions including on offers for =
the purchase or sale of=0D=0Asecurities, accuracy and completeness of infor=
mation, viruses,=0D=0Aconfidentiality, legal privilege, and legal entity di=
sclaimers,=0D=0Aavailable at http://www=2Ejpmorgan=2Ecom/pages/disclosures/=
email=2E