Michael P. Reilly
Michael.P..Reilly at p98.f112.n480.z2.fidonet.org
Wed Jun 30 10:07:14 EDT 1999
From: "Michael P. Reilly" <arcege at shore.net>
Tim Peters <tim_one at email.msn.com> wrote:
: [Reuben Sumner]
:> How can I do the equivalent of a PySequence_Check in Python?
:> GvR told me that the question was illdefined (if I explained it correctly)
:> and to ask here. It seems that some of the functions in abstract.h
:> are available from python (len(), callable()) and others are not
:> (PySequence_Check, PyNumber_Check).
:> What am I missing?
: For starters, unquestioning faith in Guido's pronouncements <wink>.
:> I have a nested structure of lists and tuple and I just want to know
:> when I have bottomed out.
: Then why not check for that directly and be done with it?
:> I can check for tuple or list indivually but that is ugly and
: Not to mention simple, obvious and unsurprising <wink>.
:> I could just try indexing and see if there was an exception but that
:> is not much better.
: OK, you tell me: what *is* a sequence? Answer that, and you'll know how to
: check for it <0.5 wink>. Before answering too fast, note that mapping types
: can be indexed without raising an exception, and so can strings. Do you
: want your nest of "lists and tuples" *not* to "bottom out" at a string?
: Probably not -- but if so, you're not really interested in testing whether
: it's a sequence! Note too that user-defined classes can support the
: __getitem__ protocol without supporting the __getslice__ protocol, or vice
: versa. Are either of those sequences, or do they need to support both?
: Whichever way you answer, half the world will disagree.
: I think that, in most cases, when people say "sequence": (A) it's
: ill-defined <wink>; and, (B) after five rounds of tedious questioning they
: end up deciding they mean "responds to slicing but isn't a string". So
: there you go:
: def issequence(x):
: return 0
: return type(x) is not type("")
My basis (in my own programming) has been:
import types, UserList
return (isinstance(x, types.ListType) or
isinstance(x, types.TupleType) or
It is very restrictive in some cases, but if I want a user defined
sequence, I usually _try_ to make one from UserList.
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