what does 'for _ in range()' mean?

[Jon Perez]

I saw this code snippet:

sock.listen(20)
for _ in range(20):
newsock, client_addr = sock.accept()
print "Client connected:", client_addr
data[newsock] = ""

why use _ for this example? Is there any
optimization to be had using it?

I know that in the interpreter _ means the
last value calculated, but what does _ mean
inside source code?

[Peter Hansen]

Matteo Dell'Amico wrote:

Peter Hansen wrote:

Sounds to me like the two discussions are unrelated.

More or less so. It was only a discussion on the etimology of the
underscore convention. :-)

In that case they would be related, but is this any more than
supposition on your part? Is the use of _ as a throw-away in
for loops, and perhaps even in the interpreter as "last result",
definitely descended from its wildcard usage in functional
languages (or, at least, Ocaml)?

-Peter

[Christopher T King]

On Thu, 29 Jul 2004, Peter Hansen wrote:

In that case they would be related, but is this any more than
supposition on your part? Is the use of _ as a throw-away in
for loops, and perhaps even in the interpreter as "last result",
definitely descended from its wildcard usage in functional
languages (or, at least, Ocaml)?

I'm pretty sure they're both descended from the use of _ as both a
throw-away and a wildcard in Prolog, as another poster pointed out.

For reference, Prolog was born in 1970, OCaml was born in 1996 (with roots
dating back to ML in 1983), and Python was born in 1991. (Data gleaned
from the Programming Language Genealogy Project at everything2.com .)

[Peter Hansen]

Christopher T King wrote:

On Thu, 29 Jul 2004, Peter Hansen wrote:

In that case they would be related, but is this any more than
supposition on your part? Is the use of _ as a throw-away in
for loops, and perhaps even in the interpreter as "last result",
definitely descended from its wildcard usage in functional
languages (or, at least, Ocaml)?

I'm pretty sure they're both descended from the use of _ as both a
throw-away and a wildcard in Prolog, as another poster pointed out.

I'm just as unsure about any of that. It seems more likely to
me that a number of people have independently found _ to be a handy
"anonymous" variable in different situations, and that trying to find
links to analogous usage in other languages is largely speculative.

Of course, that said, we'll probably hear shortly from someone
channelling the person who added it to the interpreter as the
"last result" name, contra-dicting me. :-)

-Peter

[Matteo Dell'Amico]

Peter Hansen wrote:

I'm just as unsure about any of that. It seems more likely to
me that a number of people have independently found _ to be a handy
"anonymous" variable in different situations, and that trying to find
links to analogous usage in other languages is largely speculative.
It would surprise me if the three were unrelated... there are
similarities between prolog's unification, ML's pattern matching and
python's tuple unpacking (even if the latter is considerably simpler and
less powerful), and note that the specific meaning of _ - both in Prolog
and in ML-like languages - is "throw away": if I'm not mistaken, you can
always replace an underscore with a new unused variable.
Of course, that said, we'll probably hear shortly from someone
channelling the person who added it to the interpreter as the
"last result" name, contra-dicting me. :-)

I see this one as a totally unrelated meaning, similar to perl's $_. I'm
waiting to be contradicted, too. :-)

--
Ciao,
Matteo

[Dave Benjamin]

In article <41************ ***@alcyone.com >, Erik Max Francis wrote:

Note also there's a noticeable difference between the anonymous variable
in Prolog and the use of _ in Python; in Prolog, the anonymous variable
can be used multiple times in the same expression and there is no need
for the variable to represent the same thing. In

middle(X) :- inOrder(_, X, _).

In Python, it's possible to use _ multiple times in the same expression,
too. I guess there's no obvious reason why this shouldn't work, but I
discovered the following tuple-unpacking idiom recently:

for _, val, _ in ((1, 2, 3), (4, 5, 6), (7, 8, 9)):

.... print val
....
2
5
8

--
.:[ dave benjamin: ramen/[sp00] -:- spoomusic.com -:- ramenfest.com ]:.

"When the country is confused and in chaos, information scientists appear."
Librarian's Lao Tzu: http://www.geocities.com/onelibrarian.geo/lao_tzu.html

[Terry Reedy]

"Dave Benjamin" <ra***@lackingt alent.com> wrote in message
news:slrncgj6j3 .vte.ra***@lack ingtalent.com.. .

In Python, it's possible to use _ multiple times in the same expression,
In Python, _ is a (non-keyword) name like any other. It can be used like
any other name. In interactive mode, it is the default name bound to bare
expressions, but it is still just a name.
too. I guess there's no obvious reason why this shouldn't work, but I
discovered the following tuple-unpacking idiom recently:

for _, val, _ in ((1, 2, 3), (4, 5, 6), (7, 8, 9)): ... print val
...
2
5
8

So does

for a,v,a in [[1,2,3], [4,5,6]]: print v

....
2
5

too much fuss over simplicity yours

Terry J. Reedy

[Heike C. Zimmerer]

Dave Benjamin <ra***@lackingt alent.com> writes:

In Python, it's possible to use _ multiple times in the same expression,

There is, however, one objection to the use of _ as a placeholder for
a dummy variable: it is used by the gettext module. So if a program
may be subject to internationaliz ation, it is advisable to keep this
name reserved, or you may run into problems.

[Skip Montanaro]

Heike> Dave Benjamin <ra***@lackingt alent.com> writes:

In Python, it's possible to use _ multiple times in the same expression,

Heike> There is, however, one objection to the use of _ as a placeholder
Heike> for a dummy variable: it is used by the gettext module. So if a
Heike> program may be subject to internationaliz ation, it is advisable
Heike> to keep this name reserved, or you may run into problems.

Only if you use it as both a placeholder variable and call it as a function
within the same function, or have the misfortune to declare

global _

in some function where it's used as a placeholder in a module whose other
functions call _("some string").

Skip