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?

[David Eppstein]

In article <2m************ @uni-berlin.de>,
Jon Perez <jb********@wah oo.com> wrote:

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?

AFAIK it's just a variable like any other, but by convention it means
that you don't intend to use that value, just read it and ignore it.

--
David Eppstein
Computer Science Dept., Univ. of California, Irvine
http://www.ics.uci.edu/~eppstein/

[Roy Smith]

In article <ep************ *************** *@news.service. uci.edu>,
David Eppstein <ep******@ics.u ci.edu> wrote:

In article <2m************ @uni-berlin.de>,
Jon Perez <jb********@wah oo.com> wrote:

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?

AFAIK it's just a variable like any other, but by convention it means
that you don't intend to use that value, just read it and ignore it.

I've never heard of that convention before. Is it some python-specific
thing, or is my ignorance more global in scope?

In any case, I'd vote for some more useful variable name. In the above
case, something like connectionNumbe r or whatever would be more
self-explanitory.

[Tor Iver Wilhelmsen]

Roy Smith <ro*@panix.co m> writes:

I've never heard of that convention before. Is it some python-specific
thing, or is my ignorance more global in scope?

It may derive from Prolog, where you use _ as an unbound "wildcard".
E.g. to declare that X is a parent if X is a father or mother:

parent(X) :- fatherOf(X, _), !.
parent(X) :- motherOf(X, _), !.

because you don't need to know anything about the child, just that it
exists. If you used a variable like Y, you would either also need to
say something about Y, or Y would end up in the resolved answer.

(The ! is a "cut", and means we're not interested in more than one
answer.)

[Erik Max Francis]

Tor Iver Wilhelmsen wrote:

Roy Smith <ro*@panix.co m> writes:

I've never heard of that convention before. Is it some python-specific
thing, or is my ignorance more global in scope?

It may derive from Prolog, where you use _ as an unbound "wildcard".
E.g. to declare that X is a parent if X is a father or mother:

It may, but to answer the original poster's question, it's not uncommon
to see this in other languages (where _ is a valid identifier). _ often
means a variable one's not interested in the value of, but which is
needed for syntactic reasons.

Note that _(...) as a macro call has another conventional meaning, which
comes from gettext, where one uses _("string literal") to indicate a
string that needs localization.

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, _).

there's no need for _ to map to the same object -- middle(c) would be
true even if inOrder(b, c, d) were the only known relevant fact, and b
and d are certainly not equal. That's not true in Python, where _ is
just another name with no special semantics.

--
__ Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ Never be the first to believe / Never be the last to deceive
-- Florence, _Chess_

[Matteo Dell'Amico]

Roy Smith wrote:

I've never heard of that convention before. Is it some python-specific
thing, or is my ignorance more global in scope?
I have seen it in logic and functional programming, where '_' -
differently from python - has a special meaning: assignments to '_' are
discarded in functional programming and

I guess that it's value in the interpreter, instead, comes from perl's $_.
In any case, I'd vote for some more useful variable name. In the above
case, something like connectionNumbe r or whatever would be more
self-explanitory.

In that case, I interpret is as this: that loop has to be iterated 20
times, and the looping variable is uninfluent. In this cases, again by
convention, in C programs the variable is often called "i".

--
Ciao,
Matteo

[Matteo Dell'Amico]

Whoops, part missing :-)

Matteo Dell'Amico wrote:

I have seen it in logic and functional programming, where '_' -
differently from python - has a special meaning: assignments to '_' are
discarded in functional programming and

in logic programming it is seen as a jolly "match-all" value.

--
Ciao,
Matteo

[John Roth]

"David Eppstein" <ep******@ics.u ci.edu> wrote in message
news:ep******** *************** *****@news.serv ice.uci.edu...

In article <2m************ @uni-berlin.de>,
Jon Perez <jb********@wah oo.com> wrote:

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?
AFAIK it's just a variable like any other, but by convention it means
that you don't intend to use that value, just read it and ignore it.

What convention? I have to agree with a couple
of other posters; I've never heard of it before.

If it really is a convention, it would be nice to have it
documented somewhere (like the single underscore
in front of a variable means "weak internal use").
Somewhere is most likely PEP 8 - the Python
Style Guide.

John Roth

--
David Eppstein
Computer Science Dept., Univ. of California, Irvine
http://www.ics.uci.edu/~eppstein/

[Erik Max Francis]

John Roth wrote:

What convention? I have to agree with a couple
of other posters; I've never heard of it before.

If it really is a convention, it would be nice to have it
documented somewhere (like the single underscore
in front of a variable means "weak internal use").
Somewhere is most likely PEP 8 - the Python
Style Guide.

A convention doesn't need to have official sanction to be a convention.

--
__ Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
\__/ But you're not going to be there tomorrow. And it's all about
tomorrow. -- Montgomery Brogan

[Peter Hansen]

John Roth wrote:

"David Eppstein" <ep******@ics.u ci.edu> wrote:

Jon Perez <jb********@wah oo.com> wrote:

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

AFAIK it's just a variable like any other, but by convention it means
that you don't intend to use that value, just read it and ignore it.

What convention? I have to agree with a couple
of other posters; I've never heard of it before.

I would imagine it's the same convention that suggests that
the value assigned to _ is ignored in the following type
of code, which surely most of us have seen around here a
few times:

a, b, _ = tupleWithThreeI tems

-Peter