for loop without variable

[erik gartz]

Hi. I'd like to be able to write a loop such as:
for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint
and it complains about the unused variable i. I can achieve the above
with more lines of code like:
i = 0
while (i != 10):
i += 1
Is there a concise way to accomplish this without adding extra lines
of codes? Thanks in advance for your help.

[Diez B. Roggisch]

Re: for loop without variable

erik gartz schrieb:

Hi. I'd like to be able to write a loop such as:
for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint
and it complains about the unused variable i. I can achieve the above
with more lines of code like:
i = 0
while (i != 10):
i += 1
Is there a concise way to accomplish this without adding extra lines
of codes? Thanks in advance for your help.

The underscore is used as "discarded" identifier. So maybe


for _ in xrange(10):
...


works.

Diez

[Thomas Heller]

Re: for loop without variable

erik gartz schrieb:

Hi. I'd like to be able to write a loop such as:
for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint
and it complains about the unused variable i.

Pychecker won't complain if you rename 'i' to '_', IIRC:

for _ in range(10):
pass

Thomas

[Ben Finney]

Conventions for dummy name (was: for loop without variable)

"Diez B. Roggisch" <deets@nospam.w eb.dewrites:

The underscore is used as "discarded" identifier. So maybe
>
for _ in xrange(10):
...

The problem with the '_' name is that it is already well-known and
long-used existing convention for an entirely unrelated purpose: in
the 'gettext' i18n library, the '_' function to get the
locally-translated version of a text string.

Since the number of programs that need to use something like 'gettext'
(and therefore use the '_' function) is likely only to increase, it
seems foolish to set one's program up for a conflict with that
established usage.

I've seen 'dummy' used as a "don't care about this value" name in
other Python code. That seems more readable, more explicit, and less
likely to conflict with existing conventions.

--
\ "It is forbidden to steal hotel towels. Please if you are not |
`\ person to do such is please not to read notice." -- Hotel |
_o__) sign, Kowloon, Hong Kong |
Ben Finney

[MRAB]

Re: Conventions for dummy name (was: for loop without variable)

On Jan 9, 11:17 pm, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

"Diez B. Roggisch" <de...@nospam.w eb.dewrites:
>

The underscore is used as "discarded" identifier. So maybe

>

for _ in xrange(10):
...

>
The problem with the '_' name is that it is already well-known and
long-used existing convention for an entirely unrelated purpose: in
the 'gettext' i18n library, the '_' function to get the
locally-translated version of a text string.
>
Since the number of programs that need to use something like 'gettext'
(and therefore use the '_' function) is likely only to increase, it
seems foolish to set one's program up for a conflict with that
established usage.
>
I've seen 'dummy' used as a "don't care about this value" name in
other Python code. That seems more readable, more explicit, and less
likely to conflict with existing conventions.
>

Perhaps a "discarded" identifier should be any which is an underscore
followed by digits.

A single leading underscore is already used for "private" identifiers,
but they are usually an underscore followed by what would be a valid
identifier by itself, eg. "_exit".

The convention would then be:

2 underscores + valid_by_self + 2 underscores =special

underscore + valid_by_self =private

underscore + invalid_by_self =dummy

[Dan Sommers]

Re: for loop without variable

On Wed, 09 Jan 2008 14:25:36 -0800, erik gartz wrote:

Hi. I'd like to be able to write a loop such as:
for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint and
it complains about the unused variable i ...

What does that loop do? (Not the loop you posted, but the "real" loop
in your application.) Perhaps python has another way to express what
you're doing.

For example, if you're iterating over the elements of an array, or
through the lines of a file, or the keys of a dictionary, the "in"
operator may work better:

for thing in array_or_file_o r_dictionary:
do_something_wi th(thing)

HTH,
Dan

--
Dan Sommers A death spiral goes clock-
<http://www.tombstoneze ro.net/dan/ wise north of the equator.
Atoms are not things. -- Werner Heisenberg -- Dilbert's PHB

[erik gartz]

Re: for loop without variable

On Jan 9, 8:35 pm, Dan Sommers <m...@privacy.n etwrote:

On Wed, 09 Jan 2008 14:25:36 -0800, erik gartz wrote:

Hi. I'd like to be able to write a loop such as:
for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint and
it complains about the unused variable i ...

>
What does that loop do? (Not the loop you posted, but the "real" loop
in your application.) Perhaps python has another way to express what
you're doing.
>
For example, if you're iterating over the elements of an array, or
through the lines of a file, or the keys of a dictionary, the "in"
operator may work better:
>
for thing in array_or_file_o r_dictionary:
do_something_wi th(thing)
>
HTH,
Dan
>
--
Dan Sommers A death spiral goes clock-
<http://www.tombstoneze ro.net/dan/ wise north of the equator.
Atoms are not things. -- Werner Heisenberg -- Dilbert's PHB

The loop performs some actions with web services. The particular
iteration I'm on isn't important to me. It is only important that I
attempt the web services that number of times. If I succeed I
obviously break out of the loop and the containing function (the
function which has the loop in it) returns True. If all attempts fail
the containing loop returns False.

I guess based on the replies of everyone my best bet is to leave the
code the way it is and suck up the warning from pylint. I don't want
to turn the warning off because catching unused variables in the
general is useful to me. Unfortunately, I don't *think* I can shut the
warning for that line or function off, only for the entire file.
Pylint gives you a rating of your quality of code which I think is
really cool. This is a great motivation and helps me to push to
"tighten the screws". However it is easy to get carried away with your
rating.:-)

[Basilisk96]

Re: for loop without variable

On Jan 9, 9:49 pm, erik gartz <eegun...@yahoo .comwrote:

The loop performs some actions with web services. The particular
iteration I'm on isn't important to me. It is only important that I
attempt the web services that number of times. If I succeed I
obviously break out of the loop and the containing function (the
function which has the loop in it) returns True. If all attempts fail
the containing loop returns False.

Do you think you could apply something like this:

def foo():print "fetching foo..."
actions = (foo,)*5
for f in actions:
f()

fetching foo...
fetching foo...
fetching foo...
fetching foo...
fetching foo...

...but not knowing your specific implementation, I may be off the wall
here.

Cheers,
-Basilisk96

[Carl Banks]

Re: for loop without variable

On Wed, 09 Jan 2008 14:25:36 -0800, erik gartz wrote:

Hi. I'd like to be able to write a loop such as: for i in range(10):
pass
but without the i variable. The reason for this is I'm using pylint and
it complains about the unused variable i. I can achieve the above with
more lines of code like:
i = 0
while (i != 10):
i += 1
Is there a concise way to accomplish this without adding extra lines of
codes? Thanks in advance for your help.

IIRC, in pylint you can turn off checking for a particular symbol. I had
to edit a .pylintrc file (location may vary on Windows) and there was a
declaration in the file that listed symbols to ignore.

Last time I bothered running it, I added "id" to that list, since I use
it often (bad habit) and almost never use the builtin id, but still
wanted shadowing warnings for other symbols.


Carl Banks

[Ben Finney]

Re: for loop without variable

erik gartz <eegunnar@yahoo .comwrites:

The loop performs some actions with web services. The particular
iteration I'm on isn't important to me. It is only important that I
attempt the web services that number of times. If I succeed I
obviously break out of the loop and the containing function (the
function which has the loop in it) returns True. If all attempts
fail the containing loop returns False.

When you have iteration requirements that don't seem to fit the
built-in types (lists, dicts, generators etc.), turn to 'itertools'
<URL:http://www.python.org/doc/lib/module-itertoolsin the standard
library.

>>from itertools import repeat

>>def foo():

... import random
... print "Trying ..."
... success = random.choice([True, False])
... return success
...

>>max_attempt s = 10
>>for foo_attempt in repeat(foo, max_attempts):

... if foo_attempt():
... break
...
Trying ...
Trying ...
Trying ...
Trying ...
Trying ...
Trying ...

>>>

Note that this is possibly more readable than 'for foo_attempt in
[foo] * max_attempts", and is more efficient for large values of
'max_attempts' because 'repeat' returns an iterator instead of
actually allocating the whole sequence.

I guess based on the replies of everyone my best bet is to leave the
code the way it is and suck up the warning from pylint.

I think your intent -- "repeat this operation N times" -- is better
expressed by the above code, than by keeping count of something you
don't actually care about.

I don't want to turn the warning off because catching unused
variables in the general is useful to me.

Agreed.

--
\ "Dyslexia means never having to say that you're ysror." |
`\ —anonymous |
_o__) |
Ben Finney

[Mike Meyer]

Re: for loop without variable

On Wed, 9 Jan 2008 18:49:36 -0800 (PST) erik gartz <eegunnar@yahoo .comwrote:

The loop performs some actions with web services. The particular
iteration I'm on isn't important to me. It is only important that I
attempt the web services that number of times. If I succeed I
obviously break out of the loop and the containing function (the
function which has the loop in it) returns True. If all attempts fail
the containing loop returns False.

It sounds to me like your counter variable actually has meaning, but
you've hidden that meaning by giving it the meaningless name "i". If
you give it a meaningful name, then there's an obvious way to do it
(which you listed yourself):

while retries_left:
if this_succeeds() :
return True
retries_left -= 1

return False

<mike
--
Mike Meyer <mwm@mired.or g> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.

[Sam]

Re: for loop without variable

Unfortunately, I don't *think* I can shut the
warning for that line or function off, only for the entire file.
Pylint gives you a rating of your quality of code which I think is

wrong :)

# pylint: disable-msg=XXXXX will only impact the current line!

$ cat -n dummy.py
1 for i in range(2): # pylint: disable-msg=W0612
2 print "foo"
3 for i in range(2):
4 print "foo"

pylint will not generate a warning on line 1, but will on line 3!

Cheers.

Sam

[Hrvoje Niksic]

Re: for loop without variable

Mike Meyer <mwm-keyword-python.b4bdba@m ired.orgwrites:

It sounds to me like your counter variable actually has meaning,

It depends how the code is written. In the example such as:

for meaningless_var iable in xrange(number_o f_attempts):
...

the loop variable really has no meaning. Rewriting this code only to
appease pylint is exactly that, it has nothing with making the code
more readable.

you've hidden that meaning by giving it the meaningless name "i". If
you give it a meaningful name, then there's an obvious way to do it
(which you listed yourself):
>
while retries_left:

[...]

This loop contains more code and hence more opportunities for
introducing bugs. For example, if you use "continue" anywhere in the
loop, you will do one retry too much.

[Jeroen Ruigrok van der Werven]

Re: Conventions for dummy name (was: for loop without variable)

-On [20080110 00:21], Ben Finney (bignose+hates-spam@benfinney. id.au) wrote:

>The problem with the '_' name is that it is already well-known and
>long-used existing convention for an entirely unrelated purpose: in
>the 'gettext' i18n library, the '_' function to get the
>locally-translated version of a text string.

The same applies for Babel (http://babel.edgewall.org/) where we have _() in
similar vein to the gettext implementation.

--
Jeroen Ruigrok van der Werven <asmodai(-at-)in-nomine.org/ asmodai
イェルーン ラウフãƒãƒƒ ク ヴァン デル ウェルヴェ ン
http://www.in-nomine.org/ | http://www.rangaku.org/
With a nuclear fire of Love in our Hearts, rest easy baby, rest easy...

[Torsten Bronger]

Re: Conventions for dummy name

Hallöchen!

Ben Finney writes:

"Diez B. Roggisch" <deets@nospam.w eb.dewrites:
>

>The underscore is used as "discarded" identifier. So maybe
>>
>for _ in xrange(10):
> ...

>
The problem with the '_' name is that it is already well-known and
long-used existing convention for an entirely unrelated purpose:
in the 'gettext' i18n library, the '_' function to get the
locally-translated version of a text string.

Right, that's because I've used "__" where not all returning values
are interesing to me such as

a, b, __ = function_that_r eturns_three_va lues(x, y)

However, in loops, I prefer real names, even if the loop variable
isn't used outside.

Tschö,
Torsten.

--
Torsten Bronger, aquisgrana, europa vetus
Jabber ID: bronger@jabber. org
(See http://ime.webhop.org for further contact info.)