for loop without variable

[Guest]

SV: Conventions for dummy name

Torsten Bronger wrote:

>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)

Variable name "dummy" serves the same purpose, such as:

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

According to http://linux.die.net/man/1/pylint it is also possible to use the
option

--dummy-variables-rgx=<regexp>

to further specify which variable not to report as unused. As far as I can
tell, it defaults to '_|dummy'.

..david

[Duncan Booth]

Re: for loop without variable

erik gartz <eegunnar@yahoo .comwrote:

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.

Google for: pylint unused

It pointed me at:

Question:
I've a function / method which is a callback where I do not have any
control on received argument, and pylint is complaining about unused
arguments. What can I do to avoid those warnings ?
>
Answer:
prefix (ui) the callback's name by cb_, as in cb_onclick(...) . By doing
so arguments usage won't be checked. Another solution is to use one of
the name defined in the "dummy-variables" configuration variable for
unused argument ("_" and "dummy" by default).

http://www.iaeste.or.at/doc/python2.3-pylint/html/FAQ.html

So it looks like you can use 'dummy' or add any other names you want to the
configuration.

[Torsten Bronger]

Re: Conventions for dummy name

Hallöchen!

David.Reksten@s weco.no writes:

Torsten Bronger wrote:
>

>[...]
>>
>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)

>
Variable name "dummy" serves the same purpose, such as:
>
a, b, dummy = function_that_r eturns_three_va lues(x, y)

Granted, but my rationale is that "__" is less visible in the source
code, so there is more emphasis on the actually interesting
variables.

Tschö,
Torsten.

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

[Guest]

SV: Conventions for dummy name

Torsten Bronger writes:

>David.Reksten@ sweco.no writes:
>

>Torsten Bronger wrote:
>>

>>[...]
>>>
>>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)

>>
>Variable name "dummy" serves the same purpose, such as:
>>
> a, b, dummy = function_that_r eturns_three_va lues(x, y)

>
>Granted, but my rationale is that "__" is less visible in the source
>code, so there is more emphasis on the actually interesting
>variables.

I guess it's a matter of preference. Personally, I find "dummy" to be more
explicit, and hence more readable for those that that will read my code
later. YMMV.

Regards,
..david

[Mike Meyer]

Re: for loop without variable

On Thu, 10 Jan 2008 08:42:16 +0100 Hrvoje Niksic <hniksic@xemacs .orgwrote:

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.

Except in this case, the variable *has* a meaning. You've just chosen
to obfuscate it.

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.

All correct - and I'm a big fan of minimizing code, as code you don't
write has no bugs in it. But you can still show the meaning of this
"meaningles s" variable:

for number_of_attem pts in xrange(maximum_ attempts):

Of course, the OP's request is a better solution: since he doesn't
actually need the variable, removing it completely means there's one
less variable, which is one less thing you can set to the wrong value.

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

[Basilisk96]

Re: for loop without variable

On Jan 9, 10:55 pm, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

erik gartz <eegun...@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

Neat! That is the best solution I've seen so far. I should definitely
dig into the itertools module more often.

Cheers,
-Basilisk96

[Marty]

Re: for loop without variable

Hrvoje Niksic wrote:

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.

I recently faced a similar issue doing something like this:

data_out = []
for i in range(len(data_ in)):
data_out.append ([])

This caused me to wonder why Python does not have a "foreach" statement (and
also why has it not come up in this thread)? I realize the topic has probably
been beaten to death in earlier thread(s), but does anyone have the short answer?

[Ryan Ginstrom]

RE: for loop without variable

On Behalf Of Marty

I recently faced a similar issue doing something like this:
>
data_out = []
for i in range(len(data_ in)):
data_out.append ([])
>
This caused me to wonder why Python does not have a "foreach"
statement (and also why has it not come up in this thread)?
I realize the topic has probably been beaten to death in
earlier thread(s), but does anyone have the short answer?

data_out = [[] for item in data_in]

Regards,
Ryan Ginstrom

[Mike Meyer]

Re: for loop without variable

On Thu, 10 Jan 2008 22:36:56 -0500 Marty <martyb1@earthl ink.netwrote:

I recently faced a similar issue doing something like this:
>
data_out = []
for i in range(len(data_ in)):
data_out.append ([])

More succinctly:

data_out = []
for _ in data_in:
data_out.append ([])

Or, as has already been pointed out:

data_out = [[] for _ in data_in]

This caused me to wonder why Python does not have a "foreach" statement (and
also why has it not come up in this thread)? I realize the topic has probably
been beaten to death in earlier thread(s), but does anyone have the short answer?

But I'm curious - what's the difference between the "foreach" you have
in mind and the standard python "for"?

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

[Matimus]

Re: for loop without variable

On Jan 10, 10:36 pm, Marty <mart...@earthl ink.netwrote:

Hrvoje Niksic wrote:

Mike Meyer <mwm-keyword-python.b4b...@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.

>
I recently faced a similar issue doing something like this:
>
data_out = []
for i in range(len(data_ in)):
data_out.append ([])
>
This caused me to wonder why Python does not have a "foreach" statement (and
also why has it not come up in this thread)? I realize the topic has probably
been beaten to death in earlier thread(s), but does anyone have the short answer?

Pythons `for' essentially is foreach. The code below does the same
thing as what you have posted does. Actually, I've found that if you
find yourself ever doing range(len(data_ in)) in python, it is time to
take a second look.

data_out = []
for x in data_in:
data_out.append ([])

`range' is just a function that returns a list.

Matt

[Paul Rubin]

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.

This uses an index var, but doesn't leak it outside the genexp, so
I don't know what pylint would say (untested):

def f():
return any(attempt_ser vice() for i in xrange(10))

I think the above is pretty natural. If you really insist on not
using any variables, the below might work (untested):

from itertools import imap, repeat

def f():
return any(imap(apply, repeat(attempt_ service, 10)))

it just seems way too obscure though. Python style seems to favor
spewing extra variables around.

[Paul Rubin]

Re: for loop without variable

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

data_out = [[] for _ in data_in]
...
But I'm curious - what's the difference between the "foreach" you have
in mind and the standard python "for"?

The "for" loop, like the list comprehension, pollutes the namespace
with an index variable that's not used for anything. I prefer genexps:

data_out = list([] for x in data_in)

[Carl Banks]

Re: for loop without variable

On Thu, 10 Jan 2008 22:36:56 -0500, Marty wrote:

I recently faced a similar issue doing something like this:
>
data_out = []
for i in range(len(data_ in)):
data_out.append ([])
>
This caused me to wonder why Python does not have a "foreach" statement
(and also why has it not come up in this thread)? I realize the topic
has probably been beaten to death in earlier thread(s), but does anyone
have the short answer?

Most languages that have "foreach" use it the same way Python uses
"for". The reason they use "foreach" instead of plain "for" is often
because they have a separate for statement that mimic C's for.

Perhaps you're wondering why there is no syntax for looping a given
number of times. (Languages that have this feature, e.g., Ada, often to
use "repeat" as the keyword.)

1. Looping a fixed number of times is quite uncommon.
2. A syntax for it buys you almost nothing.


Carl Banks

[Marty]

Re: for loop without variable

Mike Meyer wrote:

On Thu, 10 Jan 2008 22:36:56 -0500 Marty <martyb1@earthl ink.netwrote:

>I recently faced a similar issue doing something like this:
>>
> data_out = []
> for i in range(len(data_ in)):
> data_out.append ([])

>
More succinctly:
>
data_out = []
for _ in data_in:
data_out.append ([])
>
Or, as has already been pointed out:
>
data_out = [[] for _ in data_in]

That's nice.

>

>This caused me to wonder why Python does not have a "foreach" statement (and
>also why has it not come up in this thread)? I realize the topic has probably
>been beaten to death in earlier thread(s), but does anyone have the short answer?

>
But I'm curious - what's the difference between the "foreach" you have
in mind and the standard python "for"?
>
<mike

For example, I thought the python "equivalent " of perl's foreach might be:

data_out = [[] foreach data_in]

Trying to answer my own question, if it comes down to a choice between a unique
statement v. the anonymous variable "_", then I guess I can see why Python did
it this way.

[Basilisk96]

Re: for loop without variable

On Jan 10, 10:36 pm, Marty <mart...@earthl ink.netwrote:

Hrvoje Niksic wrote:

Mike Meyer <mwm-keyword-python.b4b...@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.

>
I recently faced a similar issue doing something like this:
>
data_out = []
for i in range(len(data_ in)):
data_out.append ([])
>
This caused me to wonder why Python does not have a "foreach" statement (and
also why has it not come up in this thread)? I realize the topic has probably
been beaten to death in earlier thread(s), but does anyone have the short answer?

But it does:

data_in = (1,2,3,4,5)
data_out = []

data_out += [[] for blah in data_in]

print data_out
[[], [], [], [], []]


Cheers,
-Basilisk96