Is using range() in for loops really Pythonic?

[Grant Edwards]

Re: Is using range() in for loops really Pythonic?

On 2008-05-12, Ben Finney <bignose+hate s-spam@benfinney. id.auwrote:

Paddy <paddy3118@goog lemail.comwrite s:
>

>I've used Fortran and C and so would tend to use either i,j,k as the
>unused loop variable above, or, for clarity, call it something
>descriptive like loop_count, if the loop body would be clearer.

>
The problem with all of these names is that they also have long
precedent as names of values that *will* be used inside the loop.

I guess people who standardize on loop_count never nest loops. :)

Because of the precedent of those names, choosing one of those
names doesn't make it clear to the reader that the value is
never used; they have no indication from you of that until
they look over the code a few times. It's implicit rather than
explicit.

And when somebody adds a nested loop things fall apart.

--
Grant Edwards grante Yow! BARBARA STANWYCK makes
at me nervous!!
visi.com

[Paddy]

Re: Is using range() in for loops really Pythonic?

On May 12, 3:46 pm, Grant Edwards <gra...@visi.co mwrote:

On 2008-05-12, Ben Finney <bignose+hate s-s...@benfinney. id.auwrote:
>

Paddy <paddy3...@goog lemail.comwrite s:

>

I've used Fortran and C and so would tend to use either i,j,k as the
unused loop variable above, or, for clarity, call it something
descriptive like loop_count, if the loop body would be clearer.

>

The problem with all of these names is that they also have long
precedent as names of values that *will* be used inside the loop.

>
I guess people who standardize on loop_count never nest loops. :)
>

Because of the precedent of those names, choosing one of those
names doesn't make it clear to the reader that the value is
never used; they have no indication from you of that until
they look over the code a few times. It's implicit rather than
explicit.

>
And when somebody adds a nested loop things fall apart.

I don't have an example to hand. A lot of casses of repeat_X_times
inside a loop of repeat_Y_times would naturally be written as
repeat_Y*X_time s.
Oh, wait a bit,
<oversimplified _example_alert>
for i in range(3):
print "Stay!"
for j in range(2):
print "Come over."
</oversimplified_ example_alert>

Which could become:

for outer_stay_repe tions in range(3):
print "Stay!"
for inner_come_over _repetions in range(2):
print "Come over."

But the second is daft. Nested repeats don't neccessarily pose a
problem to choosing meaningful names for repeat counters that are not
going to be referenced in the loop body, and most times i,j,k are fine
for used and un-used loop indices I find.

- Paddy
(Or were you just having a laugh ;-)

[Carl Banks]

Re: Is using range() in for loops really Pythonic?

On May 12, 3:42 am, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

Because of the precedent of those names, choosing one of those names
doesn't make it clear to the reader that the value is never used;

Why is it even necessary to document this? This is the thing that
baffles me the most about this thread.

Think about all the various aspect of a varibale:

* is it some common usage archetype
* value it represents
* scope
* (expected) type
* physical units of measure
* software package it belongs to
* whether it's used in the program or not
* scalar, structure, or array
* mutable or constant
* and so on

You cannot possibly document all of this pertinent information in the
name alone. A variable's name is a very limited resource: one can
only document a tiny number of things, so one must prioritize.

IMHO, whether a varibale is used or not has got to be one of the least
important things of all (in no small part because it's easily
discernable from nearby code). In fact, I'd say it's so irrelevant I
probably wouldn't have listed it if it wasn't the topic of discussion.

The first two items I listed are the most important to me. If a
variable fills the archetypal role of counter, then it ought to be
named like a counter; this usually trumps all other concerns. (Never
mind that the varibale here isn't really a counter, it still fills the
archetypal role of one, which is more important. The name of a
variable is for the reader's benefit.) So I would have to agree with
Ivan Illarionov.

Having said that, if you do think it's most important to document
whether something is used, I would suggest not using "_" (because it's
conflicts with gettext conventions) or "dummy" (because it's
misleading) for it.


Carl Banks

[Ben Finney]

Re: Is using range() in for loops really Pythonic?

Carl Banks <pavlovevidence @gmail.comwrite s:

IMHO, whether a varibale is used or not has got to be one of the least
important things of all (in no small part because it's easily
discernable from nearby code).

I couldn't disagree more.

If you're binding a name to a value that will never be used, you're
doing me (the reader of the code) a great favour if you indicate
clearly and concisely that this value is not intended to be referenced
anywhere else. Saving time for the reader is a very important job of
the writer of code.

Having said that, if you do think it's most important to document
whether something is used, I would suggest not using "_" (because
it's conflicts with gettext conventions) or "dummy" (because it's
misleading) for it.

Agreed.

Note, too, that the ideal answer to this is "don't bind names at all
to values that will never be used". It's only because the specifics of
Python's 'for' statement require us to do this that we are discussing
the ramifications of what to do when what Python requires of us is
actually unnecessary for our purposes.

--
\ "Friendship is born at that moment when one person says to |
`\ another, 'What! You too? I thought I was the only one!'" -- |
_o__) C.S. Lewis |
Ben Finney

[Carl Banks]

Re: Is using range() in for loops really Pythonic?

On May 12, 7:03 pm, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

Carl Banks <pavlovevide... @gmail.comwrite s:

IMHO, whether a varibale is used or not has got to be one of the least
important things of all (in no small part because it's easily
discernable from nearby code).

>
I couldn't disagree more.
>
If you're binding a name to a value that will never be used, you're
doing me (the reader of the code) a great favour if you indicate
clearly and concisely that this value is not intended to be referenced
anywhere else. Saving time for the reader is a very important job of
the writer of code.

I don't think knowing in advance whether a variable is going to be
used or not benefits readability.


Carl Banks

[Terry Reedy]

Re: Is using range() in for loops really Pythonic?


"Ben Finney" <bignose+hate s-spam@benfinney. id.auwrote in message
news:877ie06kdl .fsf@benfinney. id.au...
| So, when not using the values that come from the controlling iterator,
| it's good to make that explicit. If Python supported it, we might
| prefer to use no name at all for something that isn't used, but the
| 'for' syntax doesn't allow that.

But I presume it could if [target 'in'] were optional.

for range(n): <stuff to do n times>

It is possible this has been proposed and rejected,
but I have no specific memory.

tjr

[Dave Parker]

Re: Is using range() in for loops really Pythonic?

On May 10, 8:19 pm, John Salerno <johnj...@NOSPA Mgmail.comwrote :

It seems somewhat
artificial to use the for loop to do something a certain number of
times, like above.

I agree; it's a common flaw with lots of languages, not just Python.

I'd be inclined to use something like:

FOR 8 TIMES DO <statement>.

or:

REPEAT <statementFOR 8 TIMES.

as a compromise between readability and machine-parsability.

If anyone has other suggestions, please post them here and I'll
implement one of them next weekend in Flaming Thunder.

[George Sakkis]

Re: Is using range() in for loops really Pythonic?

On May 12, 7:03 pm, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

Carl Banks <pavlovevide... @gmail.comwrite s:

IMHO, whether a varibale is used or not has got to be one of the least
important things of all (in no small part because it's easily
discernable from nearby code).

>
I couldn't disagree more.
>
If you're binding a name to a value that will never be used, you're
doing me (the reader of the code) a great favour if you indicate
clearly and concisely that this value is not intended to be referenced
anywhere else. Saving time for the reader is a very important job of
the writer of code.

If you push this logic too far, you should del every name immediately
after the last statement it is used in the scope. I would generally
find less readable some code spread with del every few lines, micro-
managing the effective scope of each name. YMMV.

George

[John Salerno]

Re: Is using range() in for loops really Pythonic?

Ben Finney wrote:

I think that the idiom
>
for unused in xrange(10):
# do stuff with no reference to 'unused'
>
is quite common. Is that what you're asking about?

Yes. I was more or less asking about the specific situation of using a
for loop to do something X number of times, but I think the more
generalized problem that everyone is talking about -- using a counter
variable that is never referenced in the loop -- probably puts the point
I was trying to make in a better light.

The reason I even brought this up is because I remember someone saying a
while back (probably here on the newsgroup) that the true use of a for
loop was to iterate through a sequence (for the purpose of using that
sequence), not to do something X number of times. Once they made this
comment, I suddenly saw the for loop in a new (and I believe purer)
light. That was the first time I realized what it was really meant
to do.

Using something like:

for unused in xrange(10):
# do stuff 10 times

suddenly seemed to me like a hackish way to replace

for (int i=0; i<10; i++) {
// do stuff 10 times;
}

Not that I think the above code (C#) looks all that elegant either. But
in C# there is a distinction between the above, and this:

foreach (int i in sequence)
// do something;

which is more closely related to the Python for loop.

Now, you could easily make the argument that the Python for loop is a
much simpler tool to accomplish *both* of the above, and I suppose that
makes sense. Seems a little silly to have two separate for loops to do
these things. I just wasn't sure if the "counter" version of the Python
for loop was considered slightly unpythonic.

[Terry Reedy]

Re: Is using range() in for loops really Pythonic?


"John Salerno" <johnjsal@NOSPA Mgmail.comwrote in message
news:482912b7$0 $11641$607ed4bc @cv.net...
| The reason I even brought this up is because I remember someone saying a
| while back (probably here on the newsgroup) that the true use of a for
| loop was to iterate through a sequence (for the purpose of using that
| sequence), not to do something X number of times.

I believe the specific context was to counteract some people's tendency to
write
for i in range(len(seq)) : do stuff with seq[i]
when they would better (more Pythonically) write
for item in seq: do stuff with item
or even
for i,item in enumerate(seq): do stuff with i and item.

One subtle but real advantage is that the latter two forms work with
iterables that do not have a known-ahead length or which even continue
indefinitely.

| Once they made this
| comment, I suddenly saw the for loop in a new (and I believe purer)
| light. That was the first time I realized what it was really meant
| to do.

That is an important insight. But to me it does not negate the "do
something n times" usage when there is no iterable other than range to
iterate. Do note that range has *not* been removed from 3.0 and that its
main intended usage is for looping.

| Now, you could easily make the argument that the Python for loop is a
| much simpler tool to accomplish *both* of the above, and I suppose that
| makes sense.

Yes. Python leans toward minimalism. Proposals for various
special-purpose loopin constructs have been rejected. For-loops cover most
looping needs; while-loops cover everything else.

Terry Jan Reedy

[Marc 'BlackJack' Rintsch]

Re: Is using range() in for loops really Pythonic?

On Mon, 12 May 2008 11:23:07 -0700, Carl Banks wrote:

On May 12, 3:42 am, Ben Finney <bignose+hate s-s...@benfinney. id.au>
wrote:

>Because of the precedent of those names, choosing one of those names
>doesn't make it clear to the reader that the value is never used;

>
>
Why is it even necessary to document this? This is the thing that
baffles me the most about this thread.

I do it (with a "special" name) because static source checkers issue a
warning for unused variables.

Ciao,
Marc 'BlackJack' Rintsch

[Dave Parker]

Re: Is using range() in for loops really Pythonic?

REXX's loop construct subsumes all the common uses... And worse, it

appears that a repetition and a condition can be part of the single
statement.

Thank you for pointing out the REXX examples. I am a Kedit user, but
had forgotten about the REXX do-loops. I'll keep them in mind when I
add an easy way to "do this n times" to Flaming Thunder this weekend.

On May 11, 8:53 pm, Dennis Lee Bieber <wlfr...@ix.net com.comwrote:

On Mon, 12 May 2008 00:43:08 GMT, Jonathsn Cronin <j...@theworld. com>
declaimed the following in comp.lang.pytho n:
>

I agree in principle; the first is iteration and the second is repetition.
In Python, the common idiom for a fixed number of repetitions is iterating
over a number range.  This is true in most languages I am familiar with,
probably because fixed repetition, where you don't care about the "index"
value, is rarely used.

>

The only language I've used that does have fixed repetition is an (old)
dialect of lisp, and I'm not sure it even made it into Common Lisp.  
Smalltalk and Ruby do have fixed repetition.

>
        REXX's loop construct subsumes all the common uses... And worse, it
appears that a repetition and a condition can be part of the single
statement.
>
DO FOREVER
...
END
>
DO repeatcount
...
END
>
DO loopvar=first TO last BY step FOR reptcount
...
END
>
(where "TO last", "BY step", "FOR reptcount" are all optional clauses!)
>
DO WHILE conditional
...
END
>
DO UNTIL conditional
...
END
>
        Mixing modes
>
DO loopvar=first BY step UNTIL conditional
...
END
>
Hmmm, looks like it's been extended for container objects (those that
support MAKEARRAY):
>
DO item OVER container
...
END
--
        Wulfraed        Dennis Lee Bieber               KD6MOG
        wlfr...@ix.netc om.com              wulfr...@bestia ria.com
                HTTP://wlfraed.home.netcom.com/
        (Bestiaria Support Staff:               web-a...@bestiaria. com)
                HTTP://www.bestiaria.com/

[Larry Bates]

Re: Is using range() in for loops really Pythonic?

John Salerno wrote:

I know it's popular and very handy, but I'm curious if there are purists
out there who think that using something like:
>
for x in range(10):
#do something 10 times
>
is unPythonic. The reason I ask is because the structure of the for loop
seems to be for iterating through a sequence. It seems somewhat
artificial to use the for loop to do something a certain number of
times, like above.
>
Anyone out there refuse to use it this way, or is it just impossible to
avoid?

I use it quite often, especially if I want to implement a fixed number of
retries on a communications channel.

-Larry

[Lie]

Re: Is using range() in for loops really Pythonic?

On May 13, 11:01 am, John Salerno <johnj...@NOSPA Mgmail.comwrote :

Ben Finney wrote:

I think that the idiom

>

    for unused in xrange(10):
        # do stuff with no reference to 'unused'

>

is quite common. Is that what you're asking about?

>
Yes. I was more or less asking about the specific situation of using a
for loop to do something X number of times, but I think the more
generalized problem that everyone is talking about -- using a counter
variable that is never referenced in the loop -- probably puts the point
I was trying to make in a better light.
>
The reason I even brought this up is because I remember someone saying a
while back (probably here on the newsgroup) that the true use of a for
loop was to iterate through a sequence (for the purpose of using that
sequence), not to do something X number of times. Once they made this
comment, I suddenly saw the for loop in a new (and I believe purer)
light. That was the first time I realized what it was really meant
to do.
>
Using something like:
>
for unused in xrange(10):
    # do stuff 10 times
>
suddenly seemed to me like a hackish way to replace
>
for (int i=0; i<10; i++) {
    // do stuff 10 times;
>
}
>
Not that I think the above code (C#) looks all that elegant either. But
in C# there is a distinction between the above, and this:
>
foreach (int i in sequence)
    // do something;
>
which is more closely related to the Python for loop.
>
Now, you could easily make the argument that the Python for loop is a
much simpler tool to accomplish *both* of the above, and I suppose that
makes sense. Seems a little silly to have two separate for loops to do
these things. I just wasn't sure if the "counter" version of the Python
for loop was considered slightly unpythonic.

What was unPythonic, I think, as most people would agree, is to use
for like this:

--
for i in xrange(len(lst) ):
pass
--

In VB, my language before Python, I've never used For Each even for a
sequence (Array, in VB), it seems too messy back then. Now, with
Python that only allowed a foreach statement, I realized that I never
really needed a for i in range(10) at all. Most of the time the places
where I need to do that is where the code is just a test code that
loops a certain number of times (with randomized input data), almost
never (probably never) met that in a real program code. And even in
some of those cases, the variables are usually used too (as a test
argument to the function being tested) or for logging purpose.

On May 13, 8:25 pm, Larry Bates <larry.ba...@we bsafe.com`wrote :
(snip)

I use it quite often, especially if I want to implement a fixed number of
retries on a communications channel.
>
-Larry

That still have semantic meaning: 'tries'. If it was me, I'll go to
the trouble of giving names, since I would use it for logging or
informational purpose ('first try' 'second try' 'third try' 'no more
try').

I've never really met a real "do something n times" case, where the
variable doesn't hold any semantic meaning that I don't want to use
and is not a test case.

For me, a _ (or any other dummy variable[1]) is good enough and it's
easy to change if later I realized that I actually need to use the
variable. I agree though, that i, j, k for unused name is a bad choice
because single letter names is in common usage in mathematics.

[1] PEP 8 is useless if a certain code base has other agreed
convention, as long as the name used for a unused name in certain code
base is consistent (and probably documented), it never becomes a
problem.

[Lie]

Re: Is using range() in for loops really Pythonic?

On May 13, 9:20 pm, Lie <Lie.1...@gmail .comwrote:

On May 13, 11:01 am, John Salerno <johnj...@NOSPA Mgmail.comwrote :
>
>
>

Ben Finney wrote:

I think that the idiom

>

    for unused in xrange(10):
        # do stuff with no reference to 'unused'

>

is quite common. Is that what you're asking about?

>

Yes. I was more or less asking about the specific situation of using a
for loop to do something X number of times, but I think the more
generalized problem that everyone is talking about -- using a counter
variable that is never referenced in the loop -- probably puts the point
I was trying to make in a better light.

>

The reason I even brought this up is because I remember someone saying a
while back (probably here on the newsgroup) that the true use of a for
loop was to iterate through a sequence (for the purpose of using that
sequence), not to do something X number of times. Once they made this
comment, I suddenly saw the for loop in a new (and I believe purer)
light. That was the first time I realized what it was really meant
to do.

>

Using something like:

>

for unused in xrange(10):
    # do stuff 10 times

>

suddenly seemed to me like a hackish way to replace

>

for (int i=0; i<10; i++) {
    // do stuff 10 times;

>

}

>

Not that I think the above code (C#) looks all that elegant either. But
in C# there is a distinction between the above, and this:

>

foreach (int i in sequence)
    // do something;

>

which is more closely related to the Python for loop.

>

Now, you could easily make the argument that the Python for loop is a
much simpler tool to accomplish *both* of the above, and I suppose that
makes sense. Seems a little silly to have two separate for loops to do
these things. I just wasn't sure if the "counter" version of the Python
for loop was considered slightly unpythonic.

>
What was unPythonic, I think, as most people would agree, is to use
for like this:
>
 --
    for i in xrange(len(lst) ):
        pass
 --
>
In VB, my language before Python, I've never used For Each even for a
sequence (Array, in VB), it seems too messy back then. Now, with
Python that only allowed a foreach statement, I realized that I never
really needed a for i in range(10) at all. Most of the time the places
where I need to do that is where the code is just a test code that
loops a certain number of times (with randomized input data), almost
never (probably never) met that in a real program code. And even in
some of those cases, the variables are usually used too (as a test
argument to the function being tested) or for logging purpose.
>
On May 13, 8:25 pm, Larry Bates <larry.ba...@we bsafe.com`wrote :
(snip)
>

I use it quite often, especially if I want to implement a fixed number of
retries on a communications channel.

>

-Larry

>
That still have semantic meaning: 'tries'. If it was me, I'll go to
the trouble of giving names, since I would use it for logging or
informational purpose ('first try' 'second try' 'third try' 'no more
try').
>
I've never really met a real "do something n times" case, where the
variable doesn't hold any semantic meaning that I don't want to use
and is not a test case.
>
For me, a _ (or any other dummy variable[1]) is good enough and it's
easy to change if later I realized that I actually need to use the
variable. I agree though, that i, j, k for unused name is a bad choice
because single letter names is in common usage in mathematics.
>
[1] PEP 8 is useless if a certain code base has other agreed
convention, as long as the name used for a unused name in certain code
base is consistent (and probably documented), it never becomes a
problem.

On my last message, I'm tempted not to say: "We need to have /dev/null
for Python" though I proved to fail.