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.

[Mike Meyer]

On Fri, 11 Jan 2008 01:48:43 -0500 Marty <ma*****@earthl ink.netwrote:

Mike Meyer wrote:

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"?

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

No, python's equivalent of Perl's foreach is "for". I.e.

foreach $var (@list)

does the same thing as Python's

for var in list

(except Perl gets the scoping right).

Maybe you're thinking of Perls "default variable" feature (I don't
know what else to call it), which implicitly uses $_ as a variable in
any number of places if you fail to provide a variable? So that you
can say:

foreach (@list)

and apparently not have to use a variable, except it implicitly uses $_.

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

[Fredrik Lundh]

Paul Rubin wrote:

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

that's because (local) variables have near-zero cost, and zero overhead.
use as many as you want, and reuse them as often as you want to.

(and if you use sane naming conventions, the risk for collisions is near
zero as well).

</F>

[Paul Rubin]

Fredrik Lundh <fr*****@python ware.comwrites:

(and if you use sane naming conventions, the risk for collisions is
near zero as well).

I haven't felt that way, I'm always worried about clobbering something
by leaking a variable. Maybe collisions don't really happen much, but
it's always seemed cleaner to me to use the most restricted scopes
possible just to minimize or eliminate the possibility. This is
especially attractie in a language like Python, with no declarations
and no compile-time type safety.

[Neil Hodgson]

Marty:

I recently faced a similar issue doing something like this:

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

Another way to write this is
data_out = [[]] * len(data_in)

Neil

[Mike Meyer]

On Fri, 11 Jan 2008 22:18:22 GMT Neil Hodgson <ny************ *****@gmail.com wrote:

Marty:

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

Another way to write this is
data_out = [[]] * len(data_in)

Not quite. All the other variants give you 23 empty lists. That one
gives you 23 references to one list:

>>x = [[] for _ in range(23)]
x[1].append(23)
x

[[], [23], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], []]

>>x = [[]] * 23
x[1].append(23)
x

[[23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23], [23]]
<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.

[Tim Chase]

>I recently faced a similar issue doing something like this:

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

Another way to write this is
data_out = [[]] * len(data_in)

....if you're willing to put up with this side-effect:

>>data_in = range(10)
data_out = [[]] * len(data_in)
data_out

[[], [], [], [], [], [], [], [], [], []]

>>data_out[0].append('hello' )
data_out

[['hello'], ['hello'], ['hello'], ['hello'], ['hello'],
['hello'], ['hello'], ['hello'], ['hello'], ['hello']]

For less flakey results:

>>data_out = [[] for _ in data_in]
data_out

[[], [], [], [], [], [], [], [], [], []]

>>data_out[0].append('hello' )
data_out

[['hello'], [], [], [], [], [], [], [], [], []]
-tkc