[python] Is it possible to implement a Python for range loop without an iterator variable?

Is it possible to do following without the i?

for i in range(some_number):
    # do something

If you just want to do something N amount of times and don't need the iterator.

What about:

while range(some_number):
    #do something

#Return first n items of the iterable as a list
list(itertools.islice(iterable, n))

Taken from http://docs.python.org/2/library/itertools.html

You may be looking for

for _ in itertools.repeat(None, times): ...

this is THE fastest way to iterate times times in Python.

I generally agree with solutions given above. Namely with:

1. Using underscore in for-loop (2 and more lines)
2. Defining a normal while counter (3 and more lines)
3. Declaring a custom class with __nonzero__ implementation (many more lines)

If one is to define an object as in #3 I would recommend implementing protocol for with keyword or apply contextlib.

Further I propose yet another solution. It is a 3 liner and is not of supreme elegance, but it uses itertools package and thus might be of an interest.

from itertools import (chain, repeat)

times = chain(repeat(True, 2), repeat(False))
while next(times):
    print 'do stuff!'

In these example 2 is the number of times to iterate the loop. chain is wrapping two repeat iterators, the first being limited but the second is infinite. Remember that these are true iterator objects, hence they do not require infinite memory. Obviously this is much slower then solution #1. Unless written as a part of a function it might require a clean up for times variable.

May be answer would depend on what problem you have with using iterator? may be use

i = 100
while i:
    print i
    i-=1

or

def loop(N, doSomething):
    if not N:
        return
    print doSomething(N)
    loop(N-1, doSomething)

loop(100, lambda a:a)

but frankly i see no point in using such approaches

If you really want to avoid putting something with a name (either an iteration variable as in the OP, or unwanted list or unwanted generator returning true the wanted amount of time) you could do it if you really wanted:

for type('', (), {}).x in range(somenumber):
    dosomething()

The trick that's used is to create an anonymous class type('', (), {}) which results in a class with empty name, but NB that it is not inserted in the local or global namespace (even if a nonempty name was supplied). Then you use a member of that class as iteration variable which is unreachable since the class it's a member of is unreachable.

Here's a random idea that utilizes (abuses?) the data model (Py3 link).

class Counter(object):
    def __init__(self, val):
        self.val = val

    def __nonzero__(self):
        self.val -= 1
        return self.val >= 0
    __bool__ = __nonzero__  # Alias to Py3 name to make code work unchanged on Py2 and Py3

x = Counter(5)
while x:
    # Do something
    pass

I wonder if there is something like this in the standard libraries?

The general idiom for assigning to a value that isn't used is to name it _.

for _ in range(times):
    do_stuff()

Instead of an unneeded counter, now you have an unneeded list. Best solution is to use a variable that starts with "_", that tells syntax checkers that you are aware you are not using the variable.

x = range(5)
while x:
  x.pop()
  print "Work!"

We have had some fun with the following, interesting to share so:

class RepeatFunction:
    def __init__(self,n=1): self.n = n
    def __call__(self,Func):
        for i in xrange(self.n):
            Func()
        return Func


#----usage
k = 0

@RepeatFunction(7)                       #decorator for repeating function
def Job():
    global k
    print k
    k += 1

print '---------'
Job()

Results:

0
1
2
3
4
5
6
---------
7

If do_something is a simple function or can be wrapped in one, a simple map() can do_something range(some_number) times:

# Py2 version - map is eager, so it can be used alone
map(do_something, xrange(some_number))

# Py3 version - map is lazy, so it must be consumed to do the work at all;
# wrapping in list() would be equivalent to Py2, but if you don't use the return
# value, it's wastefully creating a temporary, possibly huge, list of junk.
# collections.deque with maxlen 0 can efficiently run a generator to exhaustion without
# storing any of the results; the itertools consume recipe uses it for that purpose.
from collections import deque

deque(map(do_something, range(some_number)), 0)

If you want to pass arguments to do_something, you may also find the itertools repeatfunc recipe reads well:

To pass the same arguments:

from collections import deque
from itertools import repeat, starmap

args = (..., my args here, ...)

# Same as Py3 map above, you must consume starmap (it's a lazy generator, even on Py2)
deque(starmap(do_something, repeat(args, some_number)), 0)

To pass different arguments:

argses = [(1, 2), (3, 4), ...]

deque(starmap(do_something, argses), 0)

What everyone suggesting you to use _ isn't saying is that _ is frequently used as a shortcut to one of the gettext functions, so if you want your software to be available in more than one language then you're best off avoiding using it for other purposes.

import gettext
gettext.bindtextdomain('myapplication', '/path/to/my/language/directory')
gettext.textdomain('myapplication')
_ = gettext.gettext
# ...
print _('This is a translatable string.')

You can use _11 (or any number or another invalid identifier) to prevent name-colision with gettext. Any time you use underscore + invalid identifier you get a dummy name that can be used in for loop.