python - Why is list.remove only removing every second item? -

in python 2.7.2 idle interpreter:

>>> mylist = [1, 2, 3, 4, 5] >>> item in mylist:         mylist.remove(item)  >>> mylist [2, 4] 

why?

it's because when iterate on list, python keeps track of index in list. consider following code instead:

for in range(len(mylist)):     if >= len(mylist):        break     item = mylist[i]     mylist.remove(item) 

if track (which python doing in code), see when remove item in list, number right shifts 1 position left fill void left when removed item. right item @ index i , never seen in iteration because next thing happens increment i next iteration of loop.

---

now little clever. if instead iterate on list backward, we'll clear out list:

for item in reversed(mylist):     mylist.remove(item) 

the reason here we're taking item off end of list @ each iteration of loop. since we're taking items off end, nothing needs shift (assuming uniqueness in list -- if list isn't unique, result same, argument gets bit more complicated).

of course, if you're looking remove items list, can easily:

del mylist[:] 

or slice assignment:

mylist[:] = [] 

(i mention latter because can useful replace segments of list other items don't need same length).