Ensure value range for python list

Question

I want a function that takes a list and returns that list with any elements less than 0 or greater than "upper" removed.

I figured out how to do it with a list comprehension, but I can't figure out why this didn't work:

dim = 4

def ensure_values(l, upper=dim**2):
    for i in l:
        if i < 0 or i >= upper:
            l.remove(i)
    return l

l = [0,2,-3,5]

ensure_values(l)
[0,2,-3,5]

I expected [0,2,5].

Answer 1

Best practice in Python would be for your function to return a new list that contains only the desired elements. This is best done with a list comprehension.

dim = 4

def ensure_values(lst, upper=dim**2):
    return [n for n in lst if 0 < n <= upper]

lst = [0,2,-3,5]

assert ensure_values(lst) == [2, 5]

Note that I was able to use the expression 0 < n <= upper . That's not legal in C or most other languages, but in Python it is legal and does what one would expect it to do.

Also, I recommend against using l as a variable name. It looks almost exactly like a 1 or an I in many fonts. Recommended practice is to use lst or L for a short name for a list.

The use of l as a variable name is specifically dis-recommended in the PEP 8 coding style guidelines for Python. Here's a link:

http://legacy.python.org/dev/peps/pep-0008/#id29

Answer 2

You could hange your loop so it iterates over a copy of the list, which avoids modifying the list you're iterating over, because that won't work in many cases.

dim = 4

def ensure_values(l, upper=dim**2):
    for i in l[:]:  # iterate over copy of list
        if i < 0 or i >= upper:
            l.remove(i)
    return l

l = [0, 2, -3, 20, 5]

print ensure_values(l)  # -> [0, 2, 5]

A more "Pythonic" way to do it — because it's shorter and doesn't need a copy of the list — would be to use a list comprehension. Note the condition had to be reversed because it's now being used to determine when to keep elements.

l = [i for i in l if i >= 0 and i < dim**2]

print l  # -> [0, 2, 5]

Answer 3

Why not use a filter?

def ensure_values(l, upper=dim**2):
   return filter(lambda x: x>0 or x<upper, l)