class FooObj:
    def __init__(self, i, a):
        self.i = i
        self.a = a
 
    def rem(self, list): #Doesn't work correctly
        list.remove(self)
 
    def __repr__(self):
        return "(obj " + str(self.i) + ", active = " + str(self.a) + ")"
 
 
if __name__ == "__main__":
    o1 = FooObj(1, 1)
    o2 = FooObj(2, 0)
    o3 = FooObj(3, 0)
    o4 = FooObj(4, 1)
    o5 = FooObj(5, 1)
 
    #Test 1
    lst = [o1, o2, o3, o4, o5]
 
    #Trying to remove objects flagged as inactive (a = 0)
    for obj in lst:
        if obj.a == 0:
            lst.remove(obj)
 
    #Prints: [(obj 1, active = 1), (obj 3, active = 0), (obj 4, active = 1), (obj 5, active = 1)]
    #^obj 3 is wrong! All objs with a=0 should be removed.
    print lst 
 
 
    #Test 2
    lst = [o1, o2, o3, o4, o5]
 
    #Trying to remove all objects. I need to be able to call a function of the object for each object in the loop.
    for obj in lst:
        obj.rem(lst)
 
    #Prints: [(obj 2, active = 0), (obj 4, active = 1)]
    #completely wrong
    print lst

In [1]: lst = ["a", "b", "b", "d"]
 
In [2]: for elem in lst:
  ....:     print "before: ", lst      # check the list
  ....:     print elem, lst.index(elem)   # check which element we are using
  ....:     if elem == "b":             # remove if element is a "b"
  ....:         lst.remove(elem)
  ....:     print "after: ", lst      # check list again
  ....:     print "-" * 20
  ....:     
before:  ['a', 'b', 'b', 'd']
a 0          
after:  ['a', 'b', 'b', 'd']
--------------------
before:  ['a', 'b', 'b', 'd']
b 1
after:  ['a', 'b', 'd']
--------------------
before:  ['a', 'b', 'd']
d 2
after:  ['a', 'b', 'd']
--------------------

before:  ['a', 'b', 'b', 'd']
a 0          
after:  ['a', 'b', 'b', 'd']

before:  ['a', 'b', 'b', 'd']
b 1
after:  ['a', 'b', 'd']

before:  ['a', 'b', 'd']
d 2
after:  ['a', 'b', 'd']

lst2 = [ elem for elem in lst if elem != "b" ]

for item in mylist[:]:
  if item_is_a_reject():  #whatever test you need to run goes here
     mylist.remove(item)

In [1]: def valid( elem ):   # very complicated function
   ...:     if elem < 0: return False
   ...:     return True

In [2]: lst = [-3, 2, -5, -10, 12]
In [3]: [ elem for elem in lst if valid(elem) ]
Out[3]: [2, 12]

In [4]: filter(valid, lst)
Out[4]: [2, 12]