Python throws ValueError: list.remove(x): x not in list

Question

Every time I run this program, I get this error:

ValueError: list.remove(x): x not in list 

I am trying to lower the health of a single alien whenever it is hit by a bolt. That single alien should also be destroyed if its health is <= 0. Similarly, the bolt would also be destroyed. Here is my code:

def manage_collide(bolts, aliens):     # Check if a bolt collides with any alien(s)     for b in bolts:         for a in aliens:             if b['rect'].colliderect(a['rect']):                 for a in aliens:                     a['health'] -= 1                     bolts.remove(b)                     if a['health'] == 0:                         aliens.remove(a)     # Return bolts, aliens dictionaries     return bolts, aliens 

The ValueError happens on the line aliens.remove(a). Just to clarify, both the aliens and bolts are lists of dictionaries.

What am I doing wrong?

Answer 1

You should not remove items from a list you are looping over. Create a copy instead:

for a in aliens[:]: 

and

for b in bolts[:]: 

Modifying a list while looping over it, affects the loop:

>>> lst = [1, 2, 3] >>> for i in lst: ...     print i ...     lst.remove(i) ...  1 3 >>> lst [2] 

Removing items from a list you are looping over twice makes things a little more complicated still, resulting in a ValueError:

>>> lst = [1, 2, 3] >>> for i in lst: ...     for a in lst: ...         print i, a, lst ...         lst.remove(i) ...  1 1 [1, 2, 3] 1 3 [2, 3] Traceback (most recent call last):   File "<stdin>", line 4, in <module> ValueError: list.remove(x): x not in list 

When creating a copy of the lists you are modifying at each level of your loops, you avoid the problem:

>>> lst = [1, 2, 3] >>> for i in lst[:]: ...     for i in lst[:]: ...         print i, lst ...         lst.remove(i) ...  1 [1, 2, 3] 2 [2, 3] 3 [3] 

When you have a collision, you only need to remove the b bolt once, not in the loop where you hurt the aliens. Clean out the aliens separately later:

def manage_collide(bolts, aliens):     for b in bolts[:]:         for a in aliens:             if b['rect'].colliderect(a['rect']) and a['health'] > 0:                 bolts.remove(b)                 for a in aliens:                     a['health'] -= 1     for a in aliens[:]:         if a['health'] <= 0:             aliens.remove(a)     return bolts, aliens 

Answer 2

There is a bug in your code that is causing this. Your code, simplified, looks like:

for b in bolts:   for a in aliens:     for a in aliens:       bolts.remove(b) 

That is causing you to loop over aliens multiple times for every entry in b. If the b is removed on the first loop over aliens then, when it loops over it a second time, you will get there error.

A few things to fix. First, change in the inner loop over aliens to use something other than a, so:

for b in bolts:   for a in aliens:     for c in aliens:       if hit:         bolts.remove(b) 

Second, only remove b from bolts once. so:

for b in bolts:   for a in aliens:     should_remove = False     for c in aliens:       if hit:         should_remove = True     if should_remove:       bolts.remove(b) 

There are other issues with this code as well, I think, but that is the cause your main problem. Martijn's post may also help.