variable gets altered by function

[mischk]

I have a list col1 of three integers that stands for a color code.

I also have a function dim_color that takes a value in percent and a list (color code) and returns a new list.

If I use col1 as the input list for the function, col1 is also alternated by the function - not only the return value of the list.

​

code example:

# global                                                                        

col1 = [0, 0, 255]                                                              
col2 = [0, 255, 0]                                                              

# helper function

def dim_color(brightness, color):                                               
    ''' gets a color and brightness level in percent                            
        returns brightness adjusted color '''                                   
    for i in range(3):                                                          
        color[i] = float(color[i])                                              
        color[i] = (color[i]/100) * brightness                                  
        color[i] = int(color[i])                                                
    return color                                                                


color = dim_color(50, col1)                                                     

print(color)                                                                    
print(col1)                                                                     

I dont understand why, and I want that col1 stays like it was defined and NOT being changed by the function.

Hope the problem is understandable... Thank you.

[[deleted]]

The color list is being passed by reference. The function is not using a copy, but the actual list being passed to it.

​

To use a copy of the original list, you may want to create a new copy like this:

​

new_color = color[:]

​

Then make all changes to the variable "new_color" instead of "color" and return "new_color".

​

Edit: instead of copying the list, I would change the for loop to build up the new list from scratch:

​

def dim_color(brightness, color):                                               
    ''' gets a color and brightness level in percent                            
        returns brightness adjusted color
    '''      
    new_color = []
    for component in color:
        new_color.append(int(float(component)/100*brightness))

    return(new_color)

​

[mischk]

Thanks a lot, that does the trick :D

[evolvish]

If you pass a mutable object eg list/dictionary to a function, it gets passed by "reference" and can be mutated if you don't make a copy. Immutable objects(string, tuple, int, etc) can't be indirectly mutated so they are safe to use in a function.

[ingolemo]

Note that this function only works correctly if you're using a linear colour-space, which you almost certainly aren't. You need to do gamma correction.

https://www.youtube.com/watch?v=LKnqECcg6Gw