Question

I’m a beginner at Python and I have a short question which I haven’t been able to find a solution to:

Is there any way to regulate what is overwritten in a list? For instance, I have a list filled with zeros which I will then gradually fill up with other elements, what I want to be able to do is to create an error when something other than a zero is overwritten. Is there any smart way to do this?

Could I use something like:

a = [0, 1, 0, 1, 0, 0, 0, 0]
b = []
[i for i, e in enumerate(a) if e != 0]
return False 

or something similar?

Answer 1

You can use a function to change the elements in the list, which checks if the element is 0.

def setElement(l, index, element):
    '''Change the element from given list(l) at given index.'''
    if l[index] != 0:
        raise Exception("Attempt to overwrite %s instead of 0" %l[index])
    else:
        l[index] = element

And now you can use it by calling setElement(<list>, <index>, <element>)

 In[1]: a = [0, 0, 0, 0, 0, 0, 0]

 In[2]: setElement(a, 2, 3)

 In[3]: setElement(a, len(a)-1, "Last Element!")

 In[4]: setElement(a, len(a)-1, 53)
Out[4]: Attempt to overwrite "Last Element!" instead of 0

 In[5]: print(a)
Out[5]: [0, 0, 3, 0, 0, 0, "Last Element"]