Dynamically allocating memory in python

I've recently begun to dabble in python. I have come across a problem where I can think of no other solution than to have a list which is custom tailored to the size I need it to be at runtime. The code I have written this far opens a text file and reads through it line by line. Grabbing paths to images and flag codes as it goes. I want to store all the data in a list of classes so I can access each class by its index number. My map will be nothing more than an array of numbers (which refer to the index of the images in this "list of classes"). I thought about using a stack but I would have to traverse the the entire stack to find the right class. This operation needs to be done often. Is there a way I can allocate more space in my lists on the fly? Or is my mapping method just the problem? Perhaps I'm not seeing a much better way to store map data. -Kris