how to create a list of lists

My Python code generates a list everytime it loops:

list = np.genfromtxt('temp.txt', usecols=3, dtype=[('floatname','float')], skip_header=1)

But I want to save each one - I need a list of lists right?

So I tried:

list[i] = np.genfromtxt('temp.txt', usecols=3, dtype=[('floatname','float')], skip_header=1)

But Python now tells me that "list" is not defined. I'm not sure how I go about defining it. Also, is a list of lists the same as an array??

Thank you!