by using numbers as subscriptsmathematicians are able to succinctly summarize computations over items in the sequence using subscript variables for examplethe sum of the sequence is written using standard summation notation as - si = similar idea can be applied to computer programs with listwe can use single variable to represent an entire sequenceand the individual items in the sequence can be accessed through subscripting wellalmostwe don' have way of typing subscriptsbut we use indexing instead suppose that our sequence is stored in variable called we could write loop to calculate the sum of the items in the sequence like thissum for in range( )sum sum [ivirtually all computer languages provide some sort of sequence structure similar to python' listin other languagesit is called an array to summarizea list or array is sequence of items where the entire sequence is referred to by single name (in this casesand individual items can be selected by indexing ( [ ]arrays in other programming languages are generally fixed size when you create an arrayyou have to specify how many items it will hold if you don' know how many items you will havethen you have to allocate large arrayjust in caseand keep track of how many "slotsyou actually fill arrays are also usually homogeneous that means they are restricted to holding objects of single data type you can have an array of ints or an array of strings but cannot mix strings and ints in single array in contrastpython lists are dynamic they can grow and shrink on demand they are also heterogeneous you can mix arbitrary data types in single list in nutshellpython lists are mutable sequences of arbitrary objects list operations because lists are sequencesyou know that all of the python built-in sequence operations also apply to lists to jog your memoryhere' summary of those operationsoperator len(for in in meaning concatenation repetition indexing length slicing iteration membership check (returns boolean