Parameters:

PY,

a = 1 # this value can be changed

MZN,

int:a = 1; # this value is fixed


Variables:


PY,

a = 1 (implicit, 'a' can be either a var or parameter)

MZN,

var int:a; % this value can be changed


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Array

It is a collection of data, each data is characterized by an index and a value.
By default, the index is a sequence of consecutive integers.
Such sequence starts from 0 in Python; starts from 1 in MZN.
In python, an array (usually called as list) can contain any kinds of data.
In contrast, the data type of MZN array must be homogeneous, and it should be declared a priori. 
You need to say, it is an array of parameters or variables; it is an array of integers or string.

PY, 

A = [] # In Python you can declare empty list (array);
A = [1,5] # or a list with instantiated values

MZN,

array[1..2] of int:A; % an array of parameters (whose values will be instantiated and fixed)
array[1..2] of var int:A % an array of variables
% it is meaningless to declare empty array in mzn,
% in CP (declarative language), when you use something, you must describe
% what it is (e.g. the index range, the type of elements it contain)


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Sum a fraction of an array:

PY,

theSum = sum(A[1:3]) # from the second to the third element (included)

MZN,

var int : theSum = sum(i in 1..3)(A[i]) % from the first to the third element (included)