Check if list contains only item x

If all items in the list are hashable:

set(A) == set([x])

This checks that all elements in A are equal to x without reference to any other variables:

all(element==x for element in A)

A=[w,y,x,z]
all(p == x for p in A)

That, or if you don't want to deal with a loop:

>>> a = [w,x,y,z]
>>> a.count(x) == len(a) and a

(and a is added to check against empty list)