def secondmax(iterable):
    """return the second largest value in iterable"""
    m = max(iterable)
    return max(i for i in iterable if i<m)

A = [[i+j for j in range(10)] for i in range(4)] # a matrix


print("List:")
col2_list = [a[2] for a in A]  # extract second column as list
print("col2 =",col2_list)
print("max =",max(col2_list))
print("secondmax =",secondmax(col2_list))
print("secondmax =",secondmax(col2_list))


print("\nGenerator:")
col2_gen  = (a[2] for a in A)  # extract as generator object
try:
    print("secondmax =",secondmax(col2_gen))
except ValueError as ex:
    print("ERROR in secondmax:",ex)



class ReusableGenerator():
    def __init__(self,g): self.g = g
    def __iter__(self):   return self.g()

col2_re = ReusableGenerator(lambda: (a[2] for a in A)) # I want this!
print("\nReusable:")
print("col2 =",col2_re)
print("max =",max(col2_re))
print("secondmax =",secondmax(col2_re))
print("secondmax =",secondmax(col2_re))