def plus_grand(L):
    
    assert isinstance(L, list) and L != []
    
    plus_grand_tmp = L[0]
    for element in L:
        if element > plus_grand_tmp:
            plus_grand_tmp = element
            
    return plus_grand_tmp
    
    
    
assert plus_grand([5, 9, 12, 6, -1, 4]) == 12
assert plus_grand([-6, -19, -2]) == -2
#assert plus_petit([5, 9, 12, 6, -1, 4]) == -1
#assert plus_petit([-6, -19, -2]) == -19