Os command output in variable shows wrong value

[paulo79]

I have this :

v_num_distinct_phy_disks_in_cells=os.system('cat /tmp/p_count_num_phy_disks_all_cell | sort -u | wc -l')

The content of file is :

cat /tmp/p_count_num_phy_disks_all_cell
12
12
12

If I run manually it shows correct as value of distinct number is 1:

cat /tmp/p_count_num_phy_disks_all_cell | sort -u | wc -l
1

If I run in Python I get this:

>>> v_num_distinct_phy_disks_in_cells=os.system('cat /tmp/p_count_num_phy_disks_all_cell | sort -u | wc -l')
1
>>> print(v_num_distinct_phy_disks_in_cells)
-1

Why it is showing -1 ?