A bit mask is just a variable that aids you with bitwise operations. A bit mask can help you turn specific bits on, turn others off, or just collect data from an integer about which bits are on or off.

num  = 0b1100
mask = 0b0100
desired = num & mask
if desired > 0:
  print "Bit was on"

In the example above, we want to see if the third bit from the right is on.

1. First, we first create a variable num containing the number 12, or 0b1100.
2. Next, we create a mask with the third bit on.
3. Then, we use a bitwise-and operation to see if the third bit from the right of num is on.
4. If desired is greater than zero, then the third bit of num must have been one.