Matlab-style mask in Python??

[Ozone Hole near South Pole]

Hi,

I am a long time Matlab user.  I just picked up Python for a short
while.
I wonder what should be the best way to do sth like:

Matlab:
a=[3,4,6,7,2,54,2,1,2]
idx=a>4      ans: [0,0,1,1,0,1,0,0,0]
a(idx)         ans: [6,7,54]

Python:
a=[3,4,6,7,2,54,2,1,2]
idx=map(lambda x: x>4,a) #assume this calculation is lengthy
                         #and we don't want to repeat
last cmd???

While we can always put that into a for loop, then append the ans
element-by-element....  I feel there should be an easier way in
Python.
Any suggestions?