[Matplotlib-users] How to save image without showing?

First of all, one of the easiest graph package yet fill with tons of 
capability and future potential. 

Now my issue:

I am trying to save an image without showing it. Here is small program

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from matplotlib.matlab import *
plot([1,2,3])
savefig('test.png')
#show()

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This does not create test.png file, but if I uncomment last line, it create 
test.png file. How can I create a test.png file without showing it?