[Tutor] How to get a key from dictionary?

[A]

Hi,
Is there a possibility to get, from a dictionary, a key according to a 
value ?
For example
I have a dictionary

dict={'aa':1,'bb':2}

and 
dict['aa']
is 1

But how can I for value 1 find out  key? (That is here  'aa')

Thank you for help
Ladislav