It's a common pythonic idiom, from http://docs.python.org/2/glossary.html?highlight=eafp#term-eafp 

    Easier to ask for forgiveness than permission. This common Python coding style 
    assumes the existence of valid keys or attributes and catches exceptions if the 
    assumption proves false. This clean and fast style is characterized by the presence 
    of many try and except statements. The technique contrasts with the LBYL style 
    common to many other languages such as C.