I'm finding myself typing the following a lot (developing for Django, if that's relevant):

if testVariable then:myVariable = testVariable
else:# something else

Alternatively, and more commonly (i.e. building up a parameters list)

if 'query' in request.POST.keys() then:myVariable = request.POST['query']
else:# something else, probably looking at other keys

Is there a shortcut I just don't know about that simplifies this? Something with the kind of logic myVariable = assign_if_exists(testVariable)?

Assuming you want to leave myVariable untouched to its previous value in the "not exist" case,

myVariable = testVariable or myVariable

deals with the first case, and

myVariable = request.POST.get('query', myVariable)

deals with the second one. Neither has much to do with "exist", though (which is hardly a Python concept;-): the first one is about true or false, the second one about presence or absence of a key in a collection.