pickle.PicklingError: Function to be pickled has free variables that are referenced before assignment in enclosing scope

The code I had that caused this problem looked like this:

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if x :
    y = x

def foo() :
    print y

cloud.pickle(foo)

If this were a normal call to foo, the error instead would be:

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UnboundLocalError: local variable 'y' referenced before assignment

The fix is to make sure that all variables used in foo have values at the time that it is pickled:

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if x :
    y = x
else :
    y = None

def foo() :
    print y

cloud.pickle(foo)

Even this would be ok:

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def foo() :
    print y

if x :
    y = x
else :
    y = None

cloud.pickle(foo)