Hello!

Is it intentional, that the default argument is ALWAYS evaluated, even if it is not needed???

Is it not a bug, that this method has no short-circuit eval (http://en.wikipedia.org/wiki/Short-circuit_evaluation)
??

Example1:
=========
infinite = 1e100

one_div_by = {0.0 : infinite}

def func(n):
    return one_div_by.setdefault(float(n), 1/float(n))

for i in [1, 2, 3, 4]:
    print i, func(i)
print one_div_by
# works!!

for i in [0, 1, 2, 3, 4]:     # added 0 -> FAIL!
    print i, func(i)
print one_div_by
# fail!!



Example2:
=========
fib_d = {0 : 0, 1 : 1}

def fibonacci(n):
    return fib_d.setdefault(n, fibonacci(n-1) + fibonacci(n-2))

for i in range(10):
    print i, fibonacci(i)
print fib_d

setdefault() is a method, its arguments are evaluated then the function is called. This is not a bug, and this behavior cannot change.

If you are trying to "cache" the computation of a function, you should try "memoizing" techniques, like the one mentioned here: http://code.activestate.com/recipes/52201-memoizing-cacheing-function-return-values/
Then you can write::

    @Memoize
    def fib(n):
        return fib(n-1) + fib(n-2)
    fib.memo = {(0,): 1, (1,): 1}

    @Memoize
    def func(n):
        return 1/float(n)
    func.memo = {(0.0,): infinite}

Or use a collections.defaultdict, which has a factory function as a constructor argument. It sort of depends on what you're trying to do.