Feb. 23 - 27

The reading assignments for this week are still from the online "DB Design" text. One of the more interesting Python examples we've looked at this week was about function defaults. Take a look at this example:

def h1 (y = []) :
assert(type(y) is types.ListType)
y += [2]
return y

Now, if I were to make these two calls:

h = h1()
h = h1()

h would be equal to [2, 2]
This mutable default array lives on as a sort of memory for the function, which isn't exactly what I would have expected.

Now, if I were to make another call to h1([1]) with an argument, the default array gets wiped away. To be clear, at the end of this sequence of statements:

h = h1()
h = h1()
h = h1([1])

h now equals [1, 2]

and one final call:

h = h1()

results in h equal to [2, 2, 2]

So it seems that the persistent array is actually persistent and surfaces any time an argument isn't give.

Although this strange behaviour might introduce a bug into an unsuspecting programmer's function, it can also be a very attractive way of giving a function some persistent storage...possibly a useful feature if you wanted to write something like a primes generator.