PL/Python prepare example's use of setdefault

I was just reading the PL/Python docs section "42.7.1 Database Access
Functions" and saw this example:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
    plan = SD.setdefault("plan", plpy.prepare("SELECT 1"))
    # rest of function
$$ LANGUAGE plpythonu;

The above example uses the plpy.prepare() function, reusing the result
across function calls uses setdefault(). Unfortunately, since
setdefault() is a method on dict objects, the values passed to it must
be evaluated before it can be called. Therefore, plpy.prepare() will be
called every time usesavedplan() executes whether a result already
exists in the SD dict or not.

I'm not sure if it's a problem that plpy.prepare() is called every time
since the result is discarded if a prepared statement had been cached by
a previous execution of usesavedplan(). It seems that some wasted
processing will occur, but maybe not enough to matter. The documentation
for SPI_prepare() does not clearly state what tasks that function
performs other than constructing a prepared statement object. It seems
to imply that parsing does occur within SPI_prepare(). It does state
that query planning occurs within SPI_execute_plan().

Can anyone clarify what occurs when plpy.prepare() is called? Is it
worth using a Python conditional to determine whether to call it rather
than using SD.setdefault()?
--
Jonathan Ross Rogers