PL/Python: How do I use result methods?

I'm having my first try at a PL/Python function. It is also a first try
at Python, so the answer to this may be obvious.

According to the docs (8.0):

Calling plpy.execute with a query string and an optional limit
argument causes that query to be run and the result to be
returned in a result object. The result object emulates a list
or dictionary object. The result object can be accessed by row
number and column name. It has these additional methods: nrows
which returns the number of rows returned by the query, and
status which is the SPI_execute() return value.

What exactly is the syntax for using the nrows and status methods?

I am trying to use it like this:

...
arec = plpy.execute(plan, [ addrno ], 1)
plpy.info("Executed plan")
n = plpy.nrows(arec)
plpy.info(" got nrows")
...

This gives the output:
advacs=# select core.formatted_address(1);
INFO: ('Prepared plan',)
INFO: ('Executed plan',)
ERROR: plpython: function "formatted_address" failed
DETAIL: exceptions.AttributeError: 'module' object has no attribute 'nrows'

If I use
n = arec.nrows

the message becomes
DETAIL: exceptions.SystemError: error return without exception set

so how do you use it?

--
Oliver Elphick olly(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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"But be ye doers of the word, and not hearers only,
deceiving your own selves." James 1:22