[SciPy-Dev] distributions.py expect

[nicky van foreest]

Hi,

I am trying to implement the expect method in rv_frozen. To understand
the normal working of the expect method I tried the following:

from scipy.stats import geom, norm, gamma

print norm.expect(loc = 3,scale =5)
print gamma.expect(None, 4.5)
print gamma.expect(lambda x: x, 4.5)
print geom.expect(lambda x: x, 1./3)

This is the result:

3.0
4.5
4.5
Traceback (most recent call last):
  File "expecttest.py", line 6, in <module>
    print geom.expect(lambda x: x, 1./3)
  File "/home/nicky/prog/scipy/scipy/stats/distributions.py", line
6375, in expect
    self._argcheck(*args) # (re)generate scalar self.a and self.b
TypeError: _argcheck() argument after * must be a sequence, not float

So the first examples work, but the rv_discrete example doesn't. One
thing is that the _argcheck is not called in rv_continuous while it is
in rv_discrete. Removing this line results in another error:

3.0
4.5
4.5
Traceback (most recent call last):
  File "expecttest.py", line 6, in <module>
    print geom.expect(lambda x: x, 1./3)
  File "/home/nicky/prog/scipy/scipy/stats/distributions.py", line
6394, in expect
    low, upp = self._ppf(0.001, *args), self._ppf(0.999, *args)
TypeError: _ppf() argument after * must be a sequence, not float

What would be actually the right way to call the expect method for
rv_discrete? Or perhaps the other way around, should this method be
changed so that my example with geom works?

----

I extended rv_frozen with this method, and this appears to work well
for continuous rvs, but also fails for discrete rvs.

    def expect(self, *args, **kwds):
        args += self.args
        kwds.update(self.kwds)
        return self.dist.expect(*args, **kwds)

Nicky