Strange behaviour with pytest + warnings

warnbug.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
Created on Wed Nov 14 01:39:53 2012

@author: Danilo J. S. Bellini
"""

from warnings import warn, catch_warnings
import pytest

class Warner(object):

  def __init__(self):
    self.data = [1, 2]

  def pop(self):
    return self.data.pop()

  def bad(self):
    """ Get warned, bro! """
    if self.data != []:
      warn("Blah", Warning)
    print "I was here!"

def test():
  bros = Warner()
  bros.pop()

  with catch_warnings(record=True) as warn_list:
    bros.bad()

  assert bros.data != []
  assert warn_list != []

# Put the code below inside a
#   if __name__ == "__main__":
# and the problem get solved...why?
pytest.main('"{0}"'.format(__file__))

warnbug_result.txt

============================= test session starts ==============================
platform linux2 -- Python 2.7.3 -- pytest-2.2.4
collecting ... collected 1 items

warnbug.py F

=================================== FAILURES ===================================
_____________________________________ test _____________________________________

    def test():
      bros = Warner()
      bros.pop()

      with catch_warnings(record=True) as warn_list:
        bros.bad()

      assert bros.data != []
>     assert warn_list != []
E     assert [] != []

warnbug.py:34: AssertionError
------------------------------- Captured stdout --------------------------------
I was here!
=========================== 1 failed in 0.02 seconds ===========================