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Date: Thu, 5 Jun 2014 13:26:48 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20140605112648.GK27366@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] qemu-iotests: Test 0-length image for
 mirror
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To: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, jcody@redhat.com, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On Thu, Jun 05, 2014 at 11:42:36AM +0800, Fam Zheng wrote:
> +class TestSingleDriveZeroLength(TestSingleDrive):
> +    def setUp(self):
> +        TestSingleDrive.image_len = 0
> +        return TestSingleDrive.setUp(self)

This is buggy since it assigns to TestSingleDrive.image_len.  It
modifies the class variable for everyone else!

I think we need something like this instead:

  >>> class Foo(object):
  ...     a = 1
  ...     def test(self):
  ...             return self.a
  ...
  >>> Foo().a
  1
  >>> Foo().test()
  1
  >>> class Bar(Foo):
  ...     a = 2
  ...
  >>> Bar().a
  2
  >>> Bar().test()
  2

Please also fix the previous patch.  It uses the same pattern.