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Re: [PATCH v10 09/10] qcow2_format.py: collect fields to dump in JSON fo

From: Andrey Shinkevich
Subject: Re: [PATCH v10 09/10] qcow2_format.py: collect fields to dump in JSON format
Date: Thu, 16 Jul 2020 18:52:31 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 16.07.2020 18:40, Vladimir Sementsov-Ogievskiy wrote:
16.07.2020 18:34, Andrey Shinkevich wrote:
On 16.07.2020 13:24, Vladimir Sementsov-Ogievskiy wrote:
14.07.2020 00:36, Andrey Shinkevich wrote:
As __dict__ is being extended with class members we do not want to
print, make a light copy of the initial __dict__ and extend the copy
by adding lists we have to print in the JSON output.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
  tests/qemu-iotests/qcow2_format.py | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/tests/qemu-iotests/qcow2_format.py b/tests/qemu-iotests/qcow2_format.py
index e0e14b5..83c3482 100644
--- a/tests/qemu-iotests/qcow2_format.py
+++ b/tests/qemu-iotests/qcow2_format.py
@@ -109,6 +109,8 @@ class Qcow2Struct(metaclass=Qcow2StructMeta):
          self.__dict__ = dict((field[2], values[i])
                               for i, field in enumerate(self.fields))
  +        self.fields_dict = self.__dict__.copy()

No, I don't like that. Keeping two copies of all the data is bad idea. If you want to select some fields, add a method (dump_json() ?) Which will collect the fields you want in a dict and return that new dict. But don't store object attributes twice.

That is what I did in the versions before but it looks clumsy to me. Every single class lists almost all the items of the __dict__ again in the additional method.


Not really. It makes a light copy that stores only references to the desired fields.

I'm not about memory consumption, I'm about the design. Keeping two representations of same thing is always painful to maintain.

      def dump(self, dump_json=None):
          for f in self.fields:
              value = self.__dict__[f[2]]
@@ -144,6 +146,7 @@ class Qcow2BitmapExt(Qcow2Struct):
          self.bitmap_directory = \
              [Qcow2BitmapDirEntry(fd, cluster_size=self.cluster_size)
               for _ in range(self.nb_bitmaps)]
+ self.fields_dict.update(bitmap_directory=self.bitmap_directory)
        def dump(self, dump_json=None):
@@ -189,6 +192,7 @@ class Qcow2BitmapDirEntry(Qcow2Struct):
          table = [e[0] for e in struct.iter_unpack('>Q', fd.read(table_size))]
          self.bitmap_table = Qcow2BitmapTable(raw_table=table,
+ self.fields_dict.update(bitmap_table=self.bitmap_table)
        def dump(self, dump_json=None):
          print(f'{"Bitmap name":<25} {self.name}')

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