Can you have a look?

The way to customize the disks of the template is to use the 'template.vm.disk_attachments.disk' attribute, not 'template.disk_attachments.disk'. For example, if you are using directly the API:

  POST /ovirt-engine/api/templates

  <template>
    <name>mytemplate</name>
    <vm id="0957c87d-c933-4d28-9ce4-635fa4d8c711">
      <disk_attachments>
        <disk_attachment>
          <disk id="db7594bd-102b-4a72-8c90-900f3609f0eb">
            <format>cow</format>
            <sparse>true</sparse>
          </disk>
        </disk_attachment>
      </disk_attachments>
    </vm>
  </template>

With the Python SDK it should be something like this:

---8<---
# Find the original virtual machine:
vms_service = system_service.vms_service()
vm = vms_service.list(search='name=myvm')[0]

# Get the identifiers of the disks attached to the virtual machine. We
# need this because we want to tell the server to create the disks of
# the template using a format different to the format used by the
# original disks.
attachments = connection.follow_link(vm.disk_attachments)
disk_ids = [
    attachment.disk.id
    for attachment in attachments
]

# Send the request to create the template. Note that the way to specify
# the original virtual machine, and the customizations, is to use the
# 'vm' attribute of the 'Template' type. In the customization we
# explicitly indicate that we want COW disks, regardless of what format
# the original disks had.
templates_service = system_service.templates_service()
template = templates_service.add(
    template=types.Template(
        name='mytemplate',
        vm=types.Vm(
            id=vm.id,
            disk_attachments=[
                types.DiskAttachment(
                    disk=types.Disk(
                        id=disk_id,
                        sparse=True,
                        format=types.DiskFormat.COW
                    )
                )
                for disk_id in disk_ids
            ]
        )
    )
)
--->8---

Please try that. If it works correctly I think you can close the bug as NOTABUG.

So that the attachments must be set in template.vm.disk_attachments and not in template.disk_attachments. I would never have guessed it.

But it works, thanks a lot!

I admit that is confusing. I am trying to clarify that in the specification of the API:

  Document template disk customization
  https://gerrit.ovirt.org/75272

I kindly invite you all to participate in the improvement of that documentation. when you have a question, and you find the answer somewhere other than the documentation, please consider updating it.

Hi Juan,

Much appreciated your DOC improvement efforts! It's much better now and the examples are also very useful.

(In reply to Juan Hernández from comment #4)
> I kindly invite you all to participate in the improvement of that
> documentation. when you have a question, and you find the answer somewhere
> other than the documentation, please consider updating it.
Thanks, I will.

There is one more thing though, I think I found a bug related to this. Since it's not the exact same subject of this BZ (DOCs) I decided to open BZ#1439970.