libvirt-python: API change List → (Named)Tuple?
Hello,
Am 25.07.20 um 23:45 schrieb Philipp Hahn:
Am 27.04.20 um 15:44 schrieb Philipp Hahn:
> I'm working on adding PEP 484 type hints
> <https://www.python.org/dev/peps/pep-0484/> to the Python binding of
> libvirt.
...
I just opened a merge request
<https://gitlab.com/libvirt/libvirt-python/-/merge_requests/9> for my
code at <https://gitlab.com/pmhahn/libvirt-python/-/tree/typing>;.
While working on that I stumbled over some annoyances in the current
Python API: There are many methods which return a List[int], for example:
virDomainGetInfo
virDomainGetState
virDomainGetControlInfo
virDomainGetBlockInfo
virDomainGetJobInfo
virStoragePoolGetInfo
virStorageVolGetInfo
virStorageVolGetInfoFlags
There are more function returning similar information as plain List:
virNodeGetSecurityModel
virNodeGetSecurityModel
virDomainGetSecurityLabelList
virDomainBlockStats
virDomainInterfaceStats
virDomainGetVcpus
virNodeGetCPUMap
The worst offender is `virNodeGetInfo`, which returns `Tuple[str,
int*7]`: The problem for type annotation is that `List` is unlimited in
the number of elements, that is you cannot specify the number of entries
the list must or will contain.
Also all elements of a list must have the same (super-)type; for "str"
and "int" of "virNodeGetInfo()" that would be "Any", which
is not very
useful here.
A better type for those `List`s would be `Tuple`, which has a fixed
length and can have different types for each position.
But that would be an API change: In most cases users of those functions
should not notice the difference as indexing tuples or lists is the same.
It would break code where someone would do something like this:
ret = virFunction_returning_list()
ret += [1, 2, 3]
which breaks if that function would return a `Tuple` in the future:
$ python -c '() + []'
Traceback (most recent call last):
File "<string>", line 1, in <module>
TypeError: can only concatenate tuple (not "list") to tuple
Even better then plain `Tuple`s would be
<https://docs.python.org/3/library/collections.html#collections.namedtuple>;,
which would allow us to use the return value of `virNodeGetInfo()` as this:
from collections import namedtuple
import libvirt
virNodeInfo = namedtuple("virNodeInfo", ["model", "memory",
"cpus", "mhz", "nodes", "sockets",
"cores", "threads"])
c = libvirt.open('test:///default')
info = virNodeInfo(*c.getInfo())
print(info.model)
print(info)
# virNodeInfo(model='i686', memory=3072, cpus=16, mhz=1400, nodes=2, sockets=2,
cores=2, threads=2)
<https://libvirt.org/html/libvirt-libvirt-host.html#virNodeInfo>
This could be improved even more by using
<https://mypy.readthedocs.io/en/stable/kinds_of_types.html?highlight=Named...;,
which allows to add type information:
from typing import NamedTuple
virNodeInfo = NamedTuple("virNodeInfo", [("model", str),
("memory", int), ("cpus", int), ("mhz", int),
("nodes", int), ("sockets", int), ("cores", int),
("threads", int)])
or with Python 3.6 the more readable form
class virNodeInfo(NamedTuple):
model: str
memory: int
cpus: int
mhz: int
nodes: int
sockets: int
cores: int
threads: int
IMHO that would be a real usability improvement as I have to lookup that
information every time I have to use those functions myself.
Indexing with `namedtuple` and `NamedTuple` still works, so you can
still use `info[0]` above to get the model.
What do you think of that?
When would be a good time to make such a change?
Philipp