[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH v3 02/16] qapi/expr.py: Check for dict instead of OrderedDict

From: John Snow
Subject: Re: [PATCH v3 02/16] qapi/expr.py: Check for dict instead of OrderedDict
Date: Thu, 25 Feb 2021 15:04:00 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 2/25/21 5:40 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:

On 2/24/21 4:30 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:

OrderedDict is a subtype of dict, so we can check for a more general
form. These functions do not themselves depend on it being any
particular type.

True.  The actual arguments can only be OrderedDict, though.  I think we
refrained from relaxing to dict in these helpers because we felt
"staying ordered" is clearer.

As a habit, I tend towards declaring the least specific type possible
for input and declaring the most specific type possible for output.

This maximimizes generality, which can be quite worthwhile.  Maximizing
generality by default is not a bad habit, I guess.  But cases exist
where generality is not needed, and other considerations take

We're *this* close to mooting the point, because

      Changed in version 3.7: Dictionary order is guaranteed to be
      insertion order. This behavior was an implementation detail of
      CPython from 3.6.


Is messing with it necessary for later work?  If not, is it a worthwhile

Not strictly necessary, but if the expression checkers here don't
*require* the type be an ordereddict, why bother to enforce that here?

It's just a bid to slacken the type (my type hints will look for Dict,
not OrderedDict) and leave the use of OrderedDict as an "implementation
detail" that only parser.py knows about.

"Orderedness" is anything but a detail only parser.py knows about.


     { 'command': 'blockdev-insert-medium',
       'data': { 'id': 'str',
                 'node-name': 'str'} }


     OrderedDict([('command', 'blockdev-insert-medium'),
                   OrderedDict([('id', {'type': 'str'}),
                                ('node-name', {'type': 'str'})]))])

For the inner dictionary, order matters, because the difference between

     void qmp_blockdev_insert_medium(const char *id, const char *node_name,
                                     Error **errp);


     void qmp_blockdev_insert_medium(const char *node_name, const char *id,
                                     Error **errp);


For the outer dictionary, order carries no semantic meaning.

My point is: parser.py fundamentally builds *ordered* dicts.  We're
certainly free to relax them to more general types wherever
"orderedness" is not needed.  However, one of the main aims of this
typing exercise is to make the code easier to read, and I doubt making
things more general helps there.

I primarily I saw the writing on the wall that we *will* be abandoning the use of OrderedDict and so I preferred to type in terms of just Dict, using the fact that Dict < OrderedDict anyway, asserting that parser.py uses OrderedDict as an "implementation detail".

Later, parser.py may abandon its use of OrderedDict without changes to the rest of the code.

The alternative is to use OrderedDict everywhere here in expr.py, but I would *prefer* not to, as it will inhibit prototyping and experimentation efforts where we might use a parser that doesn't use OrderedDict.

What I absolutely did not want to do was type in terms of Dict[str, object] but then use isinstance checks for OrderedDict.

My preference is still, I think, to just go all-in on dict. I am personally comfortable trusting that parser.py creates an ordered implementation of the type.

As for these specific checks:

- normalize_members doesn't assert that it has an OrderedDict, it only normalizes *if* it gets one. I don't think this is helpful behavior.

- check_type has an error message that doesn't square with the check: we can give it a dict and it will pretend like we didn't give it one. I don't think that's helpful either.

Related: the type aliases for the AST you define later in this series.
I figure relaxing these to more general types where possible would
actually reduce their value.  TopLevelExpression tells me more than

I'm not against relaxing types per se.  Judicious relaxation is often
useful to keep code more generally useful.  When to relax isn't always

(I needed to change it for prototyping using an off-the-shelf parser, so
it was annoying to have it check for a stronger type if it doesn't
absolutely have to.)

If your off-the-shelf parse doesn't preserve order, it's not fit for the
purpose :)

It does, but in 3.6 that might be relying on CPython details. This is a pretty frustrating place to be in, support-wise.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>

reply via email to

[Prev in Thread] Current Thread [Next in Thread]