Re: [PATCH v3 03/16] qapi/expr.py: constrain incoming expression types

[John Snow <jsnow@redhat.com>]

On 2/24/21 5:01 AM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
> 
>> mypy does not know the types of values stored in Dicts that masquerade
>> as objects. Help the type checker out by constraining the type.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>> Reviewed-by: Cleber Rosa <crosa@redhat.com>
>> ---
>>   scripts/qapi/expr.py | 25 ++++++++++++++++++++++---
>>   1 file changed, 22 insertions(+), 3 deletions(-)
>>
>> diff --git a/scripts/qapi/expr.py b/scripts/qapi/expr.py
>> index 5694c501fa3..783282b53ce 100644
>> --- a/scripts/qapi/expr.py
>> +++ b/scripts/qapi/expr.py
>> @@ -15,9 +15,17 @@
>>   # See the COPYING file in the top-level directory.
>>   
>>   import re
>> +from typing import MutableMapping, Optional
>>   
>>   from .common import c_name
>>   from .error import QAPISemError
>> +from .parser import QAPIDoc
>> +from .source import QAPISourceInfo
>> +
>> +
>> +# Expressions in their raw form are JSON-like structures with arbitrary forms.
>> +# Minimally, their top-level form must be a mapping of strings to values.
>> +Expression = MutableMapping[str, object]
> 
> MutableMapping, fancy.  It's only ever dict.  Why abstract from that?
> 

I don't know! I referenced this in the cover letter. I cannot remember 
the reason anymore. It had R-Bs on it so I left it alone.

There are some differences, but I no longer remember why I thought they 
applied. Maybe some of my more exploratory work wanted it. Dunno.

> The use of object is again owed to mypy's inability to do recursive
> types.  What we really have here is something like
> 
>     Expression = Union[bool, str, dict[str, Expression], list[Expression]]
> 
> with the root further constrained to the Union's dict branch.  Spell
> that out in a bit more detail, like you did in introspect.py?
> 

Terminology problem?

I am using "Expression" to mean very specifically a top-level object as 
returned from parser.py, which *must* be an Object, so it *must* be a 
mapping of str => yaddayadda.

The type as I intended it is Expression = Dict[str, yaddayadda]

where yaddayadda is
Union[int, str, bool, List[yaddayadda], Dict[str, yaddayadda]]
expr.py is what validates the yaddayadda, so there's no point in trying 
to type it further, I think.

Probably worth a better comment.

> Hmm, there you used Any, not object.  I guess that's because mypy lets
> you get away with object here, but not there.  Correct?
> 

Yep. I can get away with the stricter type here because of how we use 
it, so I did. That's an artifact of it not being recursive and how 
expr.py's entire raison d'etre is using isinstance() checks to 
effectively downcast for us everywhere already.

> Also, PEP 8 comment line length.
> 

Augh.

Is there a way to set emacs mode highlighting in Python such that it 
highlights when I run past the 72-col margin, but only for comments?

I have the general-purpose highlighter on for the 80-col margin.

I'm not familiar with any setting like this for any of the linters or 
pycharm right away either.

>>   
>>   
>>   # Names must be letters, numbers, -, and _.  They must start with letter,
>> @@ -287,9 +295,20 @@ def check_event(expr, info):
>>   
>>   def check_exprs(exprs):
>>       for expr_elem in exprs:
>> -        expr = expr_elem['expr']
>> -        info = expr_elem['info']
>> -        doc = expr_elem.get('doc')
>> +        # Expression
>> +        assert isinstance(expr_elem['expr'], dict)
>> +        for key in expr_elem['expr'].keys():
>> +            assert isinstance(key, str)
>> +        expr: Expression = expr_elem['expr']
> 
> You're fine with repeating exp_elem['expr'] here, and ...
> 
>> +
>> +        # QAPISourceInfo
>> +        assert isinstance(expr_elem['info'], QAPISourceInfo)
>> +        info: QAPISourceInfo = expr_elem['info']
> 
> ... expr_elem['info'] here, but ...
> 
>> +
>> +        # Optional[QAPIDoc]
>> +        tmp = expr_elem.get('doc')
>> +        assert tmp is None or isinstance(tmp, QAPIDoc)
>> +        doc: Optional[QAPIDoc] = tmp
> 
> ... you avoid repetition of expr_elem.get('doc') here.  Any particular
> reason?
> 

Because this looks like garbage written by a drunkard:

assert expr_elem.get('doc') is None or isinstance(expr_elem.get('doc'), 
QAPIDoc)
doc: Optional[QAPIDoc] = expr_elem.get('doc')

>>   
>>           if 'include' in expr:
>>               continue