Re: [PATCH v3 07/16] qapi/expr.py: Add casts in a few select cases

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> Casts are instructions to the type checker only, they aren't "safe" and
> should probably be avoided in general. In this case, when we perform
> type checking on a nested structure, the type of each field does not
> "stick".
>
> We don't need to assert that something is a str if we've already checked
> that it is -- use a cast instead for these cases.
>
> 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 | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/scripts/qapi/expr.py b/scripts/qapi/expr.py
> index afa6bd07769..f45d6be1f4c 100644
> --- a/scripts/qapi/expr.py
> +++ b/scripts/qapi/expr.py
> @@ -15,7 +15,7 @@
>  # See the COPYING file in the top-level directory.
>  
>  import re
> -from typing import MutableMapping, Optional
> +from typing import MutableMapping, Optional, cast
>  
>  from .common import c_name
>  from .error import QAPISemError
> @@ -232,7 +232,7 @@ def check_enum(expr, info):
>  
>  
>  def check_struct(expr, info):
> -    name = expr['struct']
> +    name = cast(str, expr['struct'])  # Asserted in check_exprs
>      members = expr['data']
>  

I believe you need this only so you can declare check_type()'s
allow_dict to be Optional[str].  More busy-work around allow_dict...

I'm tempted to ask for allow_dict: Any and call it a day.

>      check_type(members, info, "'data'", allow_dict=name)
> @@ -240,7 +240,7 @@ def check_struct(expr, info):
>  
>  
>  def check_union(expr, info):
> -    name = expr['union']
> +    name = cast(str, expr['union'])  # Asserted in check_exprs
>      base = expr.get('base')
>      discriminator = expr.get('discriminator')
>      members = expr['data']

Likwewise.

> @@ -337,7 +337,7 @@ def check_exprs(exprs):
>          else:
>              raise QAPISemError(info, "expression is missing metatype")
>  
> -        name = expr[meta]
> +        name = cast(str, expr[meta])  # Asserted right below:

"Checked", not "asserted".

>          check_name_is_str(name, info, "'%s'" % meta)
>          info.set_defn(meta, name)
>          check_defn_name_str(name, info, meta)


Cast before check gives me a queasy feeling.  It's lying to the type
checker.

Hamfisted way to avoid lying:

           check_name_is_str(expr[meta], ...)
           name = cast(str, expr[meta])

Something like

           name = check_name_is_str(name, ...)

might be neater, but then we'd have to find a better function name.