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

From: John Snow
Subject: Re: [PATCH v3 07/16] qapi/expr.py: Add casts in a few select cases
Date: Wed, 24 Feb 2021 17:24:31 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 2/24/21 7:32 AM, Markus Armbruster wrote:
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

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.

You're right, this is because of the signature for the allow_dict argument. Ultimately, the pragma is a collection of strings and we need to prove we are looking up a string somewhere or other.

(Or tell the type checker to leave us alone.)

Unfortunately, the "check" in check_exprs falls off almost immediately. Working with dictly-typed objects is a little annoying for this reason.

This works for now; but finding a better way to do the pragma checks is probably the more correct thing. (And not something I really want to try and get through review until we're done typing, I think.)

      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']


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

"Checked", not "asserted".

Oh, yes.

          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

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.

OK, I'll look into re-ordering this.

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