On 10/8/20 4:42 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:
On 10/7/20 7:55 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:
Annotations do not change runtime behavior.
This commit *only* adds annotations.
A note on typing of __init__: mypy requires init functions with no
parameters to document a return type of None to be considered fully
typed. In the case when there are input parameters, None may be
omitted.
Since __init__ may never return any value, it is preferred to omit the
return annotation whenever possible.
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
---
scripts/qapi/mypy.ini | 5 -----
scripts/qapi/source.py | 31 ++++++++++++++++++-------------
2 files changed, 18 insertions(+), 18 deletions(-)
diff --git a/scripts/qapi/mypy.ini b/scripts/qapi/mypy.ini
index 8ab9ac52cc4..1b8555dfa39 100644
--- a/scripts/qapi/mypy.ini
+++ b/scripts/qapi/mypy.ini
@@ -34,11 +34,6 @@ disallow_untyped_defs = False
disallow_incomplete_defs = False
check_untyped_defs = False
-[mypy-qapi.source]
-disallow_untyped_defs = False
-disallow_incomplete_defs = False
-check_untyped_defs = False
-
[mypy-qapi.types]
disallow_untyped_defs = False
disallow_incomplete_defs = False
diff --git a/scripts/qapi/source.py b/scripts/qapi/source.py
index e97b9a8e15e..1cc6a5b82dc 100644
--- a/scripts/qapi/source.py
+++ b/scripts/qapi/source.py
@@ -11,37 +11,42 @@
import copy
import sys
+from typing import List, Optional, TypeVar
class QAPISchemaPragma:
- def __init__(self):
+ def __init__(self) -> None:
# Are documentation comments required?
self.doc_required = False
# Whitelist of commands allowed to return a non-dictionary
- self.returns_whitelist = []
+ self.returns_whitelist: List[str] = []
# Whitelist of entities allowed to violate case conventions
- self.name_case_whitelist = []
+ self.name_case_whitelist: List[str] = []
class QAPISourceInfo:
- def __init__(self, fname, line, parent):
+ T = TypeVar('T', bound='QAPISourceInfo')
+
+ def __init__(self: T, fname: str, line: int, parent:
Optional[T]):
More ignorant questions (I'm abusing the review process to learn Python
type hints)...
Why do you need to annotate self here, but not elsewhere?
This is admittedly me being a little extra, but I thought it was a
good way to show a pattern for people who maybe hadn't been exposed to
it yet.
This is a pattern that allows for subclassing. I am stating that this
__init__ method takes a parent of the same type as itself, whatever
that happens to actually be.
T is a TypeVar that we can use for Generics. By declaring the type of
self as that TypeVar, we bind T to self's type. When we annotate the
parent as a T, we are enforcing that the parent we receive is of the
same type as ourselves.
(Yep, we don't actually subclass QAPISourceInfo and I don't have plans
to. It felt slightly icky to hard-code the class type name, though. I
try to avoid that whenever I can. I'm not sure I would go so far as to
call it a code smell or an antipattern, but it's something I tend to
avoid anyway.)
Say I define class QSISub as a direct subclass of QAPISourceInfo, and
let it inherit __init__(). What's the type of QSISub.__init__()'s
parameter @parent?
According to my reading of your explanation, it's QSISub. Correct?
That's right.
(I'm realizing that this is maybe not a constraint that we should even
anticipate here, because maybe we don't wish to say that the parent
should always be of the same type. but hey, it led to a good mypy lesson.
I'm going to edit it to do the simpler thing for now and leave well
enough alone. There's another chance to see an interesting pattern of
TypeVars in the error series in part 4 that I think is actually more
explicitly appropriate.)
If we used QAPISourceInfo instead of T for @parent, then it would be
QAPISourceInfo. Correct?
Yup!
Here's a little sample program that shows what this kind of typing does:
```
from typing import TypeVar, Optional
class Example:
T = TypeVar('T', bound='Example')
def __init__(self: T, parent: Optional[T] = None):
self.parent = parent
class SubExample(Example):
pass
x = Example()
y = Example(x)
z = SubExample()
a = Example(x) # OK
b = Example(z) # OK
c = SubExample(x) # BZZZT
d = SubExample(z) # OK
```
If you check this with mypy, you'll get this error:
```
test.py:17: error: Argument 1 to "SubExample" has incompatible type
"Example"; expected "Optional[SubExample]"
Found 1 error in 1 file (checked 1 source file)
```
Now, perhaps any QAPISourceInfo will do as @parent, perhaps it really
needs to be a QSISub. We can't know when we write QAPISourceInfo. But
we don't *have* to get this exactly right for all future subclasses,
because I can always override __init__() when inheritance doesn't give
me the __init__() I want. Correct?
You could, but I suggested on IRC the other day that I am not fully
comfortable with the LSP rules that mypy (sometimes?) enforces. I tend
not to want to override init with narrower types if I can avoid it, but
it is true that we do this quite a lot in the codebase already.
(I believe I have seen mypy throw errors about this on occasion, but I
can't pinpoint the exact circumstances in which it does. It's a point of
confusion for me.)
Say I override __init__(), and have it call super().__init__(). I have
to pass it a QAPISourceInfo @parent. A QSISub will do (it's a subtype).
Correct?
One more: is bound='QAPISourceInfo' strictly needed?
I'm not sure if bound is strictly required or not. mypy docs just use it
outright and don't mention why:
https://mypy.readthedocs.io/en/stable/generics.html#generic-methods-and-generic-self
Generally, this constrains a TypeVar to some upper bound, see --
https://mypy.readthedocs.io/en/stable/generics.html#type-variables-with-upper-bounds
-- but in this case, we're only using that TypeVar for an init method
that only exists for this type and below, so it might be redundant.
I modified my example program to remove the bound and it appears to fail
in the same exact way, so it might be pointless in this case. It might
have a stronger use if you want to re-use the 'T' typevar elsewhere
where it isn't implicitly bound by the 'self' argument. Maybe it has
implications for multi-inheritance too, I've not tested it to that level.