Add __setattr__ support by emmatyping · Pull Request #3451 · python/mypy

emmatyping · 2017-05-25T08:00:53Z

This is my attempt at solving #521, which allows the setting of arbitrary attributes, restricted by the typing of values in __setattr__. I did not do the same signature checking as __getattr__, as @JelleZijlstra pointed out, the check would be done based on object's method.

I want to add some more in the docs about this, as I think it deserves to be well documented, I just wasn't sure where to put it.

Also the wiki should be updated if this is merged.

ilevkivskyi

Thanks!

I have few comments.

ilevkivskyi · 2017-05-25T08:05:16Z

test-data/unit/check-classes.test

+b.at = '3'
+integer = b.at
+[out]
+main:13: error: Incompatible types in assignment (expression has type "str", variable has type "int")


Could you please use inline errors with # E: syntax?

ilevkivskyi · 2017-05-25T08:15:58Z

docs/source/cheat_sheet.rst

+       def __setattr__(self, name, value):
+           # type: (str, int) -> None
+           ...
+   a.foo = bar() # works if bar() returns int, fails otherwise


Maybe just use an integer like 42 instead of bar() here and in Python 3 version?

Fair point, and perhaps a str with a comment about it failing.

ilevkivskyi · 2017-05-25T08:19:10Z

mypy/checkmember.py

+                typ = map_instance_to_supertype(itype, setattr_func.info)
+                setattr_type = expand_type_by_instance(bound_type, typ)
+                if isinstance(setattr_type, CallableType):
+                    return setattr_type.arg_types[-1]


Should we have some requirement that __setattr__ and __getattr__ types are somehow compatible? In either case this decision should be explicitly documented with some motivation.

This is something I considered, but since we don't know what __setattr__ does in some edge cases, I think it is safer to not assume that the types are the same. Consider a class which takes str attributes, and in __setattr__ converts it to an int before binding to itself.

But I don't know what the best decision here is.

ilevkivskyi · 2017-05-25T08:25:16Z

test-data/unit/check-classes.test

+[case testSetAttr]
+from typing import Union
+class A:
+    def __setattr__(self, name: str, value: Any) -> None: ...


Two more ideas for tests:

presence of both __setattr__ and normal attributes

__setattr__ in superclass

A few others:

__setattr__ is defined, but not a callable

__setattr__ is defined but takes the wrong arguments

@ilevkivskyi could you explain 'presence of both setattr and normal attributes'
You mean something like:

class Ex: def __setattr__(self, name: str, value: int) -> None:... test = '42' # type: str e = Ex() e.test = 'hello'

Yes, something like this (plus e.whatever = 5)

JelleZijlstra · 2017-05-25T15:29:19Z

docs/source/cheat_sheet.rst

   reveal_type(c)  # -> error: Revealed type is 'builtins.list[builtins.str]'
   print(c)  # -> [4] the object is not cast

+   # if you want dynamic attributes on your class, have it override __setattr__ in a stub


And also __getattr__, depending on our decision on that matter.

Right, perhaps I should make that more clear and which one is for which case.

Looks like you put it in the py3 but not the py2 cheat sheet. It should be the same in both.

JelleZijlstra · 2017-05-25T15:33:47Z

mypy/checkmember.py

+                typ = map_instance_to_supertype(itype, setattr_func.info)
+                setattr_type = expand_type_by_instance(bound_type, typ)
+                if isinstance(setattr_type, CallableType):
+                    return setattr_type.arg_types[-1]


Is this going to crash if __setattr__ is defined to take no arguments?

Is this going to crash if __setattr__ is defined to take no arguments?

Good catch!

Ah, yes. I fixed this by checking len of the args.

JelleZijlstra · 2017-05-25T15:36:06Z

mypy/checkmember.py

+        else:
+            setattr_func = info.get_method('__setattr__')
+            if setattr_func and setattr_func.info.fullname() != 'builtins.object':
+                setattr_f = function_type(setattr_func, builtin_type('builtins.function'))


It's confusing to have separate variables called setattr_func and setattr_f. Perhaps the first one should be setattr_method.

emmatyping · 2017-05-25T22:57:38Z

I believe I resolved all of the points reviewed. If I missed something or there is something else needed, lmk.

JelleZijlstra · 2017-05-25T23:07:32Z

test-data/unit/check-classes.test

+class B:
+    def __setattr__(self, name: str, value: int) -> None: ...
+    def __getattr__(self, name: str) -> str: ...
+integer = None  # type: int


why not just integer = 0? that will work with strict-optional too

ilevkivskyi

Thanks! Just few very minor comments. Also please update PY2 cheat-sheet as proposed by @JelleZijlstra.

ilevkivskyi · 2017-05-28T11:56:37Z

docs/source/cheat_sheet_py3.rst

+   a.foo = 42  # works
+   a.bar = 'Ex-parrot'  # fails type checking
+   f = None  # type: int
+   b = None  # type: str


Why do you need these two lines with None assignments?

I believe I was going to add more examples, but I don't think its needed. Removed.

ilevkivskyi · 2017-05-28T11:59:52Z

test-data/unit/check-classes.test

+s.success = 4
+s.fail = 'fail'  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+
+


One empty line will be enough here.

ilevkivskyi · 2017-05-28T12:01:58Z

test-data/unit/check-classes.test

+class B:
+    def __setattr__(self, name, value: int): ...
+b = B()
+b.fail = 5


I think the attribute name fail in a test that passes is misleading.

ilevkivskyi · 2017-05-28T12:04:09Z

test-data/unit/check-classes.test

+c = C()
+c.check = 13
+
+[case test testAttributes]


This is probably too generic test case name. Maybe testGetAttrAndSetattr? Also there is an extra test word.

ilevkivskyi · 2017-05-28T12:04:50Z

test-data/unit/check-classes.test

+b.at = '3'  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+integer = b.at  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+
+


Again, remove the second added empty line.

ilevkivskyi

Thanks! This (finally :-) looks good to me!

emmatyping · 2017-05-29T22:26:22Z

Thank you! Sorry it took so long :^) next time I'll know more about what is needed!

* master: (23 commits) Make return type of open() more precise (python#3477) Add test cases that delete a file during incremental checking (python#3461) Parse each format-string component separately (python#3390) Don't warn about returning Any if it is a proper subtype of the return type (python#3473) Add __setattr__ support (python#3451) Remove bundled lib-typing (python#3337) Move version of extensions to post-release (python#3348) Fix None slice bounds with strict-optional (python#3445) Allow NewType subclassing NewType. (python#3465) Add console scripts (python#3074) Fix 'variance' label. Change label for variance section to just 'variance' (python#3429) Better error message for invalid package names passed to mypy (python#3447) Fix last character cut in html-report if file does not end with newline (python#3466) Print pytest output as it happens (python#3463) Add mypy roadmap (python#3460) Add flag to avoid interpreting arguments with a default of None as Optional (python#3248) Add type checking plugin support for functions (python#3299) Mismatch of inferred type and return type note (python#3428) Sync typeshed (python#3449) ...

add __setattr__ support

c09f47e

ilevkivskyi reviewed May 25, 2017

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JelleZijlstra reviewed May 25, 2017

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fix edge case of null signature, better tests for __setattr__

a3e1cf0

JelleZijlstra reviewed May 25, 2017

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emmatyping added 2 commits May 25, 2017 16:18

modify test to be strict-optional friendly

a2afab6

Merge branch 'master' of https://github.com/python/mypy

2ad776c

ilevkivskyi reviewed May 28, 2017

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emmatyping added 2 commits May 28, 2017 16:23

Merge branch 'master' of https://github.com/python/mypy

6ce347e

correct docs and test data

73def65

gvanrossum assigned ilevkivskyi May 29, 2017

ilevkivskyi approved these changes May 29, 2017

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ilevkivskyi merged commit 46e41d9 into python:master May 29, 2017

ilevkivskyi mentioned this pull request Jun 10, 2017

Support __setattr__ #521

Closed

deifactor mentioned this pull request Apr 29, 2022

Disable the special __setattr__ handling #12696

Closed

		s.success = 4
		s.fail = 'fail' # E: Incompatible types in assignment (expression has type "str", variable has type "int")

		b.at = '3' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
		integer = b.at # E: Incompatible types in assignment (expression has type "str", variable has type "int")

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emmatyping commented May 25, 2017

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emmatyping commented May 25, 2017

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emmatyping commented May 29, 2017

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