Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.44%. Comparing base (df05cae) to head (16d4e44).
Report is 22 commits behind head on master.

@@           Coverage Diff           @@
##           master   #17693   +/-   ##
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  Coverage   98.44%   98.44%           
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  Files         171      171           
  Lines       22192    22219   +27     
=======================================
+ Hits        21847    21874   +27     
  Misses        345      345           

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Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:  +168 +0.020% standard[incl +32(data)]
      stm32:  +108 +0.027% PYBV10
     mimxrt:  +112 +0.030% TEENSY40
        rp2:  +120 +0.013% RPI_PICO_W
       samd:  +136 +0.051% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:  +117 +0.026% VIRT_RV32

I like this approach, it's nice and simple, and safe.

the ability for a descriptor to add siblings or transparently delete or replace itself isn't an especially-rarely-used functionality in descriptor-heavy code.

Do you think we'd be severely limiting this feature by disallowing locals dict updates?

Do you think we'd be severely limiting this feature by disallowing locals dict updates?

The use cases aren't overwhelming, so there's definitely merit to just disallowing locals dict updates; or maybe having a feature flag for this.

Just to describe a few, though:

Some descriptors need to coordinate with other descriptor instances on the same class (e.g. a serialization or ORM libraries); in these cases, it's common to have the first descriptor create an attribute on the class to hold a data structure that they all register in. (One of my own libraries use owner class attrs like this for the tables it uses to work out what registers are contiguous, for I2C transaction pooling.)

class Registered:
    def __set_name__(self, owner, name):
        try:
            owner.__registry[name] = self
        except AttributeError:
            owner.__registry = {name: self}

In CPython, a library writer can also do this using a mixin class, with an __init_subclass__ method --- but that's extra boilerplate that users easily forget, and limits extensibility since the mixin class needs to know how all of the descriptors should be processed. (Also, Micropython doesn't support multiple inheritance, so regular mixins are already out for a lot of use cases.)

It's also potentially a performance optimization in the case where a descriptor can replace itself with a concrete value at class-creation time, to avoid the indirection of dispatching access through a getter:

class Configured:
    def __set_name__(self, owner, name):
        concrete_value = query_config_database(name)
        setattr(owner, name, concrete_value)

class MyApp:
    TIME_BETWEEN_WIDGETS: int = Configured()