Annotations Best Practices
**************************

author:
   Larry Hastings


Abstract
^^^^^^^^

This document is designed to encapsulate the best practices for
working with annotations dicts.  If you write Python code that
examines "__annotations__" on Python objects, we encourage you to
follow the guidelines described below.

The document is organized into four sections: best practices for
accessing the annotations of an object in Python versions 3.10 and
newer, best practices for accessing the annotations of an object in
Python versions 3.9 and older, other best practices for
"__annotations__" that apply to any Python version, and quirks of
"__annotations__".

Note that this document is specifically about working with
"__annotations__", not uses *for* annotations. If you're looking for
information on how to use "type hints" in your code, please see the
"typing" module.


Accessing The Annotations Dict Of An Object In Python 3.10 And Newer
====================================================================

   Python 3.10 adds a new function to the standard library:
   "inspect.get_annotations()".  In Python versions 3.10 and newer,
   calling this function is the best practice for accessing the
   annotations dict of any object that supports annotations.  This
   function can also "un-stringize" stringized annotations for you.

   If for some reason "inspect.get_annotations()" isn't viable for
   your use case, you may access the "__annotations__" data member
   manually.  Best practice for this changed in Python 3.10 as well:
   as of Python 3.10, "o.__annotations__" is guaranteed to *always*
   work on Python functions, classes, and modules.  If you're certain
   the object you're examining is one of these three *specific*
   objects, you may simply use "o.__annotations__" to get at the
   object's annotations dict.

   However, other types of callables--for example, callables created
   by "functools.partial()"--may not have an "__annotations__"
   attribute defined.  When accessing the "__annotations__" of a
   possibly unknown object,  best practice in Python versions 3.10 and
   newer is to call "getattr()" with three arguments, for example
   "getattr(o, '__annotations__', None)".

   Before Python 3.10, accessing "__annotations__" on a class that
   defines no annotations but that has a parent class with annotations
   would return the parent's "__annotations__". In Python 3.10 and
   newer, the child class's annotations will be an empty dict instead.


Accessing The Annotations Dict Of An Object In Python 3.9 And Older
===================================================================

   In Python 3.9 and older, accessing the annotations dict of an
   object is much more complicated than in newer versions. The problem
   is a design flaw in these older versions of Python, specifically to
   do with class annotations.

   Best practice for accessing the annotations dict of other objects--
   functions, other callables, and modules--is the same as best
   practice for 3.10, assuming you aren't calling
   "inspect.get_annotations()": you should use three-argument
   "getattr()" to access the object's "__annotations__" attribute.

   Unfortunately, this isn't best practice for classes.  The problem
   is that, since "__annotations__" is optional on classes, and
   because classes can inherit attributes from their base classes,
   accessing the "__annotations__" attribute of a class may
   inadvertently return the annotations dict of a *base class.* As an
   example:

      class Base:
          a: int = 3
          b: str = 'abc'

      class Derived(Base):
          pass

      print(Derived.__annotations__)

   This will print the annotations dict from "Base", not "Derived".

   Your code will have to have a separate code path if the object
   you're examining is a class ("isinstance(o, type)"). In that case,
   best practice relies on an implementation detail of Python 3.9 and
   before: if a class has annotations defined, they are stored in the
   class's "__dict__" dictionary.  Since the class may or may not have
   annotations defined, best practice is to call the "get" method on
   the class dict.

   To put it all together, here is some sample code that safely
   accesses the "__annotations__" attribute on an arbitrary object in
   Python 3.9 and before:

      if isinstance(o, type):
          ann = o.__dict__.get('__annotations__', None)
      else:
          ann = getattr(o, '__annotations__', None)

   After running this code, "ann" should be either a dictionary or
   "None".  You're encouraged to double-check the type of "ann" using
   "isinstance()" before further examination.

   Note that some exotic or malformed type objects may not have a
   "__dict__" attribute, so for extra safety you may also wish to use
   "getattr()" to access "__dict__".


Manually Un-Stringizing Stringized Annotations
==============================================

   In situations where some annotations may be "stringized", and you
   wish to evaluate those strings to produce the Python values they
   represent, it really is best to call "inspect.get_annotations()" to
   do this work for you.

   If you're using Python 3.9 or older, or if for some reason you
   can't use "inspect.get_annotations()", you'll need to duplicate its
   logic.  You're encouraged to examine the implementation of
   "inspect.get_annotations()" in the current Python version and
   follow a similar approach.

   In a nutshell, if you wish to evaluate a stringized annotation on
   an arbitrary object "o":

   * If "o" is a module, use "o.__dict__" as the "globals" when
     calling "eval()".

   * If "o" is a class, use "sys.modules[o.__module__].__dict__" as
     the "globals", and "dict(vars(o))" as the "locals", when calling
     "eval()".

   * If "o" is a wrapped callable using "functools.update_wrapper()",
     "functools.wraps()", or "functools.partial()", iteratively unwrap
     it by accessing either "o.__wrapped__" or "o.func" as
     appropriate, until you have found the root unwrapped function.

   * If "o" is a callable (but not a class), use "o.__globals__" as
     the globals when calling "eval()".

   However, not all string values used as annotations can be
   successfully turned into Python values by "eval()". String values
   could theoretically contain any valid string, and in practice there
   are valid use cases for type hints that require annotating with
   string values that specifically *can't* be evaluated.  For example:

   * **PEP 604** union types using "|", before support for this was
     added to Python 3.10.

   * Definitions that aren't needed at runtime, only imported when
     "typing.TYPE_CHECKING" is true.

   If "eval()" attempts to evaluate such values, it will fail and
   raise an exception.  So, when designing a library API that works
   with annotations, it's recommended to only attempt to evaluate
   string values when explicitly requested to by the caller.


Best Practices For "__annotations__" In Any Python Version
==========================================================

   * You should avoid assigning to the "__annotations__" member of
     objects directly.  Let Python manage setting "__annotations__".

   * If you do assign directly to the "__annotations__" member of an
     object, you should always set it to a "dict" object.

   * If you directly access the "__annotations__" member of an object,
     you should ensure that it's a dictionary before attempting to
     examine its contents.

   * You should avoid modifying "__annotations__" dicts.

   * You should avoid deleting the "__annotations__" attribute of an
     object.


"__annotations__" Quirks
========================

   In all versions of Python 3, function objects lazy-create an
   annotations dict if no annotations are defined on that object.  You
   can delete the "__annotations__" attribute using "del
   fn.__annotations__", but if you then access "fn.__annotations__"
   the object will create a new empty dict that it will store and
   return as its annotations.  Deleting the annotations on a function
   before it has lazily created its annotations dict will throw an
   "AttributeError"; using "del fn.__annotations__" twice in a row is
   guaranteed to always throw an "AttributeError".

   Everything in the above paragraph also applies to class and module
   objects in Python 3.10 and newer.

   In all versions of Python 3, you can set "__annotations__" on a
   function object to "None".  However, subsequently accessing the
   annotations on that object using "fn.__annotations__" will lazy-
   create an empty dictionary as per the first paragraph of this
   section.  This is *not* true of modules and classes, in any Python
   version; those objects permit setting "__annotations__" to any
   Python value, and will retain whatever value is set.

   If Python stringizes your annotations for you (using "from
   __future__ import annotations"), and you specify a string as an
   annotation, the string will itself be quoted.  In effect the
   annotation is quoted *twice.*  For example:

      from __future__ import annotations
      def foo(a: "str"): pass

      print(foo.__annotations__)

   This prints "{'a': "'str'"}".  This shouldn't really be considered
   a "quirk"; it's mentioned here simply because it might be
   surprising.
