PEP 769: Incorporated open issues resolutions to the rest of the document. (#4214)

Incorporated open issues resolutions to the rest of the document.

Author: facundobatista

peps/pep-0769.rst

@@ -1,5 +1,5 @@
 PEP: 769
-Title: Add a 'default' keyword argument to 'attrgetter' and 'itemgetter'
+Title: Add a 'default' keyword argument to 'attrgetter', 'itemgetter' and 'getitem'
 Author: Facundo Batista <facundo@taniquetil.com.ar>
 Discussions-To: https://discuss.python.org/t/76419
 Status: Draft
@@ -12,8 +12,8 @@ Abstract
 ========
 
 This proposal aims to enhance the :mod:`operator` module by adding a
-``default`` keyword argument to the ``attrgetter`` and ``itemgetter``
-functions. This addition would allow these functions to return a
+``default`` keyword argument to the ``attrgetter``, ``itemgetter`` and
+``getitem`` functions. This addition would allow these functions to return a
 specified default value when the targeted attribute or item is missing,
 thereby preventing exceptions and simplifying code that handles optional
 attributes or items.
@@ -31,6 +31,10 @@ Introducing a ``default`` parameter would streamline operations involving
 optional attributes or items, reducing boilerplate code and enhancing
 code clarity.
 
+A similar situation occurs with ``getitem``, with the added nuance that
+allowing the specification of a default value in this case would resolve
+a longstanding asymmetry with the :func:`getattr` built-in function.
+
 
 Rationale
 =========
@@ -58,11 +62,16 @@ Proposed behaviors:
 - **itemgetter**: ``f = itemgetter(2, default=XYZ)`` followed by
   ``f(obj)`` would return ``obj[2]`` if that is valid, else ``XYZ``.
 
-This enhancement applies to single and multiple attribute/item
-retrievals, with the default value returned for any missing attribute or
-item.
+- **getitem**: ``getitem(obj, k, XYZ)`` or
+  ``getitem(obj, k, default=XYZ)`` would return ``obj[k]`` if that is valid,
+  else ``XYZ``.
+
+In the first two cases, the enhancement applies to single and multiple
+attribute/item retrievals, with the default value returned for any
+missing attribute or item.
 
-No functionality change is incorporated if ``default`` is not used.
+No functionality change is incorporated in any case if the extra
+default (keyword) argument is not used.
 
 
 Examples for attrgetter
@@ -144,7 +153,32 @@ With this PEP, using the proposed ``default`` keyword::
     ('bar', 'XYZ')
 
 
-.. _PEP 769 About Possible Implementations:
+Examples for getitem
+--------------------
+
+The current behavior is unchanged::
+
+    >>> obj = ["foo", "bar", "baz"]
+    >>> getitem(obj, 1)
+    'bar'
+    >>> getitem(obj, 5)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    IndexError: list index out of range
+
+
+With this PEP, using the proposed extra default, positionally or with
+a keyword::
+
+    >>> getitem(obj, 1, "XYZ")
+    'bar'
+    >>> getitem(obj, 5, "XYZ")
+    'XYZ'
+    >>> getitem(obj, 1, default="XYZ")
+    'bar'
+    >>> getitem(obj, 5, default="XYZ")
+    'XYZ'
+
 
 About Possible Implementations
 ------------------------------
@@ -163,37 +197,43 @@ be impossible to distinguish what it returned on each step when an
 attribute chain is specified (e.g.
 ``attrgetter("foo.bar.baz", default=XYZ)``).
 
-The implementation for ``itemgetter`` is not that easy. The more
-straightforward way is also simple to define and
-understand: attempting ``__getitem__`` and catching a possible exception
-(any of the three indicated in ``__getitem__`` `reference`_). This way,
-``itemgetter(123, default=XYZ)(obj)`` would be equivalent to::
+The implementation for ``itemgetter`` and ``getitem`` is not that
+easy. The more straightforward way is also simple to define and
+understand: attempting ``__getitem__`` and catching a possible
+exception (any of the three indicated in ``__getitem__`` `reference`_).
+This way, ``itemgetter(123, default=XYZ)(obj)`` or
+``getitem(obj, 123, default=XYZ)`` would be equivalent to::
 
     try:
         value = obj[123]
     except (TypeError, IndexError, KeyError):
         value = XYZ
 
-However, this would be not as efficient as we'd want for certain cases,
-e.g. using dictionaries where better performance is possible. A
-more complex alternative would be::
+However, for performance reasons the implementation may look more
+like the following, which has the same exact behaviour::
 
-    if isinstance(obj, dict):
+    if type(obj) == dict:
         value = obj.get(123, XYZ)
     else:
         try:
             value = obj[123]
         except (TypeError, IndexError, KeyError):
             value = XYZ
 
-While this provides better performance, it is more complicated to implement and explain. This is
-the first case in the `Open Issues <PEP 769 Open Issues_>`__ section later.
+Note how the verification is about the exact type and not using
+``isinstance``; this is to ensure the exact behaviour, which would be
+impossible if the object is a user defined one that inherits ``dict``
+but overwrites ``get`` (similar reason to not check if the object has
+a ``get`` method).
+
+This way, performance is better but it's just an implementation detail,
+so we can keep the original explanation on how it behaves.
 
 
 Corner Cases
 ------------
 
-Providing a ``default`` option would only work when accessing the
+Providing a ``default`` option would only work if accessing the
 item/attribute would fail in the normal case. In other words, the
 object accessed should not handle defaults itself.
 
@@ -255,8 +295,8 @@ combinations of ``attrgetter`` and ``itemgetter``, e.g.::
     (1, 2)
 
 However, combining ``itemgetter`` or ``attrgetter`` is totally new
-behavior and very complex to define.  While not impossible, it is beyond the scope of
-this PEP.
+behavior and very complex to define.  While not impossible, it is beyond
+the scope of this PEP.
 
 In the end, having multiple default values was deemed overly complex and
 potentially confusing, and a single ``default`` parameter was favored for
@@ -286,49 +326,7 @@ PEP.
 Open Issues
 ===========
 
-Behavior Equivalence for ``itemgetter``
----------------------------------------
-
-For ``itemgetter``, should it just attempt to
-access the item and capture exceptions regardless of the object's API, or
-should it first validate that the object provides a ``get`` method, and if so use it to
-retrieve the item with a default? See examples in the `About Possible
-Implementations <PEP 769 About Possible Implementations_>`__ subsection
-above.
-
-This would help performance for the case of dictionaries, but would make
-the ``default`` feature somewhat more difficult to explain, and a little
-confusing if some object that is not a dictionary but still provides a ``get``
-method. Alternatively, we could call ``.get`` *only* if the
-object is an instance of ``dict``.
-
-In any case, it is desirable that we do *not* affect performance
-at all if the ``default`` is not triggered. Checking for ``.get`` would
-be faster for dicts, but implies doing a verification
-in all cases. Using the try/except model would make it less efficient as possible
-in the case of dictionaries, but only if the
-default is not triggered.
-
-
-Add a Default to ``getitem``
-----------------------------
-
-It was proposed that we could also enhance ``getitem``, as part of
-this PEP, adding the ``default`` keyword to that function as well.
-
-This will not only improve ``getitem`` itself, but we would also gain
-internal consistency in the ``operator`` module and in comparison with
-the ``getattr`` builtin function, which also has a default.
-
-The definition could be as simple as the try/except proposed above, so
-doing ``getitem(obj, name, default)`` would be equivalent to::
-
-    try:
-        result = obj[name]
-    except (TypeError, IndexError, KeyError):
-        result = default
-
-(However see previous open issue about special case for dictionaries.)
+There are no open issues at this time.
 
 
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