PEP 750: Collect spec fixes discovered during final implementation work (#4364)

Author: davepeck

peps/pep-0750.rst

@@ -209,7 +209,7 @@ Like ``Template``, it is a new class found in the :mod:`!string.templatelib` mod
         def __new__(
             cls,
             value: object,
-            expression: str,
+            expression: str = "",
             conversion: Literal["a", "r", "s"] | None = None,
             format_spec: str = "",
         ):
@@ -226,14 +226,20 @@ The ``value`` attribute is the evaluated result of the interpolation:
     template = t"Hello {name}"
     assert template.interpolations[0].value == "World"
 
-The ``expression`` attribute is the *original text* of the interpolation:
+When interpolations are created from a template string literal, the
+``expression`` attribute contains the *original text* of the interpolation:
 
 .. code-block:: python
 
     name = "World"
     template = t"Hello {name}"
     assert template.interpolations[0].expression == "name"
 
+When developers explicitly construct an ``Interpolation``, they may optionally
+provide a value for the ``expression`` attribute. Even though it is stored as
+a string, this *should* be a valid Python expression. If no value is provided,
+the ``expression`` attribute defaults to the empty string (``""``).
+
 We expect that the ``expression`` attribute will not be used in most template
 processing code. It is provided for completeness and for use in debugging and
 introspection. See both the `Common Patterns Seen in Processing Templates`_
@@ -462,7 +468,9 @@ The debug specifier, ``=``, is supported in template strings and behaves similar
 to how it behaves in f-strings, though due to limitations of the implementation
 there is a slight difference.
 
-In particular, ``t'{value=}'`` is treated as ``t'value={value!r}'``:
+In particular, ``t'{value=}'`` is treated as ``t'value={value!r}'``. The first
+static string is rewritten from ``""`` to ``"value="`` and the ``conversion``
+defaults to ``r``:
 
 .. code-block:: python
 
@@ -472,8 +480,11 @@ In particular, ``t'{value=}'`` is treated as ``t'value={value!r}'``:
     assert template.interpolations[0].value == "World"
     assert template.interpolations[0].conversion == "r"
 
-If a separate format string is also provided, ``t'{value=:fmt}`` is treated
-instead as ``t'value={value!s:fmt}'``.
+If a conversion is explicitly provided, it is kept: ``t'{value=!s}'``
+is treated as ``t'value={value!s}'``.
+
+If a format string is provided without a conversion, the ``conversion``
+is set to ``None``: ``t'{value=:fmt}'`` is treated as ``t'value={value:fmt}'``.
 
 Whitespace is preserved in the debug specifier, so ``t'{value = }'`` is
 treated as ``t'value = {value!r}'``.