should we use _PyType_Name(Py_TYPE(self)) instead of hardcoding Template here?

I've updated the patch to do that.

And I also changed the call to Py_ReprEnter/Py_ReprLeave to pass op instead of self to avoid compiler warnings.

I tried updating pprint.PrettyPrinter._pprint_template() like this

--- a/Lib/pprint.py
+++ b/Lib/pprint.py
@@ -742,13 +742,9 @@ def _pprint_template(self, object, stream, indent, allowance, context, level):
         else:
             indent += len(cls_name) + 1

-        items = (
-            ("strings", object.strings),
-            ("interpolations", object.interpolations),
-        )
         stream.write(self._format_block_start(cls_name + "(", indent))
-        self._format_namespace_items(
-            items, stream, indent, allowance, context, level
+        self._format_items(
+            object, stream, indent, allowance, context, level
         )

but somehow this gives me an extra level of indentation

>>> import pprint
>>> print(pprint.pformat(t'Hello {'world'}', width=40, indent=4, expand=True))
Template(
        'Hello ',
        Interpolation(
            value='world',
            expression="'world'",
            conversion=None,
            format_spec='',
        ),
)
>>> print(pprint.pformat(t'Hello {'world'}', width=40, indent=4, expand=False))
Template(   'Hello ',
             Interpolation(   'world',
                              "'world'",
                              None,
                              ''))

What am I doing wrong?

Hm, I believe that's the indented output! If you try

>>> print(pprint.pformat(['Hello', 'foo'*10], width=40, indent=4, expand=False))
[   'Hello',
    'foofoofoofoofoofoofoofoofoofoo']

the extra indentation is the same, even if I would not have expected it either 😅

The expand=False output is indeed unexpected, but the expand=True output is wrong, since the first indentation level is 8 spaces and the second level is 4.

Okay, I think it's because we don't need the indent += self._indent_per_level with self.format_items, otherwise we ident twice:

--- a/Lib/pprint.py
+++ b/Lib/pprint.py
@@ -739,7 +739,5 @@ def _pprint_template(self, object, stream, indent, allowance, context, level):
         cls_name = object.__class__.__name__
-        if self._expand:
-            indent += self._indent_per_level
-        else:
+        if not self._expand:
             indent += len(cls_name) + 1

That still doesn't look super nice, but not worse than today!

OK, with your change, this looks better:

>>> pprint.pprint(t"a{2}b", width=40)
Template('a',
          Interpolation(2,
                        '2',
                        None,
                        ''),
          'b')

but the first level is still indented one space too much.

For expand=True the result looks OK:

>>> pprint.pprint(t"a{2}b", width=40, expand=True, indent=4)
Template(
    'a',
    Interpolation(2, '2', None, ''),
    'b',
)

Looks like we should remove the + 1 then😅

--- a/Lib/pprint.py
+++ b/Lib/pprint.py
@@ -739,4 +739,4 @@ def _pprint_template(self, object, stream, indent, allowance, context, level):
         cls_name = object.__class__.__name__
         if not self._expand:
-            indent += len(cls_name) + 1
+            indent += len(cls_name)

Gives

• expand=False

  Template('Hello ',
           Interpolation('world',
                         "'world'",
                         None,
                         ''))

• expand=False, indent=4

  Template(   'Hello ',
              Interpolation(   'world',
                               "'world'",
                               None,
                               ''))

• expand=True

  Template(
   'Hello ',
   Interpolation(
    value='world',
    expression="'world'",
    conversion=None,
    format_spec='',
   ),
  )

• expand=True, indent=4

  Template(
      'Hello ',
      Interpolation(
          value='world',
          expression="'world'",
          conversion=None,
          format_spec='',
      ),
  )

It's still a bit strange for expand=False, indent=4, but that's because self._format_items is fundamentally made for collections (bracket/brace delimiter) and not classes. This issue already exists with Interpolation, so I think it's ok to stop here for this PR.