Footnotes improvements

* footnotes: Allow to use backlink title without footnote number
- The placeholder '{}' is optional. So a user can choose to include or
not the footnote number in the backlink text.
- The modification is backward compatible with configurations using
the old '%d' placeholder.

* footnotes: Allow to use custom superscript text

- The addition of a new SUPERSCRIPT_TEXT option allows to specify
a placeholder receiving the footnote number for the superscript text.

4 files changed, 68 insertions, 7 deletions

diff --git a/docs/change_log/release-3.4.md b/docs/change_log/release-3.4.md
index 0070da9..71f9a20 100644
--- a/docs/change_log/release-3.4.md
+++ b/docs/change_log/release-3.4.md
@@ -2,7 +2,8 @@ title: Release Notes for v3.4
 
 # Python-Markdown 3.4 Release Notes
 
-Python-Markdown version 3.4 supports Python versions 3.6, 3.7, 3.8, 3.9 and PyPy3.
+Python-Markdown version 3.4 supports Python versions 3.6, 3.7, 3.8, 3.9, 3.10 and
+PyPy3.
 
 ## Backwards-incompatible changes
 
@@ -37,8 +38,19 @@ The following new features have been included in the 3.3 release:
   inter-operation. The old behavior is available by setting `use_align_attribute=True` when
   adding the extension.
 
-## Bug fixes
+* Some new configuration options have been added to the [footnotes](../extensions/footnotes.md)
+  extension (#1218):
 
-The following bug fixes are included in the 3.4 release:
+    * Small refactor of the `BACKLINK_TITLE` option; The use of `format()` instead
+      of "old" `%d` formatter allows to specify text without the need to have the
+      number of the footnote in it (like footnotes on Wikipedia for example).
+      The modification is backward compatible so no configuration change is required.
 
+    * Addition of a new option `SUPERSCRIPT_TEXT` that allows to specify a custom
+      placeholder for the footnote itself in the text.
+      Ex: `[{}]` will give <sup>[1]</sup>, `({})` will give <sup>(1)</sup>,
+      or just by default, the current behavior: <sup>1</sup>.
 
+## Bug fixes
+
+The following bug fixes are included in the 3.4 release:
diff --git a/docs/extensions/footnotes.md b/docs/extensions/footnotes.md
index 0794b7f..e841a32 100644
--- a/docs/extensions/footnotes.md
+++ b/docs/extensions/footnotes.md
@@ -86,10 +86,14 @@ The following options are provided to configure the output:
     The text string that links from the footnote definition back to the position
     in the document. Defaults to `&#8617;`.
 
+* **`SUPERSCRIPT_TEXT`**:
+    The text string that links from the position in the document to the footnote
+    definition. Defaults to `{}`, i.e. only the footnote's number.
+
 * **`BACKLINK_TITLE`**:
     The text string for the `title` HTML attribute of the footnote definition link.
-    `%d` will be replaced by the footnote number. Defaults to `Jump back to
-    footnote %d in the text`
+    The placeholder `{}` will be replaced by the footnote number. Defaults to
+    `Jump back to footnote {} in the text`.
 
 * **`SEPARATOR`**:
     The text string used to set the footnote separator. Defaults to `:`.
diff --git a/markdown/extensions/footnotes.py b/markdown/extensions/footnotes.py
index 1cc7118..96ed5c2 100644
--- a/markdown/extensions/footnotes.py
+++ b/markdown/extensions/footnotes.py
@@ -47,6 +47,10 @@ class FootnoteExtension(Extension):
                 ["&#8617;",
                  "The text string that links from the footnote "
                  "to the reader's place."],
+            "SUPERSCRIPT_TEXT":
+                ["{}",
+                 "The text string that links from the reader's place "
+                 "to the footnote."],
             "BACKLINK_TITLE":
                 ["Jump back to footnote %d in the text",
                  "The text string used for the title HTML attribute "
@@ -170,6 +174,9 @@ class FootnoteExtension(Extension):
         ol = etree.SubElement(div, "ol")
         surrogate_parent = etree.Element("div")
 
+        # Backward compatibility with old '%d' placeholder
+        backlink_title = self.getConfig("BACKLINK_TITLE").replace("%d", "{}")
+
         for index, id in enumerate(self.footnotes.keys(), start=1):
             li = etree.SubElement(ol, "li")
             li.set("id", self.makeFootnoteId(id))
@@ -185,7 +192,7 @@ class FootnoteExtension(Extension):
             backlink.set("class", "footnote-backref")
             backlink.set(
                 "title",
-                self.getConfig("BACKLINK_TITLE") % (index)
+                backlink_title.format(index)
             )
             backlink.text = FN_BACKLINK_TEXT
 
@@ -303,7 +310,9 @@ class FootnoteInlineProcessor(InlineProcessor):
             sup.set('id', self.footnotes.makeFootnoteRefId(id, found=True))
             a.set('href', '#' + self.footnotes.makeFootnoteId(id))
             a.set('class', 'footnote-ref')
-            a.text = str(list(self.footnotes.footnotes.keys()).index(id) + 1)
+            a.text = self.footnotes.getConfig("SUPERSCRIPT_TEXT").format(
+                list(self.footnotes.footnotes.keys()).index(id) + 1
+            )
             return sup, m.start(0), m.end(0)
         else:
             return None, None, None
diff --git a/tests/test_syntax/extensions/test_footnotes.py b/tests/test_syntax/extensions/test_footnotes.py
index 1a3a2b0..9a6b32a 100644
--- a/tests/test_syntax/extensions/test_footnotes.py
+++ b/tests/test_syntax/extensions/test_footnotes.py
@@ -300,3 +300,39 @@ class TestFootnotes(TestCase):
             '</div>',
             extension_configs={'footnotes': {'SEPARATOR': '-'}}
         )
+
+    def test_backlink_title(self):
+        """Test backlink title configuration without placeholder."""
+
+        self.assertMarkdownRenders(
+            'paragraph[^1]\n\n[^1]: A Footnote',
+            '<p>paragraph<sup id="fnref:1"><a class="footnote-ref" href="#fn:1">1</a></sup></p>\n'
+            '<div class="footnote">\n'
+            '<hr />\n'
+            '<ol>\n'
+            '<li id="fn:1">\n'
+            '<p>A Footnote&#160;<a class="footnote-backref" href="#fnref:1"'
+            ' title="Jump back to footnote">&#8617;</a></p>\n'
+            '</li>\n'
+            '</ol>\n'
+            '</div>',
+            extension_configs={'footnotes': {'BACKLINK_TITLE': 'Jump back to footnote'}}
+        )
+
+    def test_superscript_text(self):
+        """Test superscript text configuration."""
+
+        self.assertMarkdownRenders(
+            'paragraph[^1]\n\n[^1]: A Footnote',
+            '<p>paragraph<sup id="fnref:1"><a class="footnote-ref" href="#fn:1">[1]</a></sup></p>\n'
+            '<div class="footnote">\n'
+            '<hr />\n'
+            '<ol>\n'
+            '<li id="fn:1">\n'
+            '<p>A Footnote&#160;<a class="footnote-backref" href="#fnref:1"'
+            ' title="Jump back to footnote 1 in the text">&#8617;</a></p>\n'
+            '</li>\n'
+            '</ol>\n'
+            '</div>',
+            extension_configs={'footnotes': {'SUPERSCRIPT_TEXT': '[{}]'}}
+        )