Pass language to Pygments formatter in CodeHilite

* Add an extra option `lang_str` to pass the language of the code block
to the specified Pygments formatter.
* Include an example custom Pygments formatter in the documentation
that includes the language of the code in the output using the new option.

Resolves #1255.

5 files changed, 199 insertions, 9 deletions

diff --git a/docs/change_log/release-3.4.md b/docs/change_log/release-3.4.md
index 6dc8401..7a30de5 100644
--- a/docs/change_log/release-3.4.md
+++ b/docs/change_log/release-3.4.md
@@ -10,7 +10,7 @@ PyPy3.
 ### The `table` extension now uses a `style` attribute instead of `align` attribute for alignment.
 
 The [HTML4 spec][spec4] specifically
-deprecates the use of the `align` attribute and it does not appear at all in the 
+deprecates the use of the `align` attribute and it does not appear at all in the
 [HTML5 spec][spec5]. Therefore, by default, the [table] extension will now use the `style`
 attribute (setting just the `text-align` property) in `td` and `th` blocks.
 
@@ -55,8 +55,10 @@ The following new features have been included in the 3.4 release:
   parameter which can be used to set the CSS class(es) on the `<div>` that contains the
   Table of Contents (#1224).
 
-* The Codehilite extension now supports a `pygments_formatter` option that can be set to
-    use a custom formatter class with Pygments.
+* The CodeHilite extension now supports a `pygments_formatter` option that can be set to
+    use a custom formatter class with Pygments (#1187). Additionally, the specified
+    Pygments formatter is passed an extra option `lang_str` to denote the language of
+    the code block (#1258).
     - If set to a string like `'html'`, we get the default formatter by that name.
     - If set to a class (or any callable), it is called with all the options to get a
       formatter instance.
diff --git a/docs/extensions/code_hilite.md b/docs/extensions/code_hilite.md
index 6fa6190..5179d0a 100644
--- a/docs/extensions/code_hilite.md
+++ b/docs/extensions/code_hilite.md
@@ -231,9 +231,6 @@ The following options are provided to configure the output:
 * **`lang_prefix`**{ #lang_prefix }:
     The prefix prepended to the language class assigned to the HTML `<code>` tag. Default: `language-`.
 
-    This option only applies when `use_pygments` is `False` as Pygments does not provide an option to include a
-    language prefix.
-
 * **`pygments_formatter`**{ #pygments_formatter }:
     This option can be used to change the Pygments formatter used for highlighting the code blocks. By default, this
     is set to the string `'html'`, which means it'll use the default `HtmlFormatter` provided by Pygments.
@@ -241,6 +238,11 @@ The following options are provided to configure the output:
     This can be set to a string representing any of the other default formatters, or set to a formatter class (or
     any callable).
 
+    The code's language is always passed to the formatter as an extra option `lang_str`, with the value formatted as
+    `{lang_prefix}{lang}`. If the language is unspecified, the language guessed by Pygments will be used. While
+    this option has no effect to the Pygments's builtin formatters, a user can make use of the language in their custom
+    formatter. See an example below.
+
     To see what formatters are available and how to subclass an existing formatter, please visit [Pygments
     documentation on this topic][pygments formatters].
 
@@ -256,6 +258,50 @@ A trivial example:
 markdown.markdown(some_text, extensions=['codehilite'])
 ```
 
+To keep the code block's language in the Pygments generated HTML output, one can provide a custom Pygments formatter
+that takes the `lang_str` option. For example,
+
+```python
+from pygments.formatters import HtmlFormatter
+from markdown.extensions.codehilite import CodeHiliteExtension
+
+
+class CustomHtmlFormatter(HtmlFormatter):
+    def __init__(self, lang_str='', **options):
+        super().__init__(**options)
+        # lang_str has the value {lang_prefix}{lang}
+        # specified by the CodeHilite's options
+        self.lang_str = lang_str
+
+    def _wrap_code(self, source):
+        yield 0, f'<code class="{self.lang_str}">'
+        yield from source
+        yield 0, '</code>'
+
+
+some_text = '''\
+    :::python
+    print('hellow world')
+'''
+
+markdown.markdown(
+    some_text,
+    extensions=[CodeHiliteExtension(pygments_formatter=CustomHtmlFormatter)],
+)
+```
+
+The formatter above will output the following HTML structure for the code block:
+
+```html
+<div class="codehilite">
+    <pre>
+        <code class="language-python">
+        ...
+        </code>
+    </pre>
+</div>
+```
+
 [html formatter]: https://pygments.org/docs/formatters/#HtmlFormatter
 [lexer]: https://pygments.org/docs/lexers/
 [spec]: https://www.w3.org/TR/html5/text-level-semantics.html#the-code-element
diff --git a/markdown/extensions/codehilite.py b/markdown/extensions/codehilite.py
index b92ebdc..4721c59 100644
--- a/markdown/extensions/codehilite.py
+++ b/markdown/extensions/codehilite.py
@@ -64,12 +64,14 @@ class CodeHilite:
     * use_pygments: Pass code to pygments for code highlighting. If `False`, the code is
       instead wrapped for highlighting by a JavaScript library. Default: `True`.
 
+    * pygments_formatter: The name of a Pygments formatter or a formatter class used for
+      highlighting the code blocks. Default: `html`.
+
     * linenums: An alias to Pygments `linenos` formatter option. Default: `None`.
 
     * css_class: An alias to Pygments `cssclass` formatter option. Default: 'codehilite'.
 
-    * lang_prefix: Prefix prepended to the language when `use_pygments` is `False`.
-      Default: "language-".
+    * lang_prefix: Prefix prepended to the language. Default: "language-".
 
     Other Options:
     Any other options are accepted and passed on to the lexer and formatter. Therefore,
@@ -81,6 +83,10 @@ class CodeHilite:
     Formatter options: https://pygments.org/docs/formatters/#HtmlFormatter
     Lexer Options: https://pygments.org/docs/lexers/
 
+    Additionally, when Pygments is enabled, the code's language is passed to the
+    formatter as an extra option `lang_str`, whose value being `{lang_prefix}{lang}`.
+    This option has no effect to the Pygments's builtin formatters.
+
     Advanced Usage:
         code = CodeHilite(
             src = some_code,
@@ -141,13 +147,17 @@ class CodeHilite:
                         lexer = get_lexer_by_name('text', **self.options)
                 except ValueError:  # pragma: no cover
                     lexer = get_lexer_by_name('text', **self.options)
+            if not self.lang:
+                # Use the guessed lexer's langauge instead
+                self.lang = lexer.aliases[0]
+            lang_str = f'{self.lang_prefix}{self.lang}'
             if isinstance(self.pygments_formatter, str):
                 try:
                     formatter = get_formatter_by_name(self.pygments_formatter, **self.options)
                 except ClassNotFound:
                     formatter = get_formatter_by_name('html', **self.options)
             else:
-                formatter = self.pygments_formatter(**self.options)
+                formatter = self.pygments_formatter(lang_str=lang_str, **self.options)
             return highlight(self.src, lexer, formatter)
         else:
             # just escape and build markup usable by JS highlighting libs
diff --git a/tests/test_syntax/extensions/test_code_hilite.py b/tests/test_syntax/extensions/test_code_hilite.py
index 41502d9..09dd523 100644
--- a/tests/test_syntax/extensions/test_code_hilite.py
+++ b/tests/test_syntax/extensions/test_code_hilite.py
@@ -354,6 +354,22 @@ class TestCodeHiliteExtension(TestCase):
         if has_pygments and pygments.__version__ != required_pygments_version:
             self.skipTest(f'Pygments=={required_pygments_version} is required')
 
+        # Define a custom Pygments formatter (same example in the documentation)
+        if has_pygments:
+            class CustomAddLangHtmlFormatter(pygments.formatters.HtmlFormatter):
+                def __init__(self, lang_str='', **options):
+                    super().__init__(**options)
+                    self.lang_str = lang_str
+
+                def _wrap_code(self, source):
+                    yield 0, f'<code class="{self.lang_str}">'
+                    yield from source
+                    yield 0, '</code>'
+        else:
+            CustomAddLangHtmlFormatter = None
+
+        self.custom_pygments_formatter = CustomAddLangHtmlFormatter
+
     maxDiff = None
 
     def testBasicCodeHilite(self):
@@ -676,3 +692,73 @@ class TestCodeHiliteExtension(TestCase):
             expected,
             extensions=[CodeHiliteExtension(pygments_style="native", noclasses=True)]
         )
+
+    def testFormatterLangStr(self):
+        if has_pygments:
+            expected = (
+                '<div class="codehilite"><pre><span></span><code class="language-python">'
+                '<span class="c1"># A Code Comment</span>\n'
+                '</code></pre></div>'
+            )
+        else:
+            expected = (
+                '<pre class="codehilite"><code class="language-python"># A Code Comment\n'
+                '</code></pre>'
+            )
+
+        self.assertMarkdownRenders(
+            '\t:::Python\n'
+            '\t# A Code Comment',
+            expected,
+            extensions=[
+                CodeHiliteExtension(
+                    guess_lang=False,
+                    pygments_formatter=self.custom_pygments_formatter
+                )
+            ]
+        )
+
+    def testFormatterLangStrGuessLang(self):
+        if has_pygments:
+            expected = (
+                '<div class="codehilite"><pre><span></span>'
+                '<code class="language-js+php"><span class="cp">&lt;?php</span> '
+                '<span class="k">print</span><span class="p">(</span>'
+                '<span class="s2">&quot;Hello World&quot;</span>'
+                '<span class="p">);</span> <span class="cp">?&gt;</span>\n'
+                '</code></pre></div>'
+            )
+        else:
+            expected = (
+                '<pre class="codehilite"><code>&lt;?php print(&quot;Hello World&quot;); ?&gt;\n'
+                '</code></pre>'
+            )
+        # Use PHP as the the starting `<?php` tag ensures an accurate guess.
+        self.assertMarkdownRenders(
+            '\t<?php print("Hello World"); ?>',
+            expected,
+            extensions=[CodeHiliteExtension(pygments_formatter=self.custom_pygments_formatter)]
+        )
+
+    def testFormatterLangStrEmptyLang(self):
+        if has_pygments:
+            expected = (
+                '<div class="codehilite"><pre><span></span>'
+                '<code class="language-text"># A Code Comment\n'
+                '</code></pre></div>'
+            )
+        else:
+            expected = (
+                '<pre class="codehilite"><code># A Code Comment\n'
+                '</code></pre>'
+            )
+        self.assertMarkdownRenders(
+            '\t# A Code Comment',
+            expected,
+            extensions=[
+                CodeHiliteExtension(
+                    guess_lang=False,
+                    pygments_formatter=self.custom_pygments_formatter,
+                )
+            ]
+        )
diff --git a/tests/test_syntax/extensions/test_fenced_code.py b/tests/test_syntax/extensions/test_fenced_code.py
index f8c3e91..be3c215 100644
--- a/tests/test_syntax/extensions/test_fenced_code.py
+++ b/tests/test_syntax/extensions/test_fenced_code.py
@@ -896,6 +896,52 @@ class TestFencedCodeWithCodehilite(TestCase):
             ]
         )
 
+    def testPygmentsAddLangClassFormatter(self):
+        if has_pygments:
+            class CustomAddLangHtmlFormatter(pygments.formatters.HtmlFormatter):
+                def __init__(self, lang_str='', **options):
+                    super().__init__(**options)
+                    self.lang_str = lang_str
+
+                def _wrap_code(self, source):
+                    yield 0, f'<code class="{self.lang_str}">'
+                    yield from source
+                    yield 0, '</code>'
+
+            expected = '''
+                <div class="codehilite"><pre><span></span><code class="language-text">hello world
+                hello another world
+                </code></pre></div>
+                '''
+        else:
+            CustomAddLangHtmlFormatter = None
+            expected = '''
+                <pre class="codehilite"><code class="language-text">hello world
+                hello another world
+                </code></pre>
+                '''
+
+        self.assertMarkdownRenders(
+            self.dedent(
+                '''
+                ```text
+                hello world
+                hello another world
+                ```
+                '''
+            ),
+            self.dedent(
+                expected
+            ),
+            extensions=[
+                markdown.extensions.codehilite.CodeHiliteExtension(
+                    guess_lang=False,
+                    pygments_formatter=CustomAddLangHtmlFormatter,
+                ),
+                'fenced_code'
+            ]
+        )
+
     def testSvgCustomPygmentsFormatter(self):
         if has_pygments:
             expected = '''