dollarmath: Add allow_blank_lines option (#46)

A port of executablebooks/markdown-it-dollarmath#8

mdit_py_plugins/dollarmath/index.py

@@ -13,6 +13,7 @@ def dollarmath_plugin(
     allow_labels: bool = True,
     allow_space: bool = True,
     allow_digits: bool = True,
+    allow_blank_lines: bool = True,
     double_inline: bool = False,
     label_normalizer: Optional[Callable[[str], str]] = None,
     renderer: Optional[Callable[[str, Dict[str, Any]], str]] = None,
@@ -30,6 +31,10 @@ def dollarmath_plugin(
     :param allow_digits: Parse inline math when there is a digit
         before/after the opening/closing ``$``, e.g. ``1$`` or ``$2``.
         This is useful when also using currency.
+    :param allow_blank_lines: Allow blank lines inside ``$$``. Note that blank lines are
+        not allowed in LaTeX, executablebooks/markdown-it-dollarmath, or the Github or
+        StackExchange markdown dialects. Hoever, they have special semantics if used
+        within Sphinx `..math` admonitions, so are allowed for backwards-compatibility.
     :param double_inline: Search for double-dollar math within inline contexts
     :param label_normalizer: Function to normalize the label,
         by default replaces whitespace with `-`
@@ -47,7 +52,9 @@ def dollarmath_plugin(
         math_inline_dollar(allow_space, allow_digits, double_inline),
     )
     md.block.ruler.before(
-        "fence", "math_block", math_block_dollar(allow_labels, label_normalizer)
+        "fence",
+        "math_block",
+        math_block_dollar(allow_labels, label_normalizer, allow_blank_lines),
     )
 
     # TODO the current render rules are really just for testing
@@ -246,6 +253,7 @@ def _math_inline_dollar(state: StateInline, silent: bool) -> bool:
 def math_block_dollar(
     allow_labels: bool = True,
     label_normalizer: Optional[Callable[[str], str]] = None,
+    allow_blank_lines: bool = False,
 ) -> Callable[[StateBlock, int, int, bool], bool]:
     """Generate block dollar rule."""
 
@@ -299,15 +307,14 @@ def _math_block_dollar(
                 start = state.bMarks[nextLine] + state.tShift[nextLine]
                 end = state.eMarks[nextLine]
 
-                if end - start < 2:
-                    continue
-
                 lineText = state.src[start:end]
 
                 if lineText.strip().endswith("$$"):
                     haveEndMarker = True
                     end = end - 2 - (len(lineText) - len(lineText.strip()))
                     break
+                if lineText.strip() == "" and not allow_blank_lines:
+                    break  # blank lines are not allowed within $$
 
                 # reverse the line and match
                 if allow_labels:

tests/fixtures/dollar_math.md

@@ -226,6 +226,18 @@ b = 2
 </div>
 .
 
+display equation with blank lines. (valid=False)
+.
+$$
+1+1=2
+
+$$
+.
+<p>$$
+1+1=2</p>
+<p>$$</p>
+.
+
 equation followed by a labelled equation (valid=True)
 .
 $$

tests/test_dollarmath.py

@@ -92,7 +92,11 @@ def test_custom_renderer(data_regression):
 )
 def test_dollarmath_fixtures(line, title, input, expected):
     md = MarkdownIt("commonmark").use(
-        dollarmath_plugin, allow_space=False, allow_digits=False, double_inline=True
+        dollarmath_plugin,
+        allow_space=False,
+        allow_digits=False,
+        double_inline=True,
+        allow_blank_lines=False,
     )
     md.options.xhtmlOut = False
     text = md.render(input)