matplotlib · ahmed5145 · Sep 9, 2026 · Sep 10, 2026 · Sep 11, 2026
diff --git a/doc/api/next_api_changes/behavior/23315-AM.rst b/doc/api/next_api_changes/behavior/23315-AM.rst
@@ -0,0 +1,12 @@
+mathtext no longer spaces operators at the edges of an expression
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+mathtext puts a space on each side of relation, binary and arrow operators.
+Following TeX, which discards the glue at the boundaries of a math list, such an
+operator is no longer spaced on a side that abuts the start or end of the
+expression (or of a ``{...}`` group or delimiter).
+
+For example, in ``"a$=b$"`` there is now no space before the ``=`` (only after
+it), matching ``usetex``, and a lone operator such as ``$\doteq$`` no longer has
+surrounding space.  This slightly changes the horizontal metrics of math strings
+that begin or end with such an operator.
diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
@@ -2452,21 +2452,33 @@ def symbol(self, s: str, loc: int,
                                       "Unknown symbol: %s" % c) from err
 
         if c in self._spaced_symbols:
-            # iterate until we find previous character, needed for cases
-            # such as $=-2$, ${ -2}$, $ -2$, or $   -2$.
+            # Find the previous and next characters (skipping spaces), needed
+            # for cases such as $=-2$, ${ -2}$, $ -2$, or $   -2$.  loc points
+            # at the start of this symbol, so the next character is looked for
+            # after its source token, which may be a multi-character command
+            # such as "\doteq".
             prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
-            # Binary operators at start of string should not be spaced
-            # Also, operators in sub- or superscripts should not be spaced
+            next_char = next(
+                (c for c in s[loc + len(toks["sym"]):] if c != ' '), '')
+            # A binary operator acting as a unary operator -- at the start of
+            # the string, or following another operator or an opening delimiter
+            # -- is not spaced.  Operators in sub- or superscripts are not
+            # spaced either.
             if (self._needs_space_after_subsuper or (
                     c in self._binary_operators and (
-                    len(s[:loc].split()) == 0 or prev_char in {
-                        '{', *self._left_delims, *self._relation_symbols}))):
+                    prev_char in {'', '{', *self._left_delims,
+                                  *self._relation_symbols}))):
                 return [char]
-            else:
-                return [Hlist([self._make_space(0.2),
-                               char,
-                               self._make_space(0.2)],
-                              do_kern=True)]
+            # Otherwise a spaced symbol gets a space on each side, except on a
+            # side where it abuts the start or end of the (sub-)expression: TeX
+            # discards the glue at the boundaries of a math list, so e.g. in
+            # "a$=b$" there is no space before the "=" (gh-23315).
+            left_space = prev_char not in {'', '{', *self._left_delims}
+            right_space = next_char not in {'', '}', *self._right_delims}
+            return [Hlist([*([self._make_space(0.2)] if left_space else []),
+                           char,
+                           *([self._make_space(0.2)] if right_space else [])],
+                          do_kern=True)]
         elif c in self._punctuation_symbols:
             prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
             next_char = next((c for c in s[loc + 1:] if c != ' '), '')

diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_01.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_01.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_cm_71.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_71.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_71.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stix_71.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_stixsans_71.svg
diff --git a/lib/matplotlib/tests/test_mathtext.py b/lib/matplotlib/tests/test_mathtext.py
@@ -114,7 +114,7 @@
     r'$\mathring{A}  \AA$',
     r'$M \, M \thinspace M \/ M \> M \: M \; M \ M \enspace M \quad M \qquad M \! M$',
     r'$\Cap$ $\Cup$ $\leftharpoonup$ $\barwedge$ $\rightharpoonup$',
-    r'$\hspace{-0.2}\dotplus\hspace{-0.2}$ $\hspace{-0.2}\doteq\hspace{-0.2}$ $\hspace{-0.2}\doteqdot\hspace{-0.2}$ $\ddots$',
+    r'$\dotplus$ $\doteq$ $\doteqdot$ $\ddots$',  # github issue #23315
     r'$xyz^kx_kx^py^{p-2} d_i^jb_jc_kd x^j_i E^0 E^0_u$',  # github issue #4873
     r'${xyz}^k{x}_{k}{x}^{p}{y}^{p-2} {d}_{i}^{j}{b}_{j}{c}_{k}{d} {x}^{j}_{i}{E}^{0}{E}^0_u$',
     r'${\int}_x^x x\oint_x^x x\int_{X}^{X}x\int_x x \int^x x \int_{x} x\int^{x}{\int}_{x} x{\int}^{x}_{x}x$',
@@ -596,6 +596,24 @@ def test_text_escaped_braces(fig_test, fig_ref):
     fig_ref.text(0.1, 0.2, r"$\text{\{example\}}$")
 
 
+@check_figures_equal()
+def test_spaced_operator_at_boundary(fig_test, fig_ref):
+    # A spaced operator (relation, binary, or arrow) at the start or end of a
+    # math expression is not spaced on the boundary side, matching TeX, which
+    # discards glue at the boundaries of a math list (gh-23315).  The reference
+    # cancels the would-be boundary space with a matching negative \hspace.
+    fig_test.text(0.5, 0.1, r"$=b$")             # relation at start
+    fig_test.text(0.5, 0.3, r"$b=$")             # relation at end
+    fig_test.text(0.5, 0.5, r"$\doteq$")         # named relation, both ends
+    fig_test.text(0.5, 0.7, r"$b\doteq$")        # named relation at end
+    fig_test.text(0.5, 0.9, r"$\leftharpoonup$")  # arrow, both ends
+    fig_ref.text(0.5, 0.1, r"$\hspace{-0.2}=b$")
+    fig_ref.text(0.5, 0.3, r"$b=\hspace{-0.2}$")
+    fig_ref.text(0.5, 0.5, r"$\hspace{-0.2}\doteq\hspace{-0.2}$")
+    fig_ref.text(0.5, 0.7, r"$b\doteq\hspace{-0.2}$")
+    fig_ref.text(0.5, 0.9, r"$\hspace{-0.2}\leftharpoonup\hspace{-0.2}$")
+
+
 @check_figures_equal()
 def test_boldsymbol(fig_test, fig_ref):
     fig_test.text(0.1, 0.2, r"$\boldsymbol{\mathrm{abc0123\alpha}}$")