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Fix mathtext spacing of operators at expression boundaries
Relation, binary and arrow operators are "spaced symbols": mathtext puts a space on each side of them. TeX, however, discards the glue at the boundaries of a math list, so an operator at the very start or end of an expression (or of a {...} group) is not spaced on that side -- e.g. in "a$=b$" there is no space before the "=", only after it. mathtext instead spaced both sides, and only binary operators were special-cased at the start of the string. Decide the left and right spacing of a spaced symbol independently, and drop the space on any side that abuts the start/end of the expression or an opening/closing delimiter. The lookahead for the next character now skips over the source token (which may be a multi-character command such as "\doteq") instead of just loc + 1. This restores the simpler \ddots test that had to be worked around with negative \hspace, and adds a check_figures_equal regression test. Closes #23315 Co-Authored-By: Claude Opus 4.8 <[email protected]>
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2 files changed

Lines changed: 42 additions & 12 deletions

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‎lib/matplotlib/_mathtext.py‎

Lines changed: 23 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -2452,21 +2452,33 @@ def symbol(self, s: str, loc: int,
24522452
"Unknown symbol: %s" % c) from err
24532453

24542454
if c in self._spaced_symbols:
2455-
# iterate until we find previous character, needed for cases
2456-
# such as $=-2$, ${ -2}$, $ -2$, or $ -2$.
2455+
# Find the previous and next characters (skipping spaces), needed
2456+
# for cases such as $=-2$, ${ -2}$, $ -2$, or $ -2$. loc points
2457+
# at the start of this symbol, so the next character is looked for
2458+
# after its source token, which may be a multi-character command
2459+
# such as "\doteq".
24572460
prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
2458-
# Binary operators at start of string should not be spaced
2459-
# Also, operators in sub- or superscripts should not be spaced
2461+
next_char = next(
2462+
(c for c in s[loc + len(toks["sym"]):] if c != ' '), '')
2463+
# A binary operator acting as a unary operator -- at the start of
2464+
# the string, or following another operator or an opening delimiter
2465+
# -- is not spaced. Operators in sub- or superscripts are not
2466+
# spaced either.
24602467
if (self._needs_space_after_subsuper or (
24612468
c in self._binary_operators and (
2462-
len(s[:loc].split()) == 0 or prev_char in {
2463-
'{', *self._left_delims, *self._relation_symbols}))):
2469+
prev_char in {'', '{', *self._left_delims,
2470+
*self._relation_symbols}))):
24642471
return [char]
2465-
else:
2466-
return [Hlist([self._make_space(0.2),
2467-
char,
2468-
self._make_space(0.2)],
2469-
do_kern=True)]
2472+
# Otherwise a spaced symbol gets a space on each side, except on a
2473+
# side where it abuts the start or end of the (sub-)expression: TeX
2474+
# discards the glue at the boundaries of a math list, so e.g. in
2475+
# "a$=b$" there is no space before the "=" (gh-23315).
2476+
left_space = prev_char not in {'', '{', *self._left_delims}
2477+
right_space = next_char not in {'', '}', *self._right_delims}
2478+
return [Hlist([*([self._make_space(0.2)] if left_space else []),
2479+
char,
2480+
*([self._make_space(0.2)] if right_space else [])],
2481+
do_kern=True)]
24702482
elif c in self._punctuation_symbols:
24712483
prev_char = next((c for c in s[:loc][::-1] if c != ' '), '')
24722484
next_char = next((c for c in s[loc + 1:] if c != ' '), '')

‎lib/matplotlib/tests/test_mathtext.py‎

Lines changed: 19 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -114,7 +114,7 @@
114114
r'$\mathring{A} \AA$',
115115
r'$M \, M \thinspace M \/ M \> M \: M \; M \ M \enspace M \quad M \qquad M \! M$',
116116
r'$\Cap$ $\Cup$ $\leftharpoonup$ $\barwedge$ $\rightharpoonup$',
117-
r'$\hspace{-0.2}\dotplus\hspace{-0.2}$ $\hspace{-0.2}\doteq\hspace{-0.2}$ $\hspace{-0.2}\doteqdot\hspace{-0.2}$ $\ddots$',
117+
r'$\dotplus$ $\doteq$ $\doteqdot$ $\ddots$', # github issue #23315
118118
r'$xyz^kx_kx^py^{p-2} d_i^jb_jc_kd x^j_i E^0 E^0_u$', # github issue #4873
119119
r'${xyz}^k{x}_{k}{x}^{p}{y}^{p-2} {d}_{i}^{j}{b}_{j}{c}_{k}{d} {x}^{j}_{i}{E}^{0}{E}^0_u$',
120120
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):
596596
fig_ref.text(0.1, 0.2, r"$\text{\{example\}}$")
597597

598598

599+
@check_figures_equal()
600+
def test_spaced_operator_at_boundary(fig_test, fig_ref):
601+
# A spaced operator (relation, binary, or arrow) at the start or end of a
602+
# math expression is not spaced on the boundary side, matching TeX, which
603+
# discards glue at the boundaries of a math list (gh-23315). The reference
604+
# cancels the would-be boundary space with a matching negative \hspace.
605+
fig_test.text(0.5, 0.1, r"$=b$") # relation at start
606+
fig_test.text(0.5, 0.3, r"$b=$") # relation at end
607+
fig_test.text(0.5, 0.5, r"$\doteq$") # named relation, both ends
608+
fig_test.text(0.5, 0.7, r"$b\doteq$") # named relation at end
609+
fig_test.text(0.5, 0.9, r"$\leftharpoonup$") # arrow, both ends
610+
fig_ref.text(0.5, 0.1, r"$\hspace{-0.2}=b$")
611+
fig_ref.text(0.5, 0.3, r"$b=\hspace{-0.2}$")
612+
fig_ref.text(0.5, 0.5, r"$\hspace{-0.2}\doteq\hspace{-0.2}$")
613+
fig_ref.text(0.5, 0.7, r"$b\doteq\hspace{-0.2}$")
614+
fig_ref.text(0.5, 0.9, r"$\hspace{-0.2}\leftharpoonup\hspace{-0.2}$")
615+
616+
599617
@check_figures_equal()
600618
def test_boldsymbol(fig_test, fig_ref):
601619
fig_test.text(0.1, 0.2, r"$\boldsymbol{\mathrm{abc0123\alpha}}$")

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