Factor out latex ifpackageloaded pattern.

... which avoids having to explain twice its point, as well.

Author: anntzer

lib/matplotlib/backends/backend_ps.py

@@ -1150,13 +1150,9 @@ def convert_psfrags(tmpfile, psfrags, font_preamble, custom_preamble,
     with mpl.rc_context({
             "text.latex.preamble":
             mpl.rcParams["text.latex.preamble"] +
-            # Only load these packages if they have not already been loaded, in
-            # order not to clash with custom packages.
-            r"\makeatletter"
-            r"\@ifpackageloaded{color}{}{\usepackage{color}}"
-            r"\@ifpackageloaded{graphicx}{}{\usepackage{graphicx}}"
-            r"\@ifpackageloaded{psfrag}{}{\usepackage{psfrag}}"
-            r"\makeatother"
+            mpl.texmanager._usepackage_if_not_loaded("color") +
+            mpl.texmanager._usepackage_if_not_loaded("graphicx") +
+            mpl.texmanager._usepackage_if_not_loaded("psfrag") +
             r"\geometry{papersize={%(width)sin,%(height)sin},margin=0in}"
             % {"width": paper_width, "height": paper_height}
     }):

lib/matplotlib/texmanager.py

@@ -37,6 +37,23 @@
 _log = logging.getLogger(__name__)
 
 
+def _usepackage_if_not_loaded(package, *, option=None):
+    """
+    Output LaTeX code that loads a package (possibly with an option) if it
+    hasn't been loaded yet.
+
+    LaTeX cannot load twice a package with different options, so this helper
+    can be used to protect against users loading arbitrary packages/options in
+    their custom preamble.
+    """
+    option = f"[{option}]" if option is not None else ""
+    return (
+        r"\makeatletter"
+        r"\@ifpackageloaded{%(package)s}{}{\usepackage%(option)s{%(package)s}}"
+        r"\makeatother"
+    ) % {"package": package, "option": option}
+
+
 class TexManager:
     """
     Convert strings to dvi files using TeX, caching the results to a directory.
@@ -176,16 +193,10 @@ def _get_preamble(self):
             self.get_custom_preamble(),
             # Use `underscore` package to take care of underscores in text
             # The [strings] option allows to use underscores in file names
-            r"\makeatletter"
-            r"\@ifpackageloaded{underscore}{}"
-            r"{\usepackage[strings]{underscore}}"
-            r"\makeatother",
-            # textcomp is loaded last (if not already loaded by the custom
-            # preamble) in order not to clash with custom packages (e.g.
-            # newtxtext) which load it with different options.
-            r"\makeatletter"
-            r"\@ifpackageloaded{textcomp}{}{\usepackage{textcomp}}"
-            r"\makeatother",
+            _usepackage_if_not_loaded("underscore", option="strings"),
+            # Custom packages (e.g. newtxtext) may already have loaded textcomp
+            # with different options.
+            _usepackage_if_not_loaded("textcomp"),
         ])
 
     def make_tex(self, tex, fontsize):