Cache is_projective_planar() method for graphs

.gitignore

@@ -462,6 +462,7 @@ src/sage/graphs/graph_decompositions/__init__.py
 src/sage/graphs/generators/__init__.py
 src/sage/graphs/__init__.py
 src/sage/graphs/base/__init__.py
+src/sage/graphs/tests/__init__.py
 src/sage/databases/__init__.py
 src/sage/stats/hmm/__init__.py
 src/sage/stats/__init__.py

meson.build

@@ -195,6 +195,7 @@ file_paths = [
   'src/sage/graphs/generators/__init__.py',
   'src/sage/graphs/__init__.py',
   'src/sage/graphs/base/__init__.py',
+  'src/sage/graphs/tests/__init__.py',
   'src/sage/databases/__init__.py',
   'src/sage/stats/hmm/__init__.py',
   'src/sage/stats/__init__.py',

src/sage/graphs/generators/smallgraphs.py

@@ -4189,6 +4189,7 @@ def PetersenGraph():
     from sage.graphs.generators.families import GeneralizedPetersenGraph
     P = GeneralizedPetersenGraph(5, 2)
     P.name("Petersen graph")
+    P.is_projective_planar.set_cache(True)
     return P
 
 

src/sage/graphs/graph.py

@@ -428,6 +428,7 @@
 from sage.parallel.decorate import parallel
 from sage.misc.lazy_import import lazy_import, LazyImport
 from sage.features.mcqd import Mcqd
+from sage.misc.cachefunc import cached_method
 
 lazy_import('sage.graphs.mcqd', ['mcqd'],
             feature=Mcqd())
@@ -9501,6 +9502,7 @@ def bipartite_double(self, extended=False):
         G.name("%sBipartite Double of %s" % (prefix, self.name()))
         return G
 
+    @cached_method
     @doc_index("Graph properties")
     def is_projective_planar(self, return_map=False):
         r"""
@@ -9528,7 +9530,7 @@ def is_projective_planar(self, return_map=False):
         The Petersen graph is a known projective planar graph::
 
             sage: P = graphs.PetersenGraph()
-            sage: P.is_projective_planar()  # long time
+            sage: P.is_projective_planar()
             True
 
         `K_{4,4}` has a projective plane crossing number of 2. One of the

src/sage/graphs/meson.build

@@ -91,6 +91,7 @@ py.install_sources(
   subdir: 'sage/graphs',
 )
 
+
 extension_data = {
   'asteroidal_triples' : files('asteroidal_triples.pyx'),
   'centrality' : files('centrality.pyx'),
@@ -178,3 +179,4 @@ py.extension_module(
 subdir('base')
 subdir('generators')
 subdir('graph_decompositions')
+subdir('tests')

src/sage/graphs/tests/is_projective_planar_test.py

@@ -0,0 +1,13 @@
+import pytest
+
+
+def test_petersen_graph_is_projective_planar():
+    r"""
+    Ensure that the Petersen graph is projective planar without
+    using the cache.
+    """
+    from sage.graphs.generators.smallgraphs import PetersenGraph
+    P = PetersenGraph()
+    P.is_projective_planar.clear_cache()
+    assert not P.is_projective_planar.is_in_cache()
+    assert P.is_projective_planar()

src/sage/graphs/tests/meson.build

@@ -0,0 +1,5 @@
+py.install_sources(
+  '__init__.py',
+  'is_projective_planar_test.py',
+  subdir: 'sage/graphs/tests',
+)