informative error message when chrom length > gc length (closes #1405)

stdpopsim/genomes.py

@@ -620,6 +620,14 @@ def species_contig(
                 ploidy=ploidy,
             )
 
+        if contig.gene_conversion_length is not None:
+            if contig.length <= contig.gene_conversion_length:
+                raise ValueError(
+                    "Cannot simulate a contig whose length is shorter "
+                    "than the gene conversion length "
+                    f"({contig.gene_conversion_length}bp)."
+                )
+
         return contig
 
     @property

tests/test_genomes.py

@@ -77,6 +77,11 @@ def test_gc_errors(self):
                     gene_conversion_length=bad_len,
                 )
 
+    def test_gc_length_error(self):
+        sp = stdpopsim.get_species("StrAga")
+        with pytest.raises(ValueError, match="shorter than the gene conv"):
+            _ = sp.get_contig(length=100000)
+
     def test_default_dfe(self):
         contig = stdpopsim.Contig.basic_contig(length=100)
         assert len(contig.dfe_list) == 1

tests/test_species.py

@@ -182,7 +182,7 @@ def test_default_ploidy(self):
         for chrom in self.species.genome.chromosomes:
             contig = self.species.get_contig(chrom.id)
             assert contig.ploidy is not None
-        contig = self.species.get_contig(length=1000)
+        contig = self.species.get_contig(length=200000)
         assert contig.ploidy == self.species.ploidy