Trac #20823: parse_label() in sage/modular/modform/constructor.py fai…

…ls in GammaH case

Currently, it does not return index in the GammaH case though the Gamma0
and Gamma1 case does.

In Newform(), both the group and the index is expected L485 in
sage/modular/modform/constructor.py

URL: http://trac.sagemath.org/20823
Reported by: klui
Ticket author(s): Kevin Lui
Reviewer(s): William Stein, Frédéric Chapoton

src/sage/modular/modform/constructor.py

@@ -505,6 +505,11 @@ def parse_label(s):
         (Congruence Subgroup Gamma1(11), 0)
         sage: sage.modular.modform.constructor.parse_label('11wG1')
         (Congruence Subgroup Gamma1(11), 22)
+
+    GammaH labels should also return the group and index (:trac:`20823`)::
+
+        sage: sage.modular.modform.constructor.parse_label('389cGH[16]')
+        (Congruence Subgroup Gamma_H(389) with H generated by [16], 2)
     """
     m = re.match(r'(\d+)([a-z]+)((?:G.*)?)$', s)
     if not m:
@@ -522,9 +527,7 @@ def parse_label(s):
         if G[2] != '[' or G[-1] != ']':
             raise ValueError("Invalid congruence subgroup label: %s" % G)
         gens = [int(g.strip()) for g in G[3:-1].split(',')]
-        return arithgroup.GammaH(N, gens)
+        return arithgroup.GammaH(N, gens), index
     else:
         raise ValueError("Invalid congruence subgroup label: %s" % G)
     return G, index
-
-