fix comparisons (issue #797) (#975)

This PR fixes #797, it is, handles comparisons involving one side with a
numeric expression and the other one with an expression involving
strings.

CHANGES.rst

@@ -47,7 +47,8 @@ Bugs
 * ``Switch[]`` involving ``Infinity`` Issue #956
 * ``Outer[]`` on ``SparseArray`` Issue #939
 * ``ArrayQ[]`` detects ``SparseArray`` PR #947
-
+* Numeric comparisons against expressions involving ``String``s (Issue #797).
+  
 Package updates
 +++++++++++++++
 

mathics/eval/testing_expressions.py

@@ -20,7 +20,11 @@ def do_cmp(x1, x2) -> Optional[int]:
             return None
 
     s1 = x1.to_sympy()
+    if s1 is None:
+        return None
     s2 = x2.to_sympy()
+    if s2 is None:
+        return None
 
     # Use internal comparisons only for Real which is uses
     # WL's interpretation of equal (which allows for slop

test/builtin/test_comparison.py

@@ -78,6 +78,14 @@
         ("g[a]<g[a]", "g[a] < g[a]", "not comparable expressions (like in WMA)"),
         ("g[1]<g[1]", "g[1] < g[1]", "not comparable expressions (like in WMA)"),
         #
+        ('Wo["x"]>3', "Wo[x] > 3", "isue #797"),
+        ('Wo["x"]<3', "Wo[x] < 3", "isue #797"),
+        ('Wo["x"]==3', "Wo[x] == 3", "isue #797"),
+        ('3>Wo["x"]', "3 > Wo[x]", "isue #797"),
+        ('3<Wo["x"]', "3 < Wo[x]", "isue #797"),
+        ('3==Wo["x"]', "3 == Wo[x]", "isue #797"),
+        ('Wo[f["x"],2]>0', "Wo[f[x], 2] > 0", "isue #797"),
+        #
         # chained compare
         ("a != a != b", "False", "Strange MMA behavior"),
         ("a != b != a", "a != b != a", "incomparable values should be unchanged"),