Some code fixes

Change some `is` comparison to `==`. Also, modify the `render_atom` method.

common/lib/calc/calc.py

@@ -177,9 +177,9 @@ def eval_sum(parse_result):
     total = 0.0
     current_op = operator.add
     for token in parse_result:
-        if token is '+':
+        if token == '+':
             current_op = operator.add
-        elif token is '-':
+        elif token == '-':
             current_op = operator.sub
         else:
             total = current_op(total, token)
@@ -195,9 +195,9 @@ def eval_product(parse_result):
     prod = 1.0
     current_op = operator.mul
     for token in parse_result:
-        if token is '*':
+        if token == '*':
             current_op = operator.mul
-        elif token is '/':
+        elif token == '/':
             current_op = operator.truediv
         else:
             prod = current_op(prod, token)

common/lib/calc/preview.py

@@ -253,16 +253,14 @@ def render_atom(children):
     """
     Properly handle parens, otherwise this is trivial.
     """
-    parens = None
-    if children[0].latex in "([{":
-        # then len(children) == 3
-        parens = children[0].latex
-        children = [children[1]]
+    if len(children) == 3:
         return LatexRendered(
-        children[0].latex,
-        parens,
-        children[0].tall
+            children[1].latex,
+            parens=children[0].latex,
+            tall=children[1].tall
         )
+    else:
+        return children[0]
 
 
 def latex_preview(math_expr, variables=(), functions=(), case_sensitive=False):