Minor changes to the main webapp

app.py

 from math import sin, pi
 
-from flask import request
-from flask import Flask
+from flask import Flask, Response, request
 
 app = Flask(__name__)
 
@@ -20,9 +19,11 @@ def integrate(fn, lower, upper, n=10):
 @app.route("/")
 def main():
     lower = float(request.args.get("lower", 0))
-    upper = float(request.args.get("upper", 1))
+    upper = float(request.args.get("upper", pi))
+    if lower >= upper:
+        return Response(f"lower={lower} must be smaller than upper={upper}", 400)
 
-    res_strs = []
+    res_strs = [f"Integrating abs(sin(x)) from {lower} to {upper}:"]
     for n in [10, 100, 1000, 10000, 100000, 1000000]:
         res = integrate(abs_sin,
                         lower=lower,
@@ -31,4 +32,3 @@ def main():
         res_strs.append(f"result={res}, n={n}")
     res_strs.append("")
     return "\n".join(res_strs)
-