added a max_attempts parameter to work()

proofow.py

@@ -12,10 +12,12 @@ class pow_base:
         self._base = urandom(self._bits//8) if not base else base
         self._target = 2 ** (self._bits - difficulty)
 
-    def work(self):
-        for nonce in count():
+    def work(self, max_attempts=0):
+        cycles = count() if max_attempts == 0 else range(max_attempts)
+        for nonce in cycles:
             if self.verify(nonce):
                 return nonce
+        raise RuntimeError('Failed to find a nonce in {} iterations!'.format(max_attempts))
 
     def verify(self, nonce):
         hexresult = self._hexdigest(self._base + str(nonce).encode())