title: Count number of records in lmdb databse with python
body: <p>I open a <code>lmdb</code> database using this code: </p>
  
  <pre><code>    lmdb_env = lmdb.open(source_path, readonly=True)
  </code></pre>
  
  <p>How can I count the number of records in this database?    </p>