title: how to pass variable values to a string in python
body: <p>I have requirement where I am reading a string from json file and assign values to it to make it as a configurable file name.</p>
  
  <p>String I am reading from json:</p>
  
  <pre><code>data_file_format =  "sourcesystem_batchid_extractname_loadtype"
  </code></pre>
  
  <p>I have variable which holds the values in my code like</p>
  
  <pre><code>sourcesystem ="xyz"
    batchid = "101"
    extractname = "abc"
    loadtype = "Delta"
  </code></pre>
  
  <p>so my data_file_format should yeild value like</p>
  
  <pre><code>data_file_format = "xyz_101_abc_Delta"
  </code></pre>