title: matlab to python code conversion
body: <p>I am trying to convert a matlab code to python due to lack of matlab. I will be grateful if you kindly tell me the python equivalents of the following functions which I could not find:</p>
  
  <pre><code>letter2number_map('A') = 1;
  number2letter_map(1) = 'A';
  str2num()
  
  strcmp()
  trace()
  eye()
  getenv('STRING')
  
  [MI_true, ~, ~] = function() What does ~ mean?
  mslice
  ones()
  </code></pre>
  
  <p>Thank you very much for your kind assistance.</p>