title: How to check in python that at least one of the default parameters of the function specified
body: <p>What is the best practice in python to check if at least one of the default parameters of the function is specified?</p>
  
  <p>Let's suppose we have some function:</p>
  
  <pre><code>def some_function(arg_a=None, arg_b=None, arg_c=False)
  </code></pre>
  
  <p>with some default parameters. In my case, I need to check if either <code>arg_a</code> or <code>arg_b</code> is specified. So I thought of implementing something like this:</p>
  
  <pre><code>def some_function(arg_a=None, arg_b=None, arg_c=False):
      ...
      if arg_a is not None:
          ...
      elif arg_b is not None:
          ...
      else:
          raise ValueError('Expected either arg_a or arg_b args')
      ...
      ...
  </code></pre>
  
  <p>So, what is more pythonic way to implement such kind of functionality?</p>