title: Combination function in Python
body: <p>I want to write a function which gives me a combination of numbers (with the given number of them) as a coefficients, that sum of them gives us 4.</p>
  <p>Let's say the given variable number is 2. So we need to say find combination of <code>a+b=4</code>. It'll be <code>[1,3], [2,2], [3,1], [0,4], [4,0]</code>. Or if the given variable number is 3, so need to find combination of <code>a+b+c=4</code> which will be <code>[1,2,1], [1,1,2]</code>, etc.</p>
  <p>How can I do this?</p>