How does this Python function work?

Question

I'm going through an MIT open courseware class right now for Python, and I'm not understanding how this function is returning 9.

def a(x):
   '''
   x: int or float.
   '''
   return x + 1

a(a(a(6)))

The above returns 9. I've went through it step by step using pythontutor (Visualize Python) and I'm still not understanding.

I understand the function. It has a name of a, and takes one argument, x. If I did a(6) I'd expect 7 to be returned. It's the a(a(a(6))) that confuses me - all of the a's and parentheses.

How is this working? Maybe a step by step sequence of what each a means, etc.

Based from your replies, is this what you mean?

Answer 1

You can see it as

x = a(6) # returns 7
y = a(x) # returns 8
z = a(y) # returns 9

In both cases, the result of the function is used für the next function call, and this result for the next again.

The first function call turns 6 into 7, the second 7 into 8 and the third and last turns 8 into 9.

The image included in your question exactly describes this.