How to explain in layman's terms for this function (function, list, in and isinstance)?

Question

I came across this function:

 def count_list(l):          
      count = 0          
      for e in l:               
           if isinstance(e, list):                    
                count = count + 1 + count_list(e)                    
      return count

I couldn't understand it especially the for e in 1 and isinstance(e, list), wonder if any pro can explain? Just started learning function, list, in and isinstance

Answer 1

Let's read it:

# define the method count_list that take one argument l (lower L)
def count_list(l):
    # define count to be a int with value 0
    count = 0
    # for each item e in l (we supose that l can be itered)
    for e in l:
        # if e is a list
        if isinstance(e, list):
            # add one to count and the result of count_list called on e (recursive call)
            count = count + 1 + count_list(e)                    
    return count

Remarks:

• If e is not a list count doesn't change. This method increment count by one only when a list is found inside l.
• It also increment by the number of list in that list in a list.
• Finally it returns the number of nested list in l. For each list it meet, it add 1+number of list in that list using recursive calls.

Example:

Here a representation of the content of l:

• Element
• ListA
  • Element
  • Element
  • ListB
    • Element
    • Element
  • ListC
    • Element
• Element

What happen when we call count_list(l):

• count_list is called with the parameter l (call 1).
• The element of l are iterated.
• The element ListA is a list. count (in call 1) is incremented by 1 and the result of the call to count_list with the parameter ListA.
• count_list is called with the parameter ListA (call 2).
  • The element of ListA are iterated.
  • The element ListB is a list. count (in call 2) is incremented by 1 and the result of the call to count_list with the parameter ListB.
    • count_list is called with the parameter ListB (call 3).
    • The element of ListB are iterated.
    • No list are found 0 is returned (call 3)
  • count (in call 2) value is the result of 0+1+0. It's 1.
  • The element ListC is a list. count (in call 2) is incremented by 1 and the result of the call to count_list with the parameter ListC.
    • count_list is called with the parameter ListC (call 4).
    • The element of ListC are iterated.
    • No list are found 0 is returned (call 4)
  • count (in call 2) value is the result of 1+1+0. It's 2.
  • There is no list left in ListA.
  • 2 is returned (call 2)
• count (in call 1) value is the result of 0+1+2. It's 3.
• There is no list left in l.
• 3 is returned (call 1)