Twiddle if not statement

I need help turning this pseudocode into actual python code: if not child already in exploredSet: make a new node, using child and how and info about the current board. frontier.enqueue(newNode)

This is what is right above the pseudocode:

`expansion = children(currentNode.getBoard())   
 for index in range (len(expansion)):
    child = expansion[index]
    how = moves[index]`