The goal for this step is to select a random falling piece in a random color, to position it in the top-middle of the board, and draw it over the board.  Note that the "falling piece" will not be falling after this step.  That comes later.  For now, it will remain at the top-middle of the board.  Also, for testing purposes, we will add temporary code that changes the falling piece whenever any key is pressed.  This code will be removed after this step.
 

Recall that the falling piece is not part of the board, but drawn over the board.  It becomes part of the board when it can no longer fall and we place it on the board, which will not happen until several steps from now in our design process.
 

Defining the piece types:
In this design, the falling piece is represented by a 2-dimensional list of booleans, indicating whether the given cell is or is not painted in this piece.  For example, here is the definition of an S piece:
  sPiece = [
    [ False, True, True],
    [ True,  True, False ]
  ]
This defines a 2-dimensional boolean list with 2 rows and 3 columns.  The "True" values have been highlighted to make it clear that they form an S pattern.  This is how we know which cells are part of the falling piece.
 

Here are all the piece types, with the "True" values highlighted to make the shapes easier to discern (note that they are provided in their standard configurations -- how they should enter the board from the top -- so, for example, the T piece is upside down):

  #Seven "standard" pieces (tetrominoes)
  iPiece = [
    [ True,  True,  True,  True]
  ]
  
  jPiece = [
    [ True, False, False ],
    [ True, True,  True]
  ]
  
  lPiece = [
    [ False, False, True],
    [ True,  True,  True]
  ]
  
  oPiece = [
    [ True, True],
    [ True, True]
  ]
  
  sPiece = [
    [ False, True, True],
    [ True,  True, False ]
  ]
  
  tPiece = [
    [ False, True, False ],
    [ True,  True, True]
  ]

  zPiece = [
    [ True,  True, False ],
    [ False, True, True]
  ]

For this design, we need to place all 7 of these piece types into a single list, tetrisPieces.  That will be a list of 2-dimensional lists, so it is in fact a 3-dimensional list!  Here it is:
  tetrisPieces = [ iPiece, jPiece, lPiece, oPiece, sPiece, tPiece, zPiece ]
 

We also need to define colors corresponding to each of these pieces, and place them in a list of the same size, which we do as such:
  tetrisPieceColors = [ "red", "yellow", "magenta", "pink", "cyan", "green", "orange" ]
 

Store these values in the canvas's data:
We should define the pieces and the piece colors in our init function, and then we should also store them in the canvas's data, with this code:
  canvas.data["tetrisPieces"] = tetrisPieces
  canvas.data["tetrisPieceColors"] = tetrisPieceColors

Writing newFallingPiece:
The newFallingPiece function is responsible for randomly choosing a new piece, setting its color, and positioning it in the middle of the top row.  The first step is to randomly choose an index from the tetrisPieces list (using the random.randint function), then to set the canvas data values holding the fallingPiece and the fallingPieceColor to the respective elements from the lists of tetrisPieces and tetrisColors.

Next, we set the top row of the falling piece (fallingPieceRow) to the top row of the board (that is, to zero).  And then we set the left column of the new falling piece (fallingPieceCol).  As we want the piece to emerge at the top-middle of the board, we might set this to the center, or cols/2.  However, this will place the new falling piece a bit off to the right, since its full width would extend beyond the middle.  To compensate for this fact, we subtract half the width of the falling piece, or fallingPieceCols/2, from the middle of the board
 

Drawing the falling piece:
After calling drawBoard, you should then call drawFallingPiece, so the falling piece is drawn over the board (in this way, to the user it looks like the falling piece is on the board, but in reality it is stored separately and drawn separately).  To draw the falling piece (in the drawFallingPiece method), we iterate over each cell in the fallingPiece, and if the value of that cell is True, then we should draw it reusing the same drawCell function that drawBoard uses, but in the color of the fallingPiece (rather than the color stored in the board for that row and column).  However, we have to add the offset of the left-top row and column (that is, fallingPieceRow and fallingPieceCol) so that the fallingPiece is properly positioned on the board.
 

Initial falling piece:
We must add one line to our init function to select the first falling piece of the game.
 

Temporary test code:
As noted above, for testing purposes, we will add temporary code that changes the falling piece whenever any key is pressed.  This code will be removed after this step.  This involves the following simple key event handler:
def keyPressed(event):
  # for now, for testing purposes, just choose a new falling piece
  # whenever ANY key is pressed!
  canvas = event.widget.canvas
  newFallingPiece(canvas)
  redrawAll(canvas)