Beginnings of a language for programming ASCII art

ASCIICanvas.py

class CanvasError(Exception):
    pass

class Canvas:
    def __init__(self, grid=None, default=" "):
        if grid:
            self.grid = grid
        else:
            self.grid = [[]]
        self.offsetx = 0
        self.offsety = 0
        self.default = default
        self.ptrx = 0
        self.ptry = 0
        self.direction = (1,0)

    def __getitem__(self, coords):
        x, y = coords
        x += self.offsetx
        y += self.offsety
        if 0 <= y < len(self.grid) and 0 <= x < len(self.grid[0]):
            return self.grid[y][x]
        else:
            return None

    def __setitem__(self, coords, value):
        if value is not None and (type(value) is not str or len(value) != 1):
            raise CanvasError("Canvas values must be single char or None, not "
                              "%r" % value)
        x, y = coords
        x += self.offsetx
        y += self.offsety
        while y < 0:
            # Add row to the top of the grid
            row = [None] * len(self.grid[0])
            self.grid = [row] + self.grid
            self.offsety += 1
            y += 1
        while y >= len(self.grid):
            # Add row to the bottom of the grid
            row = [None] * len(self.grid[0])
            self.grid += [row]
        while x < 0:
            # Add column to the left side of the grid
            self.grid = [[None] + row for row in self.grid]
            self.offsetx += 1
            x += 1
        while x >= len(self.grid[0]):
            # Add row to the right side of the grid
            self.grid = [row + [None] for row in self.grid]
        self.grid[y][x] = value

    def __str__(self):
        return "\n".join("".join(c if c else "\0" for c in row)
                         .rstrip("\0").replace("\0", self.default)
                         for row in self.grid)

    def changeDirection(self, dirCode="."):
        if dirCode == "↑":
            self.direction = (0,-1)
        elif dirCode == "↓":
            self.direction = (0,1)
        elif dirCode == "→":
            self.direction = (1,0)
        elif dirCode == "←":
            self.direction = (-1,0)
        elif dirCode == "↖":
            self.direction = (-1,-1)
        elif dirCode == "↗":
            self.direction = (1,-1)
        elif dirCode == "↘":
            self.direction = (1,1)
        elif dirCode == "↙":
            self.direction = (-1,1)
        else:
            # Unrecognized direction code; for now, ignore silently
            pass
        return self
    
    def put(self, char):
        self[self.ptrx, self.ptry] = char
        return self

    ### Functions that implement language commands start here ###
    
    def printString(self, string, direction=None):
        for char in string:
            self.put(char).move(direction)
        return self

    def multiPrint(self, string, directions):
        replacements = {ord(key): value
                        for key, value in [("+", "←↑→↓"),
                                           ("x", "↖↗↘↙"),
                                           ("*", "←↑→↓↖↗↘↙")]}
        directions = directions.translate(replacements)
        for direction in directions:
            savedPtr = self.ptrx, self.ptry
            self.printString(string, direction)
            self.ptrx, self.ptry = savedPtr
        return self

    def move(self, direction=None, length=1):
        if direction is not None:
            self.changeDirection(direction)
        self.ptrx += self.direction[0] * length
        self.ptry += self.direction[1] * length
        return self
    
    def turn(self, direction):
        pass

    def jump(self, dx, dy, relative=True):
        if relative:
            self.ptrx += dx
            self.ptry += dy
        else:
            self.ptrx = dx
            self.ptry = dy
        return self

    def readCanvasChar(self, x=0, y=0, relative=True):
        if relative:
            result = self[x+self.ptrx, y+self.ptry]
        else:
            result = self[x, y]
        return result

    def readCanvasString(self, direction=None, length=1):
        result = ""
        for i in range(length):
            char = self.readCanvasChar()
            # TBD: use the canvas default character for empty spaces, or some
            # other sentinel value (probably not in the printable ASCII range)?
            result += self.default if char is None else char
            self.move(direction)
        return result


if __name__ == "__main__":
    c=Canvas()
    c.multiPrint("ascii", "x")
    c.jump(5, -1)
    c.printString("art", "↓")
    print(c)