《Python编程快速上手2 趣味小项目轻松学》第23个项目《蚀刻绘图器》。运行代码后输入SD，为什么输出
函数getCanvasString(canvasData, cx, cy)里面的一长串if, elif和set()的逻辑没有理解到。请指点。谢谢！
整个项目代码如下：
一个艺术程序，它在屏幕周围使用WASD 键画一条连续线。
import shutil, sys
# 为行字符设置常量：
UP_DOWN_CHAR = chr(9474) # 字符 9474 是“│”
LEFT_RIGHT_CHAR = chr(9472) # 字符 9472 是 '─'
DOWN_RIGHT_CHAR = chr(9484) # 字符 9484 是 '┌'
DOWN_LEFT_CHAR = chr(9488) # 字符 9488 是 '┐'
UP_RIGHT_CHAR = chr(9492) # 字符 9492 是 '└'
UP_LEFT_CHAR = chr(9496) # 字符 9496 是 '┘'
UP_DOWN_RIGHT_CHAR = chr(9500) # 字符 9500 是 '├'
UP_DOWN_LEFT_CHAR = chr(9508) # 字符 9508 是 '┤'
DOWN_LEFT_RIGHT_CHAR = chr(9516) # 字符 9516 是 '┬'
UP_LEFT_RIGHT_CHAR = chr(9524) # 字符 9524 是 '┴'
CROSS_CHAR = chr(9532) # 字符 9532 是 '┼'
# chr() 代码列表位于 https://inventwithpython.com/chr
# 获取终端窗口的大小：
CANVAS_WIDTH, CANVAS_HEIGHT = shutil.get_terminal_size()
# 我们无法在 Windows 上打印到最后一列而不添加
# 自动换行，所以宽度减一：
CANVAS_WIDTH -= 1
# 在命令信息行的底部几行留出空间。
CANVAS_HEIGHT -= 5
"""The keys for canvas will be (x, y) integer tuples for the coordinate,
and the value is a set of letters W, A, S, D that tell what kind of line
should be drawn."""
canvas = {}
cursorX = 0
cursorY = 0
def getCanvasString(canvasData, cx, cy):
"""返回在 canvasData 中绘制的线的多行字符串。"""
canvasStr = ''
"""canvasData is a dictionary with (x, y) tuple keys and values that
are sets of 'W', 'A', 'S', and/or 'D' strings to show which
directions the lines are drawn at each xy point."""
for rowNum in range(CANVAS_HEIGHT):
for columnNum in range(CANVAS_WIDTH):
if columnNum == cx and rowNum == cy:
canvasStr += '#'
continue
# 将此点的行字符添加到canvasStr。
cell = canvasData.get((columnNum, rowNum))
if cell in (set(['W', 'S']), set(['W']), set(['S'])):
canvasStr += UP_DOWN_CHAR
elif cell in (set(['A', 'D']), set(['A']), set(['D'])):
canvasStr += LEFT_RIGHT_CHAR
elif cell == set(['S', 'D']):
canvasStr += DOWN_RIGHT_CHAR
elif cell == set(['A', 'S']):
canvasStr += DOWN_LEFT_CHAR
elif cell == set(['W', 'D']):
canvasStr += UP_RIGHT_CHAR
elif cell == set(['W', 'A']):
canvasStr += UP_LEFT_CHAR
elif cell == set(['W', 'S', 'D']):
canvasStr += UP_DOWN_RIGHT_CHAR
elif cell == set(['W', 'S', 'A']):
canvasStr += UP_DOWN_LEFT_CHAR
elif cell == set(['A', 'S', 'D']):
canvasStr += DOWN_LEFT_RIGHT_CHAR
elif cell == set(['W', 'A', 'D']):
canvasStr += UP_LEFT_RIGHT_CHAR
elif cell == set(['W', 'A', 'S', 'D']):
canvasStr += CROSS_CHAR
elif cell == None:
canvasStr += ' '
canvasStr += '\n' # 在每一行的末尾添加一个换行符。
return canvasStr
moves = []
while True: # 主程序循环。
# 根据画布中的数据绘制线条:
print(getCanvasString(canvas, cursorX, cursorY))
print('WASD keys to move, H for help, C to clear, '
+ 'F to save, or QUIT.')
response = input('> ').upper()
if response == 'QUIT':
print('Thanks for playing!')
sys.exit() # 退出程序。
elif response == 'H':
print('Enter W, A, S, and D characters to move the cursor and')
print('draw a line behind it as it moves. For example, ddd')
print('draws a line going right and sssdddwwwaaa draws a box.')
print()
print('You can save your drawing to a text file by entering F.')
input('Press Enter to return to the program...')
continue
elif response == 'C':
canvas = {} # 擦除画布数据。
moves.append('C') # 记录这一举动。
elif response == 'F':
# 将画布字符串保存为一个文本文件:
try:
print('Enter filename to save to:')
filename = input('> ')
# 确保文件名以.txt结尾:
if not filename.endswith('.txt'):
filename += '.txt'
with open(filename, 'w', encoding='utf-8') as file:
file.write(''.join(moves) + '\n')
file.write(getCanvasString(canvas, None, None))
except:
print('ERROR: Could not save file.')
for command in response:
if command not in ('W', 'A', 'S', 'D'):
continue # 忽略这封信，继续下一封信。
moves.append(command) # 记录这一举动。
# 我们添加的第一行需要形成一个完整的行:
if canvas == {}:
if command in ('W', 'S'):
# 使第一行是水平的:
canvas[(cursorX, cursorY)] = set(['W', 'S'])
elif command in ('A', 'D'):
# 让第一行是垂直的:
canvas[(cursorX, cursorY)] = set(['A', 'D'])
# 更新x和y:
if command == 'W' and cursorY > 0:
canvas[(cursorX, cursorY)].add(command)
cursorY = cursorY - 1
elif command == 'S' and cursorY < CANVAS_HEIGHT - 1:
canvas[(cursorX, cursorY)].add(command)
cursorY = cursorY + 1
elif command == 'A' and cursorX > 0:
canvas[(cursorX, cursorY)].add(command)
cursorX = cursorX - 1
elif command == 'D' and cursorX < CANVAS_WIDTH - 1:
canvas[(cursorX, cursorY)].add(command)
cursorX = cursorX + 1
else:
# 如果光标没有移动，因为它会移动到
# 画布的边缘，然后不要更改
# 画布的设置[(cursorX, cursorY)]。
continue
# 如果没有设置（cursorX，cursorY），请添加一个空集：
if (cursorX, cursorY) not in canvas:
canvas[(cursorX, cursorY)] = set()
# 将方向弦添加到xy点的集合中:
if command == 'W':
canvas[(cursorX, cursorY)].add('S')
elif command == 'S':
canvas[(cursorX, cursorY)].add('W')
elif command == 'A':
canvas[(cursorX, cursorY)].add('D')
elif command == 'D':
canvas[(cursorX, cursorY)].add('A')
