python之为什么 `letter=="A"或 "a"` 总是评估为 True

请看代码。我正在使用机器人汽车绘制字母，在此代码中，当我键入 b 时，它仍会绘制小写字母 a。

import create 
 
# Draw a: 
def drawa(): 
 #create robot 
 robot = create.Create(4) 
 #switch robot to full mode 
 robot.toFullMode() 
 for i in range(1280): 
  robot.go(20,30) 
 robot.stop() 
 robot.move(-40,20) 
 
# Draw b: 
def drawb(): 
 #create robot 
 robot = create.Create(4) 
 #switch robot to full mode 
 robot.toFullMode() 
 robot.move(-100,20) 
 for i in range(1270): 
  robot.go(20,-30) 
 robot.stop() 
 
# Draw c: 
def drawc(): 
 #create robot 
 robot = create.Create(4) 
 #switch robot to full mode 
 robot.toFullMode() 
 for i in range(700): 
  robot.go(20,30) 
 robot.stop() 
 
# Define Main Function 
def main(): 
 # While loop 
 while(True): 
  # Prompt user to enter a letter 
  letter = raw_input("Please enter the letter you want to draw: ") 
  # If user enters the letter a, draw a 
  if letter=="A" or "a": 
   drawa() 
  # If user enters the letter b, draw b 
  elif letter=="B" or "b": 
   drawb(); 
  # If user enters the letter c, draw c 
  elif letter=="C" or "c": 
   drawc(); 
  # If user enters anything other than a letter from a-z, 
  # ask them to enter a valid input 
  else: 
   print("Please enter a letter from a-z.") 
 
main() 

请帮忙。