Python编写键盘程序时函数调用变量取值异常问题咨询

问题原因 #

这是Python变量作用域规则导致的：

• 函数内部直接赋值的变量默认属于当前函数的局部变量，仅在函数内部生效，不会覆盖同名的全局变量
• 你在letter001()中赋值的letter1是该函数的局部变量，全局作用域下的letter1始终是初始值"a"，而letter002()中读取的letter1就是全局变量，因此打印结果始终带a

解决方案 #

方案1：用global声明修改全局变量 #

如果需要在函数内部修改全局变量，提前用global关键字声明即可，保留原有初始赋值逻辑，修改后代码如下：

baseLetter = ("a")

letter1 = (baseLetter)
letter2 = (baseLetter)

def letter002():
  global letter1
  letter2 = input("")
  if letter2 == ("a"):
    letter2 = ("a")
    print("{}{}".format(letter1, letter2))
  elif letter2 == ("b"):
    letter2 = ("b")
    print("{}{}".format(letter1, letter2))
  elif letter2 == ("c"):
    letter2 = ("c")

def letter001():
  global letter1
  letter1 = input("")
  if letter1 == ("a"):
    letter1 = ("a")
    print("output - {}".format(letter1))
    letter002()
  elif letter1 == ("b"):
    letter1 = ("b")
    print("output - {}".format(letter1))
    letter002()
  elif letter1 == ("c"):
    letter1 = ("c")
    print("output - {}".format(letter1))
    letter002()

方案2：通过参数传值（更推荐，避免全局变量污染） #

不需要修改全局变量，直接把letter1作为参数传递给letter002()即可，代码修改如下：

baseLetter = ("a")

letter1 = (baseLetter)
letter2 = (baseLetter)

def letter002(l1):
  letter2 = input("")
  if letter2 == ("a"):
    letter2 = ("a")
    print("{}{}".format(l1, letter2))
  elif letter2 == ("b"):
    letter2 = ("b")
    print("{}{}".format(l1, letter2))
  elif letter2 == ("c"):
    letter2 = ("c")

def letter001():
  letter1 = input("")
  if letter1 == ("a"):
    letter1 = ("a")
    print("output - {}".format(letter1))
    letter002(letter1)
  elif letter1 == ("b"):
    letter1 = ("b")
    print("output - {}".format(letter1))
    letter002(letter1)
  elif letter1 == ("c"):
    letter1 = ("c")
    print("output - {}".format(letter1))
    letter002(letter1)

两种方案运行后，输入c作为letter1、输入b作为letter2时，都会输出你预期的cb。

内容的提问来源于stack exchange，提问作者Cameron