关于python：UnboundLocalError：在赋值之前引用的局部变量’user_a’

UnboundLocalError: local variable 'user_a' referenced before assignment

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我真的不知道如何使下面的代码工作。我得到这个例外：

UnboundLocalError: local variable 'u' referenced before assignment

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user_a ="No selection"
def if_statement():
user_choice = input("Pick 1 or 2
")
if user_choice =="1":
user_a = input("What would you like A to equal?
")
if_statement()
elif user_choice =="2":
print("A equals:" + user_a)
if_statement()
if_statement()

有人能帮我吗？我必须指定我是Python的新手。提前谢谢。

解决方案(S)：

使用一些默认值作为参数：

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def if_statement(user_a='no selection'):
user_choice = raw_input("Pick 1 or 2
")
if user_choice =="1":
u = input("What would you like A to equal?
")
if_statement(user_a=u)
elif user_choice =="2":
print("A equals:" + user_a)
if_statement(user_a=user_a)

if_statement()

或者，您也可以这样使用global：

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user_a ="No selection"
def if_statement():
global user_a # here is the trick ;-)
user_choice = raw_input("Pick 1 or 2
")
if user_choice =="1":
user_a = input("What would you like A to equal?
")
if_statement()
elif user_choice =="2":
print("A equals:" + user_a)
if_statement()

if_statement()