UnboundLocalError: local variable … referenced bef

问题:

I get an UnboundLocalError because I use a template value inside an if statement which is not executed. What is the standard way to handle this situation?

class Test(webapp.RequestHandler):
    def get(self):      
        user = users.get_current_user()
        if user:
            greeting = ('Hello, ' + user.nickname())
        else:
            self.redirect(users.create_login_url(self.request.uri))
...

        template_values = {"greeting": greeting,
                       }

Error:

UnboundLocalError: local variable 'greeting' referenced before assignment

回答1:

Just Switch:

class Test(webapp.RequestHandler):
    def err_user_not_found(self):
        self.redirect(users.create_login_url(self.request.uri))
    def get(self):      
        user = users.get_current_user()
        # error path
        if not user:
            self.err_user_not_found()
            return

        # happy path
        greeting = ('Hello, ' + user.nickname())
        ...
        template_values = {"greeting": greeting,}

回答2:

I guess I need to explain the problem first: in creating template_values, you use a greeting variable. This variable will not be set if there is no user.

There isn't a standard way to handle this situation. Common approaches are:

1. make sure that the variable is initialized in every code path (in your case: including the else case)
2. initialize the variable to some reasonable default value at the beginning
3. return from the function in the code paths which cannot provide a value for the variable.

Like Daniel, I suspect that after the redirect call, you are not supposed to produce any output, anyway, so the corrected code might read

class Test(webapp.RequestHandler):
  def get(self):      
    user = users.get_current_user()
    if user:
        greeting = ('Hello, ' + user.nickname())
    else:
        self.redirect(users.create_login_url(self.request.uri))
        return
...

    template_values = {"greeting": greeting,
                   }