UnboundLocalError，分配前访问了局部变量，但不是吗？

Question

I'm doing something pretty simple in Django and I'm getting this really weird error:

UnboundLocalError at /me/profile/edit/
local variable 'form' referenced before assignment

Here's my code:

if request.method == "POST":
    form = MyForm(request.POST)
    if form.is_valid():
        print "Yes"
else:
    form = MyForm(user=request.user)

Why is this code throwing that error? It's pretty straightforward, yet if I take out the if form.is_valid() stuff, it works. What's going wrong?

Answer 1

The simplest solution to this problem is to remove the else clause:

form = MyForm(request.POST or None)

if request.method == 'POST':
    if form.is_valid():
        print 'Yes'

Danny Greenfeld's Advanced Django Form Usage presentation is a great example of this: http://www.slideshare.net/pydanny/advanced-django-forms-usage (slide 33 is what I'm referencing specifically).

Answer 2

如前所述，代码看起来是正确的，因此我希望在发布问题时会丢失一些内容（也许第二个if语句并未真正缩进）。