why UnboundLocalError occure

[jyoti thorat]

def post_edit(reque st,pk):
post =get_object_or_ 404(Post, pk=pk)
if request.method= ="POST":
form=PostForm(r equest.POST,ins tance=post)
if form.is_valid() :
post=form.save( commit=False)
post.author= request.user
post.published_ date=timezone.n ow()
post.save()
return redirect('post_ detail',pk=post .pk)
else:
form = PostForm(instan ce=post)
return render (request,'blog/post_edit.html' ,{'form':form})