How do I access the request object or any other variable in a form's clean() method?

The answer by Ber - storing it in threadlocals - is a very bad idea. There's absolutely no reason to do it this way.

A much better way is to override the form's __init__ method to take an extra keyword argument, request. This stores the request in the form, where it's required, and from where you can access it in your clean method.

class MyForm(forms.Form):

    def __init__(self, *args, **kwargs):
        self.request = kwargs.pop('request', None)
        super(MyForm, self).__init__(*args, **kwargs)


    def clean(self):
        ... access the request object via self.request ...

and in your view:

myform = MyForm(request.POST, request=request)

UPDATED 10/25/2011: I'm now using this with a dynamically created class instead of method, as Django 1.3 displays some weirdness otherwise.

class MyModelAdmin(admin.ModelAdmin):
    form = MyCustomForm
    def get_form(self, request, obj=None, **kwargs):
        ModelForm = super(MyModelAdmin, self).get_form(request, obj, **kwargs)
        class ModelFormWithRequest(ModelForm):
            def __new__(cls, *args, **kwargs):
                kwargs['request'] = request
                return ModelForm(*args, **kwargs)
        return ModelFormWithRequest

Then override MyCustomForm.__init__ as follows:

class MyCustomForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        self.request = kwargs.pop('request', None)
        super(MyCustomForm, self).__init__(*args, **kwargs)

You can then access the request object from any method of ModelForm with self.request.

For what it's worth, if you're using Class Based Views, instead of function based views, override get_form_kwargs in your editing view. Example code for a custom CreateView:

from braces.views import LoginRequiredMixin

class MyModelCreateView(LoginRequiredMixin, CreateView):
    template_name = 'example/create.html'
    model = MyModel
    form_class = MyModelForm
    success_message = "%(my_object)s added to your site."

    def get_form_kwargs(self):
        kw = super(MyModelCreateView, self).get_form_kwargs()
        kw['request'] = self.request # the trick!
        return kw

    def form_valid(self):
        # do something

The above view code will make request available as one of the keyword arguments to the form's __init__ constructor function. Therefore in your ModelForm do:

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel

    def __init__(self, *args, **kwargs):
        # important to "pop" added kwarg before call to parent's constructor
        self.request = kwargs.pop('request')
        super(MyModelForm, self).__init__(*args, **kwargs)