How do I preserve a pre-set form instance throughout rendering and cleaning?

I am assigning a model instance to my form using the following code:

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form.instance = MyModel.Objects.get(pk=a_number)

However, when the form's clean() method is called, printing the instance via:

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print(self.instance)

reveals that the instance is not the one I assigned. How can I ensure that the form retains the specified instance during both rendering and cleaning?