'Create a new formatting context.
The format context is used for when an explanation wants to
have a variable value formatted in the assertion message.  In
this case the value required can be added using
.explanation_param().  Finally .pop_format_context() is used
to format a string of %-formatted values as added by
.explanation_param().'

def push_format_context(self):

    self.explanation_specifiers = {}
    self.stack.append(self.explanation_specifiers)
