def push_format_context(self):
self.explanation_specifiers = {}<EOL>self.stack.append(self.explanation_specifiers)<EOL>
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().