by using self.with_context you can add context value.

Passing single value:
    self = self.with_context(default_partner_id: partner_id)

Passing multiple value as a dictionary (ctx)
  self = self.with_context(ctx) 

Code available @ github: https://github.com/odoo/odoo/blob/8.0/openerp/models.py#L5266-L5283

def with_context(self, *args, **kwargs): 
""" with_context([context][, **overrides]) -> records
        Returns a new version of this recordset attached to an extended
        context.
        The extended context is either the provided ``context`` in which
        ``overrides`` are merged or the *current* context in which
        ``overrides`` are merged e.g.::
            # current context is {'key1': True}
            r2 = records.with_context({}, key2=True)
            # -> r2._context is {'key2': True}
            r2 = records.with_context(key2=True)
            # -> r2._context is {'key1': True, 'key2': True}
        """