Modifying "default_section" in the configparser at runtime does not behave as described.

The documentation says about default_section: 

When default_section is given, it specifies the name for the special section holding default values for other sections and interpolation purposes (normally named "DEFAULT"). This value can be retrieved and changed on runtime using the default_section instance attribute.
[https://docs.python.org/3/library/configparser.html]

So, if I modify default_section at runtime, the default values for other sections should then come from the new default_section. But this is not the case. Instead, the default-values still come from self._default, which was set by self._read.

So, this is either a bug in the library or a bug in the documentation.

I've attached a testcase.