There are "append" options in the DNF (excludepkgs, includepkgs, installonlypkgs, tsflags). The "append" options differs from others in setting process - the new values are appended to the original value, they do not override it.

All examples below assumes "excludepkgs = iftop" in the configuration file.

Example 1:
"dnf  update --exclude=kernel*,ansible --exclude=mc" is executed.
The "--exclude" argument from command line appends values. So iftop, kernel*, ansible, mc are excluded.
But sometimes an user wants to clear/replace values defined in "append" options.  So, all "append" options have the way of clearing an existing value. -> Setting empty value clears all existing values.

Example 2:
"dnf  update --exclude= --exclude=kernel*,ansible --exclude=mc" is executed.
Arguments are solved from left to right. The first "--exclude=" clears existing excludes and the second and the thirds "--exclude" add new excludes. So kernel*, ansible, mc are excluded.

Example 3:
"dnf  update --exclude=kernel*,ansible --exclude= --exclude=mc" is executed.
Arguments are solved from left to right. The second "--exclude=" clears existing excludes and the thirds "--exclude" add new excludes. So mc is excluded.

Example 4:
"dnf  update --exclude=kernel*,ansible --exclude=mc --exclude=" is executed.
Arguments are solved from left to right. The last "--exclude=" clears existing excludes. So nothing is excluded.

A notation of command can be shortened (only one "--exclude").

Example 5:
Instead of "dnf  update --exclude= --exclude=kernel*,ansible --exclude=mc" user can write "dnf  update --exclude=,kernel*,ansible,mc". Before first ',' character is empty item.

Example 6:
Instead of "dnf  update --exclude=kernel*,ansible --exclude= --exclude=mc" user can write "dnf  update --exclude=kernel*,ansible,,mc". Before "mc" is empty value (two commas).

Example 7:
Instead of "dnf  update --exclude=kernel*,ansible --exclude=mc --exclude=" user can write "dnf  update --exclude=kernel*,ansible,mc,". The empty value is behind the last ','. Nothing is excluded.

The example 7 is the reported problem. It is not a problem but a feature of DNF. But I think that the feature is not correctly described in the documentation.

--------
We suggest:
1. to check/improve documentation
2. to remove support for Example 6 and Example 7 from DNF. It is redundant. The "--exclude=kernel*,ansible,,mc" is equivalent to "--exclude=,mc". The "--exclude=kernel*,ansible,mc," is equivalent to "--exclude=". And to keep only first shortened notation (Example 5) "--exclude=,value".

PRs with chages:
https://github.com/rpm-software-management/libdnf/pull/879
https://github.com/rpm-software-management/dnf/pull/1574

CI tests PR:
https://github.com/rpm-software-management/ci-dnf-stack/pull/764

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (yum bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2020:4510