Yes I totally understand your frustration. And I totally admit that I am the guilty person. However , I follow a different policy when I see that people are asking stupid questions in a thread. I just don't reply them in the thread. If I know the answer I just try to help directly otherwise I don't assume anything(home work etc). Because I don't know if the person is doing his home work or not. That's my policy though.But you are totally right , I will try to do more research from now on.

If it sounds like homework, it probably is. And regardless of whether it is or not, when you are POSTING, you need to provide details and answer questions fully. When you continually post as you're doing, it doesn't make ANYONE want to help you or even try to.

You keep saying sorry, keep saying you're the 'guilty person'...and KEEP ON DOING IT. Do you not see how this is not a good thing?

LEON71 and TB0ne, please stop arguing with each other.

TB0ne, can you please cite why you feel that their problem with python is related to this problem? Or if you feel there is not sufficient information, please indicate exactly what you feel will illustrate the needed information? If you feel you cannot continue without additional hostilities, then please refrain.

LEON71, please stay on topic, and also do not argue with other members. If information is requested and you do not understand the request, then please ask for a clarification. Do not post a lot of information if you are unsure what was requested. That clearly doesn't help. I.e if I were to ask you to show me the output of a certain command, and you then replied with your command history, it wouldn't answer the query for information.

I have to be honest. I have no idea what the problem is. It does seem that the installation is completely "unsat" meaning un-satisfactory, and either something needs to be done to reset the repository files completely, if not a full reinstall of the distribution. After data backup.

Meanwhile, the back and forth talk/arguments help no one.

If you all can't be civil, then I'll close the thread.

The python error is fairly well-known with DNF, and related to the posters other python-related thread that I noted. They upgraded from python 2 to 3, and had they put the error message into Google, would have found ample documentation on it and what it relates to. The OP was even directed to look at the beginning of the dnf file, to see what was there. So far, that has gone unheeded.
I absolutely can continue; however, posting dozens of lines of command history with ZERO context, and no output of commands isn't helpful at all. Asking what command-line options do rather than looking them up shows little effort, and this pattern has been repeated in many of the OP's previous threads. The OP has been directed to the "Question Guidelines" in the past, and has repeatedly apologized for such behavior, but it does not abate.

I apologize for any ruffled feathers.

from my point of view it started before this thread.
And as I see this system is now corrupted. Would be better to reinstall. OP cannot tell us what did he really make and looks like there is no easy way to restore.

Problem solved finally!!!

I just have a fresh installation of Fedora Workstation version 28.

Here is what I did :

1.Create a USB Live media with Fedora Media Writer.I used my 15GB USB drive for that.

2.Boot the live media from BIOS.At first the USB was not showing up in BIOS menu.After couple of frustrating hours I swap the USB port with my external 1TB hard drive and it worked !

3.Launch the live workstation version 28 and select the 1TB external hard drive for installation.

4.Successfully installed version 28.

Lesson Learned : Before playing with Fedora remember how to reinstall the system just in case something goes wrong.In my case I think I did something wrong with Python version. I think I have installed something with conda & pip which mostly likely broke my Python and thus also yum/dnf (both use Python).And without a working dnf the system is basically broken.

or, if you want a faster solution use docker (or VM) to play and/or destroy an os.

Yes ! That's another good solution. Not sure how much resource( RAM ) it will take though. I will certainly try that option in future.