When I try to upgrade pip, I get an infinite loop after failing to upgrade

environment

OS: Ubuntu16.04(Xenial Xerus)

Status

When I try to use pip3, I get the error "The version is old, please upgrade". The error says that you should use "pip install --install pip", but even if you type this command, After installing the old version, I get an error saying "Please upgrade".

In short, I'm trying to upgrade with pip, but I can't upgrade because it's an older version Is in a state where a mysterious error appears.

Specific error content

You are using pip version 8.1.1, however version 9.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command

Dealing with errors

1. Follow the instructions obediently.

pip install --upgrade pip
→ For some reason pip8.1.1 is installed and I get the same error

2. Try it with pip3

pip3 install -U pip
→ [Error]
  Not uninstalling pip at /usr/lib/python3/dist-packages, outside environment /usr

Did something go wrong because you couldn't uninstall the existing pip?

3. Try uninstalling pip itself

sudo apt-get remove python-pip
sudo apt-get remove python3-pip

↓ I tried "pip --version" and "pip3 --version", but the version of 8.1.1 is output.

4. Try uninstalling with another method

sudo python -m pip uninstall pip setuptools
sudo apt-get remove python3-pip

↓

pip3 --version
> pip 9.0.1
pip --version
> pip 9.0.1

I was able to do it this time for some reason. Moreover, when I thought that it would only be deleted, the version I was looking for was installed as 9.0.1. This completes the version upgrade of pip.

If the version upgrade was not completed, an error prompting for the version upgrade would continue to appear, so pip itself could not be used. I'm glad I was able to upgrade the version safely.