You are using pip3 to install flask-script which is associated with python 3.5. However, you are trying to upgrade pip associated with the python 2.7, try running pip3 install --upgrade pip.

It might be a good idea to take some time and read about virtual environments in Python. It isn't a best practice to install all of your packages to the base python installation. This would be a good start: http://docs.python-guide.org/en/latest/dev/virtualenvs/

1. Is pip the same for 3.4+

   No, it's not. A single pip installation serves a single Python distribution (pip2.7/pip3.4/pip3.5 etc).

2. Since Python 3.5, pip is already bundled with the python distribution, so you can just run python3.6 -m pip instead of pip.

3. Python 3.6 is not available in CentOS 7 vanilla repo. I usually resort to IUS repo when needing to install a fresh Python on CentOS. It always has the most recent Python version, the current one being 3.6.5. It also offers a correspondent pip package.

   $ yum install https://centos7.iuscommunity.org/ius-release.rpm
   $ yum install python36u python36u-devel python36u-pip
   

   Unfortunately, IUS doesn't offer a package for Python 3.7 yet so if you are looking for Python 3.7 on CentOS 7, building from source is your only option.

Edit: when yum is not an option

You should prefer the bootstrapping solution described in this answer as it is the most reliable way to get a working pip installed.

I usually just run the following commands to upgrade both pip2 (=pip by default) and pip3:

sudo -H pip3 install --upgrade pip
sudo -H pip2 install --upgrade pip

You must make sure that you upgrade the version (for Python 2 or 3), which you want to react on the command pip without number, last.

Also please note that this keeps the old packaged versions installed through apt-get or any other package manager, but adds new versions which have nothing to do with the system packages. The pip-installed packages will be preferred, but you should not remove the apt-get-installed ones either, because the package manager can't know that any pip version is installed otherwise.

The easiest way I've found to install pip3 (for python3.x packages) on CentOS 7 is:

$ sudo yum install python34-setuptools
$ sudo easy_install-3.4 pip

You'll need to have the EPEL repository enabled before hand, of course.

You should now be able to run commands like the following to install packages for python3.x:

$ pip3 install foo

Summarised from another site:

Core package repositories for CentOS 7 does not have python-pip. For that you need to enable an EPEL ("Extra Packages for Enterprise Linux") repository. You do that with

sudo yum install epel-release

After that, you should be able to install pip with

sudo yum install python-pip

Also possibly related:

• How to install pip in CentOS 7? (on StackOverflow, old)
• Recommended way to install pip(3) on centos7 (on StackOverflow, newer)

In particular, one answer there states that

Since Python 3.5, pip is already bundled with the python distribution, so you can just run python3.6 -m pip instead of pip.

You should have taken the default available python3, that is the python3.6 package in centos7 that would have been easier to setup rather than compile an unsupported version. Suggest you install the supported python3 package in centos

Try doing yum install python36 from repository

sudo yum install -y https://repo.ius.io/ius-release-el7.rpm

Update yum package

 sudo yum update

Install python36 along with pip

sudo yum install -y python36u python36u-libs python36u-devel python36u-pip

Below steps are for python3.7, suggest avoiding unsupported packages. Alternate Steps for pip setup for Centos You need to install pip for python3.7 series Step 1: First install the EPEL Repository

sudo yum install epel-release

Step 2: Installing pip

python37 -m pip

Step 3: Verify if pip was installed properly pip --version

If the command not found error shows up, try

python37 -m ensurepip