I ran into the same issue that you are facing and this is how I got it fixed:

1. Just want to be careful, you need to exist the Terminal and open a fresh one to start. This way, you are not under any devtoolset's bash.
2. Go to /opt/rh folder, run command ls -la to see if you have any devtoolset-* folder there. Let's say you have devtoolset-8, proceed step 2.
3. Go to /etc/scl/prefixes folder, if you don't see devtoolset-8 file, you can create a new one as devtoolset-8, and type 1 line: /opt/rh, then save and quit that file.
4. Once you are done, you can call: scl enable devtoolset-8 -- bash w/o any error. Good luck

It is because of your settings in .bashrc and/or .bash_profile. scl appends its path to gcc7 before starting your new bash shell, but the path to gcc is default to the old gcc4 again because your .bashrc appends /usr/bin to the path when starting the new bash, and it supersedes the scl settings. The solution is to use the "enable" script directly. That way no further path settings annoy you.

Enable the software collection in the answer is only effective in the current shell. The scl utility will create a "child-shell" that set the PATH variables properly, so that in the new child-shell, the enabled software collections will be firstly searched. These settings obviously only take effective temporarily in the current shell.

To make it permanently effective, add the command, source /opt/rh/devtoolset-7/enable to the user's profile (~/.bash_profile or ~/.bashrc for RHEL based OS, like CentOS 7). Then, start a new shell and you will have the right tools available.

After execute scl enable devtoolset-7 bash, you will need to execute exit twice to exit the opened shell window, which verifies that the scl command created a new shell instance as a child process. There might be side-effect with creating a child-shell, so do not put this command in the ~/.bashrc profile, otherwise it will repeatedly create child-shell (non-login shell) as each shell will load the profile, resulting in a endless recursive loop. Put it in ~/.bash_profile, it will be loaded for only once (for the login shell), but you will need to exit twice every time.

But for development purpose, scl enable devtoolset-7 bash would be preferred, as you can exit the created child-shell, and then switch between different versions of the same software.

More details about the GCC version in python terminal:

The version info of the built-in Python in CentOS 7:

[root@conda condabuilder]# python
Python 2.7.5 (default, Nov 16 2020, 22:23:17) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-44)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

The version info of the user installed (via conda) Python on a system even without higher version of GCC installed:

[root@conda condabuilder]# conda activate jupyter
(jupyter) [root@conda condabuilder]# python -VV
Python 3.10.9 | packaged by conda-forge | (main, Feb  2 2023, 20:20:04) [GCC 11.3.0]

From the results, we can see that the GCC version contained in Python's version info is not related to the system's GCC. The system's default Python (2.7.5) should have been compiled with the GCC version distributed with CentOS 7, so the version info show the same GCC version. But for user installed python, the GCC version info actually depends on what version of GCC is used for building and packging the python binary.

Update: Installing latest version of gcc 9: (gcc 9.3.0) - released March 12, 2020:

Same method can be applied to gcc 10 (gcc 10.1.0) - released May 7, 2020

Download file: gcc-9.3.0.tar.gz or gcc-10.1.0.tar.gz

Compile and install:

//required libraries: (some may already have been installed)
dnf install libmpc-devel mpfr-devel gmp-devel

//if dnf install libmpc-devel is not working try:
dnf --enablerepo=PowerTools install libmpc-devel

//install zlib
dnf install zlib-devel*

./configure --with-system-zlib --disable-multilib --enable-languages=c,c++

make -j 8 <== this may take around an hour or more to finish
              (depending on your cpu speed)

make install

Tested under CentOS 7.8.2003 for gcc 9.3 and gcc 10.1

Tested under CentOS 8.1.1911 for gcc 10.1 (may take more time to compile)

Results: gcc/g++ 9.3.0/10.1.0

Installing gcc 7.4 (gcc 7.4.0) - released December 6, 2018:

Download file: https://ftp.gnu.org/gnu/gcc/gcc-7.4.0/gcc-7.4.0.tar.gz

Compile and install:

//required libraries:
yum install libmpc-devel mpfr-devel gmp-devel

./configure --with-system-zlib --disable-multilib --enable-languages=c,c++

make -j 8 <== this may take around 50 minutes or less to finish with 8 threads
              (depending on your cpu speed)


make install

Result:

Notes:

1. This Stack Overflow answer will help to see how to verify the downloaded source file.

2. Use the option --prefix to install gcc to another directory other than the default one. The toplevel installation directory defaults to /usr/local. Read about gcc installation options

devtoolset-8 was only released a short while ago. The linked installation instructions may be of use. However, your question pertains to CentOS, and this does not yet appear to have been made available yet. You can see some evidence of it being build for CentOS here, but it's not been updated for the final release yet.

You could ask on the SCL mailing list for an ETA, or wait until it appears in its final form. In the meantime, you could download the RPMs from koji directly.

Following commands in Dockerfile worked for me:

RUN yum install -y centos-release-scl
RUN yum install -y devtoolset-7-gcc-*
RUN echo "source scl_source enable devtoolset-7" >> /etc/bashrc
RUN source /etc/bashrc