It's hard if you're trying to update the system Python and need to upgrade every...

It's hard if you're trying to update the system Python and need to upgrade every dependency for that but if you just want to have, say, “python3.7” in your path it's just a couple of commands:

https://docs.python-guide.org/starting/install3/linux/

    ~ $ docker run -it --rm ubuntu:xenial
    root@b8cda311d766:/# apt-get update -qqy
    root@b8cda311d766:/# apt-get install -qqy software-properties-common
    …
    root@b8cda311d766:/# add-apt-repository --yes ppa:deadsnakes/ppa
    gpg: keyring `/tmp/tmpe354n0av/secring.gpg' created
    gpg: keyring `/tmp/tmpe354n0av/pubring.gpg' created
    gpg: requesting key 6A755776 from hkp server keyserver.ubuntu.com
    gpg: /tmp/tmpe354n0av/trustdb.gpg: trustdb created
    gpg: key 6A755776: public key "Launchpad PPA for deadsnakes" imported
    gpg: Total number processed: 1
    gpg:               imported: 1  (RSA: 1)
    OK
    root@b8cda311d766:/# apt-get update -qqy
    root@b8cda311d766:/# apt-get install -qqy python3.7
    debconf: delaying package configuration, since apt-utils is not installed
    Selecting previously unselected package libpython3.7-minimal:amd64.
    (Reading database ... 7603 files and directories currently installed.)
    Preparing to unpack .../libpython3.7-minimal_3.7.3-1+xenial1_amd64.deb ...
    Unpacking libpython3.7-minimal:amd64 (3.7.3-1+xenial1) ...
    Selecting previously unselected package python3.7-minimal.
    Preparing to unpack .../python3.7-minimal_3.7.3-1+xenial1_amd64.deb ...
    Unpacking python3.7-minimal (3.7.3-1+xenial1) ...
    Selecting previously unselected package libpython3.7-stdlib:amd64.
    Preparing to unpack .../libpython3.7-stdlib_3.7.3-1+xenial1_amd64.deb ...
    Unpacking libpython3.7-stdlib:amd64 (3.7.3-1+xenial1) ...
    Selecting previously unselected package python3.7-lib2to3.
    Preparing to unpack .../python3.7-lib2to3_3.7.3-1+xenial1_all.deb ...
    Unpacking python3.7-lib2to3 (3.7.3-1+xenial1) ...
    Selecting previously unselected package python3.7-distutils.
    Preparing to unpack .../python3.7-distutils_3.7.3-1+xenial1_all.deb ...
    Unpacking python3.7-distutils (3.7.3-1+xenial1) ...
    Selecting previously unselected package python3.7.
    Preparing to unpack .../python3.7_3.7.3-1+xenial1_amd64.deb ...
    Unpacking python3.7 (3.7.3-1+xenial1) ...
    Processing triggers for mime-support (3.59ubuntu1) ...
    Setting up libpython3.7-minimal:amd64 (3.7.3-1+xenial1) ...
    Setting up python3.7-minimal (3.7.3-1+xenial1) ...
    Setting up libpython3.7-stdlib:amd64 (3.7.3-1+xenial1) ...
    Setting up python3.7-lib2to3 (3.7.3-1+xenial1) ...
    Setting up python3.7-distutils (3.7.3-1+xenial1) ...
    Setting up python3.7 (3.7.3-1+xenial1) ...
    root@b8cda311d766:/# python3.7
    Python 3.7.3 (default, Mar 26 2019, 01:59:45) 
    [GCC 5.4.0 20160609] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> 
    root@b8cda311d766:/# exit
    ~ $