Updated October 2026: this page replaces six separate posts I wrote between 2019 and 2021 about upgrading Python on Debian 9 and 10, Ubuntu 19.10 and Kali, and switching the python command from 2.7 to 3. The old URLs redirect here. Everything in the first half was re-tested on 8 October 2026 in fresh Debian 12 and Ubuntu 22.04 containers. The original 2019 from-source walkthrough is kept at the end because it still works, with one important correction.

Quick answer

Pick the row that matches what you’re trying to do. The one rule behind all of them: never replace the system python3 that apt installed. Debian and Ubuntu tooling (apt itself, lsb_release, cloud-init, lots of system scripts) is written against that exact interpreter, and pointing it somewhere else is how you end up with a box that can’t update itself.

You wantDo this
python says “command not found”sudo apt install python-is-python3
A newer Python for one projectuv python install 3.13 then uv venv --python 3.13
A newer Python system-wide on Ubuntudeadsnakes PPA, sudo apt install python3.12 python3.12-venv
A newer Python system-wide on Debianbuild from source with make altinstall
To switch what python points at on an old 2.7 boxupdate-alternatives (end of this post)

Check what you have

python3 --version
ls -l /usr/bin/python*

On a fresh Debian 12 that’s Python 3.11.2. Ubuntu 22.04 ships 3.10, 24.04 ships 3.12. Kali tracks Debian testing, so it’s whatever Debian testing has that week. On all of them the interpreter is python3. Whether a bare python command exists at all is a separate question, which is the next section.

Make the python command work

If scripts or tutorials call python and you get “command not found”, you don’t need another interpreter. You need the one-line package that adds the symlink:

sudo apt install python-is-python3
python --version    # now the same as python3 --version

That’s the whole fix on Debian 12, Ubuntu 20.04 and newer, and Kali. It’s what the old update-alternatives dance at the bottom of this post was trying to achieve by hand.

A newer Python for a project: uv

For most of what I do now, I don’t change the system Python at all. uv downloads a standalone build of whichever version I ask for and keeps it out of the way of apt:

curl -LsSf https://astral.sh/uv/install.sh | sh
uv python install 3.13
uv venv --python 3.13 .venv
source .venv/bin/activate
python --version    # Python 3.13.x

Tested on Debian 12: uv python install 3.13 pulled 3.13.16 in a few seconds and uv run --python 3.13 python --version reported it. pyenv does the same job if you’d rather compile. Either way the system interpreter is untouched and each project pins its own.

A newer Python system-wide on Ubuntu: deadsnakes

Ubuntu has a well-maintained PPA that packages every current Python properly, so you get python3.12 alongside the system python3 with no compiling:

sudo apt install software-properties-common
sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt update
sudo apt install python3.12 python3.12-venv
python3.12 --version
python3.12 -m venv ~/venv312

Tested on Ubuntu 22.04: python3.12 --version gave 3.12.15 while python3 --version still reported the system 3.10.12, which is exactly what you want. Note python3.12 is a separate command. Don’t alias python3 to it.

A newer Python system-wide on Debian: build from source

Debian doesn’t have a deadsnakes equivalent, so if you need a system-wide newer interpreter the honest answer is still to compile it. The install step is where my 2019 post was wrong: use make altinstall, not make install. altinstall installs python3.13 and leaves python3 alone. install overwrites the python3 symlink and that’s the apt-breaking mistake from the first section.

sudo apt install build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev \
  libsqlite3-dev libffi-dev liblzma-dev wget
cd /tmp
wget https://www.python.org/ftp/python/3.13.7/Python-3.13.7.tar.xz
tar xf Python-3.13.7.tar.xz
cd Python-3.13.7
./configure --enable-optimizations
make -j"$(nproc)"
sudo make altinstall
python3.13 --version

The dependency list and ./configure were checked on Debian 12 on 8 October 2026. Pick the current release from python.org/ftp/python. make takes a while with --enable-optimizations, which is the trade for a faster interpreter. If the box is small, drop that flag.

Kali Linux

Kali is Debian testing underneath, so every method above applies as written. Kali 2020.1 was the release that moved to Python 3 by default, and the Kali-specific post I wrote back then was only ever the update-alternatives trick below. I haven’t re-tested on a full Kali install this time round (the minimal container image doesn’t ship Python at all), so treat the Debian results as the reference.

Python 2.7

Python 2.7 reached end of life on 1 January 2020 and the final release, 2.7.18, came in April 2020. Nothing current ships it. If you’ve inherited a 2.7 project, the official porting guide is still the place to start, and python-is-python2 exists on Debian for the brief period you need both side by side.

Troubleshooting

apt or apt-get throws Python errors after an upgrade You replaced the system python3. Restore the symlink to the distro version (ls /usr/bin/python3.* shows which one) with sudo ln -sf /usr/bin/python3.11 /usr/bin/python3, substituting your distro’s version.

lsb_release fails with ImportError Same cause, a non-system interpreter answering to python3. Fix as above. The 2019 workaround of symlinking lsb_release.py into site-packages papered over this.

pip installs into the wrong Python Always call pip through the interpreter you mean: python3.12 -m pip install ..., or better, work inside a venv.

add-apt-repository: command not found Install software-properties-common first.

The original 2019 method (Debian 9, Python 3.5 to 3.7)

Kept for reference. This is what the page said from February 2019, lightly tidied. The one correction, as above: use make altinstall.

Check your version

python3 -V

Download and extract

wget https://www.python.org/ftp/python/3.7.3/Python-3.7.3.tar.xz
tar xf Python-3.7.3.tar.xz
cd ./Python-3.7.3

Make and install

./configure
make
make altinstall    # was "make install" in the original, which clobbers python3

Switching the python command with update-alternatives

On the 2019-era systems where python still meant 2.7, this registered both interpreters and gave 3.x the higher priority:

update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives --install /usr/bin/python python /usr/bin/python3.7 2
update-alternatives --config python

update-alternatives --config python also lets you flip between them later. On anything current, python-is-python3 does this in one package.

Fixing pip and lsb_release afterwards

ln -s /usr/share/pyshared/lsb_release.py /usr/local/lib/python3.7/site-packages/lsb_release.py
pip3 install --upgrade pip

That symlink was a symptom of having replaced python3. With altinstall it isn’t needed.

Six posts, one page, and the advice is shorter than it used to be: don’t touch the system interpreter, let uv or a venv own the version your project wants. That rule would have saved me the 30 minutes in 2019 that started this whole series.

Or just hand this page to your agent

Read https://exitcode0.net/posts/debian-9-how-to-upgrade-python-3-5-to-python-3-7/ and install a newer Python on this machine. First run cat /etc/os-release, python3 --version and ls -l /usr/bin/python* and show me the output, then tell me which method on the page applies and ask me which Python version I want. Never replace /usr/bin/python3; use python-is-python3, uv, deadsnakes or make altinstall as the page describes.

The page gives the agent the methods and the rule. Only you know which distro this is and whether the version is for one project or the whole machine.

Enjoy. ✌️