Disclaimer: ATM tested only in Ubuntu and Windows (see comments to this answer).

Jupyter runs the user's code in a separate process called kernel. The kernel can be a different Python installation (in a different conda environment or virtualenv or Python 2 instead of Python 3) or even an interpreter for a different language (e.g. Julia or R). Kernels are configured by specifying the interpreter and a name and some other parameters (see Jupyter documentation) and configuration can be stored system-wide, for the active environment (or virtualenv) or per user. If nb_conda_kernels is used, additional to statically configured kernels, a separate kernel for each conda environment with ipykernel installed will be available in Jupyter notebooks.

In short, there are three options how to use a conda environment and Jupyter:

Option 1: Run Jupyter server and kernel inside the conda environment

Do something like:

conda create -n my-conda-env         # creates new virtual env
conda activate my-conda-env          # activate environment in terminal
conda install jupyter                # install jupyter + notebook
jupyter notebook                     # start server + kernel inside my-conda-env

Jupyter will be completely installed in the conda environment. Different versions of Jupyter can be used for different conda environments, but this option might be a bit of overkill. It is enough to include the kernel in the environment, which is the component wrapping Python which runs the code. The rest of Jupyter notebook can be considered as editor or viewer and it is not necessary to install this separately for every environment and include it in every env.yml file. Therefore one of the next two options might be preferable, but this one is the simplest one and definitely fine.

Option 2: Create special kernel for the conda environment

Do something like:

conda create -n my-conda-env                               # creates new virtual env
conda activate my-conda-env                                # activate environment in terminal
conda install ipykernel                                    # install Python kernel in new conda env
ipython kernel install --user --name=my-conda-env-kernel   # configure Jupyter to use Python kernel

Then run jupyter from the system installation or a different conda environment:

conda deactivate          # this step can be omitted by using a different terminal window than before
conda install jupyter     # optional, might be installed already in system e.g. by 'apt install jupyter' on debian-based systems
jupyter notebook          # run jupyter from system

Name of the kernel and the conda environment are independent from each other, but it might make sense to use a similar name.

Only the Python kernel will be run inside the conda environment, Jupyter from system or a different conda environment will be used - it is not installed in the conda environment. By calling ipython kernel install the jupyter is configured to use the conda environment as kernel, see Jupyter documentation and IPython documentation for more information. In most Linux installations this configuration is a *.json file in ~/.local/share/jupyter/kernels/my-conda-env-kernel/kernel.json:

{
"argv": [
"/opt/miniconda3/envs/my-conda-env/bin/python",
"-m",
"ipykernel_launcher",
"-f",
"{connection_file}"
],
"display_name": "my-conda-env-kernel",
"language": "python"
}

Option 3: Use nb_conda_kernels to use a kernel in the conda environment

When the package nb_conda_kernels is installed, a separate kernel is available automatically for each conda environment containing the conda package ipykernel or a different kernel (R, Julia, ...).

conda activate my-conda-env    # this is the environment for your project and code
conda install ipykernel
conda deactivate


conda activate base            # could be also some other environment
conda install nb_conda_kernels
jupyter notebook

You should be able to choose the Kernel Python [conda env:my-conda-env]. Note that nb_conda_kernels seems to be available only via conda and not via pip or other package managers like apt.

Troubleshooting

Using Linux/Mac the command which on the command line will tell you which jupyter is used, if you are using option 1 (running Jupyter from inside the conda environment), it should be an executable from your conda environment:

$ which jupyter
/opt/miniconda3/envs/my-conda-env/bin/jupyter
$ which jupyter-notebook   # this might be different than 'which jupyter'! (see below)
/opt/miniconda3/envs/my-conda-env/bin/jupyter-notebook

Inside the notebook you should see that Python uses Python paths from the conda environment:

[1] !which python
/opt/miniconda3/envs/my-conda-env/bin/python
[2] import sys; sys.executable
'/opt/miniconda3/envs/my-conda-env/bin/python'
['/home/my_user',
'/opt/miniconda3/envs/my-conda-env/lib/python37.zip',
'/opt/miniconda3/envs/my-conda-env/lib/python3.7',
'/opt/miniconda3/envs/my-conda-env/lib/python3.7/lib-dynload',
'',
'/opt/miniconda3/envs/my-conda-env/lib/python3.7/site-packages',
'/opt/miniconda3/envs/my-conda-env/lib/python3.7/site-packages/IPython/extensions',
'/home/my_user/.ipython']

Jupyter provides the command jupyter-troubleshoot or in a Jupyter notebook:

!jupyter-troubleshoot

This will print a lot of helpful information about including the outputs mentioned above as well as installed libraries and others. When asking for help regarding Jupyter installations questions, it might be good idea to provide this information in bug reports or questions.

To list all configured Jupyter kernels run:

jupyter kernelspec list

Common errors and traps

Jupyter notebook not installed in conda environment

Note: symptoms are not unique to the issue described here.

Symptoms: ImportError in Jupyter notebooks for modules installed in the conda environment (but not installed system wide), but no error when importing in a Python terminal

Explaination: You tried to run jupyter notebook from inside your conda environment (option 1, see above), there is no configuration for a kernel for this conda environment (this would be option 2) and nb_conda_kernels is not installed (option 3), but jupyter notebook is not (fully) installed in the conda environment, even if which jupyter might make you believe it was.

In GNU/Linux you can type which jupyter to check which executable of Jupyter is run.

This means that system's Jupyter is used, probably because Jupyter is not installed:

(my-conda-env) $ which jupyter-notebook
/usr/bin/jupyter

If the path points to a file in your conda environment, Jupyter is run from inside Jupyter:

(my-conda-env) $ which jupyter-notebook
/opt/miniconda3/envs/my-conda-env/bin/jupyter-notebook

Note that when the conda package ipykernel is installed, an executable jupyter is shipped, but no executable jupyter-notebook. This means that which jupyter will return a path to the conda environment but jupyter notebook will start system's jupyter-nootebook (see also here):

 $ conda create -n my-conda-env
$ conda activate my-conda-env
$ conda install ipykernel
$ which jupyter            # this looks good, but is misleading!
/opt/miniconda3/envs/my-conda-env/bin/jupyter
$ which jupyter-notebook   # jupyter simply runs jupyter-notebook from system...
/usr/bin/jupyter-notebook

This happens because jupyter notebook searches for jupyter-notebook, finds /usr/bin/jupyter-notebook and calls it starting a new Python process. The shebang in /usr/bin/jupyter-notebook is #!/usr/bin/python3 and not a dynamic #!/usr/bin/env python. Therefore Python manages to break out of the conda environment. I guess jupyter could call python /usr/bin/jupyter-notebook instead to overrule the shebang, but mixing system's bin files and the environment's python path can't work well anyway.

Solution: Install jupyter notebook inside the conda environment:

 conda activate my-conda-env
conda install jupyter
jupyter notebook

Wrong kernel configuration: Kernel is configured to use system Python

Note: symptoms are not unique to the issue described here.

Symptoms: ImportError in Jupyter notebooks for modules installed in the conda environment (but not installed system wide), but no error when importing in a Python terminal

Explanation: Typically the system provides a kernel called python3 (display name "Python 3") configured to use /usr/bin/python3, see e.g. /usr/share/jupyter/kernels/python3/kernel.json. This is usually overridden by a kernel in the conda environment, which points to the environments python binary /opt/miniconda3/envs/my-conda-env/bin/python. Both are generated by the package ipykernel (see here and here).

A user kernel specification in ~/.local/share/jupyter/kernels/python3/kernel.json might override the system-wide and environment kernel. If the environment kernel is missing or the user kernel points to a python installation outside the environment option 1 (installation of jupyter in the environment) will fail.

For occurrences and discussions of this problem and variants see here, here, here and also here, here and here.

Solution: Use jupyter kernelspec list to list the location active kernel locations.

$ conda activate my-conda-env
$ jupyter kernelspec list
Available kernels:
python3 /opt/miniconda3/envs/my-conda-env/share/jupyter/kernels/python3

If the kernel in the environment is missing, you can try creating it manually using ipython kernel install --sys-prefix in the activated environment, but it is probably better to check your installation, because conda install ipykernel should have created the environment (maybe try re-crate the environment and re-install all packages?).

If a user kernel specification is blocking the environment kernel specification, you can either remove it or use a relative python path which will use $PATH to figure out which python to use. So something like this, should be totally fine:

$ cat ~/.local/share/jupyter/kernels/python3/kernel.json
{
"argv": [
"python",
"-m",
"ipykernel_launcher",
"-f",
"{connection_file}"
],
"display_name": "Python 3",
"language": "python"
}

Correct conda environment not activated

Symptoms: ImportError for modules installed in the conda environment (but not installed system wide) in Jupyter notebooks and Python terminals

Explanation: Each terminal has a set of environment variables, which are lost when the terminal is closed. In order to use a conda environment certain environment variables need to be set, which is done by activating it using conda activate my-conda-env. If you attempted to run Jupyter notebook from inside the conda environment (option 1), but did not activate the conda environment before running it, it might run the system's jupyter.

Solution: Activate conda environment before running Jupyter.

 conda activate my-conda-env
jupyter notebook

Broken kernel configuration

Symptoms: Strange things happening. Maybe similar symptoms as above, e.g. ImportError

Explanation: If you attempted to use option 2, i.e. running Jupyter from system and the Jupyter kernel inside the conda environment by using an explicit configuration for the kernel, but it does not behave as you expect, the configuration might be corrupted in some way.

Solution: Check configuration in ~/.local/share/jupyter/kernels/my-kernel-name/kernel.json and fix mistakes manually or remove the entire directory and re-create it using the command provided above for option 2. If you can't find the kernel configuration there run jupyter kernelspec list.

Python 2 vs 3

Symptoms: ImportError due to wrong Python version of the Jupyter kernel or other problems with Python 2/3

Explanation: The kernel configuration can have all sorts of confusing and misleading effects. For example the default Python 3 kernel configuration will allow me to launch a Jupyter notebook running on Python 2:

conda create -n my-conda-env
conda activate my-conda-env
conda install python=2
conda install jupyter
jupyter notebook

The default Python 3 kernel:

$ cat ~/.local/share/jupyter/kernels/python3/kernel.json
{
"argv": [
"python",
"-m",
"ipykernel_launcher",
"-f",
"{connection_file}"
],
"display_name": "Python 3",
"language": "python"
}

After creating a new Jupyter Notebook with the Python 3 kernel, Python 2 from the conda environment will be used even if "Python 3" is displayed by Jupyter.

Solution: Don't use Python 2 ;-)

Following worked for me :

1. Activate the environment that you want to use : conda activate <env_name>

2. pip install ipykernel (if you don't already have it)

3. python -m ipykernel install --user --name=<env_name>

The following command can also be used as a one liner to create your Conda environment running the latest version of Python and the latest version of Jupyter Notebooks,

conda create -n <env-name> python jupyter

If you want to install specific versions of Python or Jupyter, you can do,

conda create -n <env-name> python=<version> jupyter=<version>

For instance,

conda create -n <env-name> python=3.10.4 jupyter=1.0.0

If there are other packages that you want to use in this environment with your notebooks you can do the following,

conda create -n <env-name> python jupyter <another-package> <another-package> ...

For instance,

conda create -n <env-name> python jupyter scikit-learn

Note that similar to before, these commands will install the latest versions of Python and the relevant packages. If you want specific versions, you can use the =<version> syntax.

Also, you can still install any of the packages that you need using either pip install or conda install once the environment has been created.

After you have created your environment (using any of the methods given above), you can simply run the following commands to activate your environment and run Jupyter Notebooks,

conda activate <env-name>
jupyter notebook