# Troubleshooting¶

Here you can find some tips to try to find out why your installation of NEST didn’t work.

## 1. CMAKE error says a <package> was not found or <package> is too old¶

Please make sure you have followed the installation instructions found here and have installed the required dependencies.

1. Install the missing package or update the package to a more recent version.

2. Remove the contents of the build directory. This step ensures the old build-cache, which may be missing some of the recently installed or updated packages, is cleaned out and a fresh build is triggered.

rm -r /path/to/nest-simulator-x.y.z-build/*

3. Compile NEST again:

cmake -DCMAKE_INSTALL_PREFIX:PATH=</install/path> </path/to/NEST/src>


If the error still persists, you may have more than one installation of the <package>. A conflict may occur between different package binaries: Your system may prefentially choose a system-wide installation of a package (e.g., /usr/bin/<package>), rather than a local environment installation (e.g., /home/user/ENV/bin/<package>).

1. Determine the path and version of the <package>:

which <package>

searches the path of executables set in the \$PATH environment variable. It will tell you the path to the <package> binary that is being used in the current environment.

type -a <package>

shows the complete list of directories that your system found the binary file. The first result should be the location to your active environment.

<package> –version

will tell you the version of the <package> binary.

Here is an example,

which python


The terminal will display the path to the binary it found:

/home/user/ENVNAME/bin/python

type -a python


The terminal will list the paths it found to the package binaries:

python is /home/user/ENVNAME/bin/python
python is /usr/bin/python

python --version


The terminal will display the version number:

Python 3.6.5

1. If it looks like you have an older version on your system:

• Remove or update old versions of <package> (You may need to uninstall and reinstall the package)

If you do not have the <package> in your active environment:

• Install the <package> while in your active environment.

2. Remove the contents of the build directory

rm -r /path/to/nest-simulator-x.y.z-build/*

3. Compile NEST again

cmake -DCMAKE_INSTALL_PREFIX:PATH=</install/path> </path/to/NEST/src>


## 2. When I try to import nest, I get an error in Python that says ‘No Module named NEST’ or ‘ImportError’¶

This error message means something in your environment is not set correctly, depending on how you installed NEST.

### 1. Check which Python version you are running¶

You must use Python 3 if you installed NEST with

1. the Ubuntu PPA,

2. the conda-forge package,

3. the Live Media, or

4. if you compiled NEST with Python 3 bindings

Type python or ipython in the terminal. The python version that is used will be displayed.

If the Python version displayed is 2.X, you need to run python3 or ipython3 instead of python or ipython.

If your Python version is correct and you still have the same error, then try one of the following options:

### 2a. If you compiled NEST from source¶

• Your path variables may not be set correctly, in that case run:

source </path/to/nest_install_dir>/bin/nest_vars.sh


### 2b. If you installed NEST via the conda-forge package¶

• Make sure you have activated the correct environment

To get a list of all your environments, run:

conda info -e


An asterisk (*) indicates the active environment.

Activate the correct environment if it’s not already:

conda activate ENVNAME


Try to import nest in Python.

• Check that the correct package binary is used for NEST and Python: for example, in a terminal type:

which python
which nest


These commands will show you the path to the Python and NEST binary that your environment is using. You may have more than one installation on your system. The path to the binary should be within your active environment:

/path/to/conda/envs/ENVNAME/bin/python
/path/to/conda/envs/ENVNAME/bin/nest


You can also view the list of packages in the active environment, by running:

conda list


If the package is not in your environment, then it needs to be installed.

If something is missing, you can try to conda install <package> BUT be aware that this may break pre-installed packages!

You may be better off creating a new Conda environment and install NEST with all needed packages at one time! See the section on installation for Conda.