How do I check whether a module is installed in Python, and install it if needed?

How to know if a python module is installed or not in the system: You can do a very easy test in terminal,

$ python -c "import math"
$ echo $?
0                                # math module exists in system

$ python -c "import numpy"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named numpy
$ echo $?
1                                # numpy module does not exist in system

How will I install it if it is not installed

You can install specific module by downloading respective packages from repository, for example you can install scipy as,

sudo apt-get install python-scipy ## for Python2
sudo apt-get install python3-scipy ## for Python3

Alternately You can also install a python module using python-pip as suggested by Zack Titan in the comment below, To install numpy you can use

pip install numpy

Warning: It is highly recommended to install python-modules using official Ubuntu repository only and not to use the pip method as superuser(i.e., as root or using sudo). In some cases it may leave your system unusable by breaking system python.

How to install packages using pip into local virtual environment.

In case we do not want to unwantedly import a module in question (which would happen in a try statement) we can make use of sys.modules to test modules that are installed and were imported before.

In the python shell issue:

>>> import sys

Then test for installed modules:

>>> 'numpy' in sys.modules
True
>>> 'scipy' in sys.modules
False

Note that only those modules that were imported before give True on this test, all other modules (even if installed) result in False.

Another alternative to try an import statement in the python console is calling the inbuilt help() function. This will not give a documentation for non-installed modules, e.g.

>>> help('scipy')
no Python documentation found for 'scipy'

The output of very long help documents of installed modules can be interrupted with Q.

Now to install missing modules it is recommended to use the Ubuntu package management (and not the Python pip way) because we need root access and also to prevent messing up our heavily Python-dependend system. For the module in question this would e.g. be:

sudo apt-get install python-scipy ## for Python2
sudo apt-get install python3-scipy ## for Python3

After installation we then can add them to the sys.modules dictionary by importing them once.

Another way is the pkgutil module. Works with both Python 2 & 3:

python -c 'import pkgutil; print(1 if pkgutil.find_loader("module") else 0)'

You need to replace module with the name of your module, example:

$ python -c 'import pkgutil; print(1 if pkgutil.find_loader("math") else 0)'
1