89

In terminal, after I start Python, how will I know what are the modules present in python? Suppose I need to learn the modules NumPy and SciPy.

  • How will I install it if it is not installed?
  • How will I know if it is already installed?

11 Answers 11

73

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.

  • Like in your answer math module exists and for numby the traceback didnt appear and for echo $? output was 0, Does it mean numpy is also present in my system? – TechJhola Feb 22 '15 at 3:34
  • 1
    yes, it is there in your system. – souravc Feb 22 '15 at 3:35
  • In case you don't have the module numpy how will you import it? When we do the coding we call the import statement by from numpy import *, will it install the module ? If not how will we install a new module? – TechJhola Feb 22 '15 at 3:39
  • 1
    It will be a septate question how to install a module. Need different packages for different module. For example to install vpython you need to install it as sudo apt-get install python-visual libgtkglextmm-x11-1.2-dev – souravc Feb 22 '15 at 3:48
  • 1
    don't use sudo pip; it may break system python. Use apt-get to install packages for system python. You could use pip --user option or virtualenv to install Python packages for yourself. – jfs Feb 23 '15 at 7:03
21

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.

  • 1
    I think this is most Pythonic answer.... – Grijesh Chauhan Feb 22 '15 at 16:48
  • 3
    The first part of this is wrong: sys.modules only contains modules that have already been imported, so it is not a reliable way to test whether a module has been installed. The most reliable test is to use a try/except and trap the ImportError, as several others have already suggested. – ekhumoro Feb 22 '15 at 19:16
  • @ekhumoro - thanks, I'll edit my answer to make that clearer. – Takkat Feb 22 '15 at 20:05
  • 4
    It still seems not sufficiently clear. Given that the question is asking how to find out if a module is installed (not imported), sys.modules is entirely irrelevant and I think any mention of it is misleading. But maybe that's just me. – David Z Feb 23 '15 at 7:32
  • 1
    sys.modules is not any help here. – cat Jul 6 '16 at 23:35
19

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
  • 2
    I saw you undid some of my changes; print() will also work fine in Python 2; It's treated as a the group-with-parens syntax, and doesn't have any side effects. Only when you want to add multiple items (as in print('a', 'b') it will be treated as a tuple, in which you do need the from __future__ import print_function, but that's not applicable in this case, you can just write forward-compatible code by adding the parens (I tested my changes in Python 2 and 3). – Martin Tournoij Feb 23 '15 at 1:24
  • @Carpetsmoker: Thanks for pointing out. I rollback to the old version. – cuonglm Feb 23 '15 at 1:28
  • This is a great answer. I added a derivative answer that returns an exit/status code (for shell script flow control) and takes the module name as a command line argument. askubuntu.com/a/891384/146273 – Bruno Bronosky Dec 14 '17 at 18:09
  • That worked! - Just change to exit so it can be used in shell context: python -c 'import pkgutil; exit(0 if pkgutil.find_loader("math") else 2)' – A T Sep 25 at 11:24
12

I know the OP originally asked for a solution after starting Python, but outside of python I use pip. On ubuntu: sudo apt-get install python-pip, if it's not already installed.

Then to see what third party modules are available, just run:

pip freeze

Or even

pip list

And both will show you all modules installed and their versions.

If the module you're looking for is not installed, most of the time you can easily install it with pip:

pip install <module-name>

If you're not sure of whether a module exists or what its PyPI name is, use pip search:

pip search <keyword>
  • 1
    Also, pip show <module-name> will show whether a package is installed, e.g. pip show numpy. – Herpes Free Engineer Jul 4 '18 at 18:20
10

You could put the code inside try, except block.

$ python3 -c "\
try:
    import cow  
    print('\nModule was installed')
except ImportError:
    print('\nThere was no such module installed')"

There was no such module installed

$ python3 -c "\
try:
    import regex
    print('\nModule was installed')
except ImportError:
    print('\nThere was no such module installed')"

Module was installed
6

To provide another answer, for completion's sake:

You can (ab)use the -m option. From Python's manpage:

   -m module-name
          Searches sys.path for the named module and runs the  correspond‐
          ing .py file as a script.

Which will give us:

$ python2 -m numpy
/sbin/python2: No module named numpy.__main__; 'numpy' is a package and cannot be directly executed

$ python2 -m math
/sbin/python2: No code object available for math

But for non-existent modules, it will give us:

$ python2 -m doesnt_exist
/sbin/python2: No module named doesnt_exist

We could use grep to match for this:

$ python2 -m doesnt_exist |& grep -q 'No module named' && echo 'Nope' || echo 'Yup'
Nope

$ python2 -m math |& grep -q 'No module named' && echo 'Nope' || echo 'Yup'
Yup

This is slightly hack-ish, and not what -m was intended for; but it is the method that requires the least typing if you want a quick test :-)

  • 4
    grep -q makes redirection to null unnecessary. – muru Feb 23 '15 at 1:13
  • 3
    @muru Yikes! I have typed a lot of useless redirects to /dev/null over the years :-/ – Martin Tournoij Feb 23 '15 at 1:25
4

I wrote an example in Python:

import pip
import sys
from bigml.api import BigML


if not 'bigml' in sys.modules.keys():
    pip.main(['install', 'bigml'])
3

Shell oneliner check (useful for provisioning)

I found that in order to make my infrastructure provisioning** idempotent, I need to be able to check for a package from the shell in a oneliner. I built on @cuonglm's answer. I had to reverse the 1 and 0 because I'm producing an exit status rather than printing a string.

python -c "import sys, pkgutil; sys.exit(0 if pkgutil.find_loader(sys.argv[1]) else 1)" pymongo

You could replace the sys.argv[1] with the single quoted name of your package, but for my provisioning scripts I like the readability of having it at the end.

python -c "import sys, pkgutil; sys.exit(0 if pkgutil.find_loader('pymongo') else 1)"

** I realize that chef, puppet, and ansible all have plugins for managing python packages, but you may find yourself in a situation like me where you are using an outdated version and don't want to use deprecated plugins.

1

One can also use pydoc modules, which can be filtered with grep to find a specific module. The output is displayed in columnated format. The only disadvantage of this approach is that it also will include python files in the current working directory. Nonetheless,I use it myself most of the time and it's one of the highly cited approaches on this related question: https://stackoverflow.com/q/739993/3701431

0

From the Ubuntu Shell, by default bash, as simple as

pip list | grep <package-name-Case-Matters>

Examples

pip list | grep pywinrm

pip list | grep numpy

And, if you have doubts about the case (although I think all the package names are always lowercase):

pip list | grep [Nn]um[Pp]y  # it works with numpy, Numpy, numPy, and NumPy
0

As of writing this, pip show ... seems to be the easiest way: https://pip.pypa.io/en/stable/reference/pip_show/

But it is silent (ie. returns nothing) when the package is not installed.

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