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using virtualenv (Part 2)

September 21, 2013 Leave a comment

In Part 1 we saw how to use virtualenv.

Now let’s see how to colorize the bash prompt and how to activate a virtual environment easily.

Colorize the prompt
When a virtual env. is activated, the prompt changes. However, this change is not very visible because it’s not colorized. The name of the virtual env. should be printed with a different color, thus it would be visible immediately if an env. is activated or not.

Fortunately someone else also had this problem :) Here I found an excellent solution. I only changed the PS1 line the following way:

  PS1="${PYTHON_VIRTUALENV}${GREEN}\u@\h ${YELLOW}\w${COLOR_NONE} ${BRANCH}${PROMPT_SYMBOL} "

This way the prompt and the cursor are in one line. I made a fork, my slightly modified version is available here.

Update (20131001): I updated the script above to support light background too. Instructions are in the header in a comment.

Usage

Save the file above as ~/.bash_prompt and add the following line to the end of your ~/.bashrc:

source ~/.bash_prompt

The resulting prompt is way cooler than the default bash prompt, thus you can use it even if you don’t work with virtual environments!

See the end of the post for a screenshot.

Activating a virtual environment easily
The standard way for activating a virtual env. is to source the script “activate“:

jabba@jabba-uplink ~/python/mystuff $ ls -al
total 24
drwxrwxr-x   3 jabba jabba  4096 Sep 21 16:16 .
drwxrwxr-x 234 jabba jabba 12288 Sep 21 15:58 ..
-rwxrw-r--   1 jabba jabba    53 Sep 21 16:16 hello.py
drwxrwxr-x   6 jabba jabba  4096 Sep 21 16:13 venv
jabba@jabba-uplink ~/python/mystuff $ . venv/bin/activate
[venv] jabba@jabba-uplink ~/python/mystuff $

However, using the command “workon venv” would be much easier. I wanted to do it with an alias, but bash aliases do not accept arguments. There is a workaround: use functions instead.

Add the following lines to your ~/.bashrc:

func_workon()
{
if [[ -z "$1" ]]
then
    echo "Usage: workon <venv>"
else
    . $1/bin/activate
fi
}
alias workon=func_workon

alias workoff='deactivate'

Now you can activate a virtual env. much easier:

jabba@jabba-uplink ~/python/mystuff $ ls -al
total 24
drwxrwxr-x   3 jabba jabba  4096 Sep 21 16:16 .
drwxrwxr-x 234 jabba jabba 12288 Sep 21 15:58 ..
-rwxrw-r--   1 jabba jabba    53 Sep 21 16:16 hello.py
drwxrwxr-x   6 jabba jabba  4096 Sep 21 16:13 venv
jabba@jabba-uplink ~/python/mystuff $ workon venv
[venv] jabba@jabba-uplink ~/python/mystuff $

For deactivation use the command “deactivate“, or my alias “workoff“.

Screenshot
workon

Next step
There is virtualenvwrapper, which is “a set of extensions to Ian Bicking’s virtualenv tool. The extensions include wrappers for creating and deleting virtual environments and otherwise managing your development workflow, making it easier to work on more than one project at a time without introducing conflicts in their dependencies.

I haven’t used it yet.

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using virtualenv (Part 1)

September 21, 2013 Leave a comment

Here I won’t explain what virtualenv is. There are lots of tutorials on this topic (see the “virtualenv” subsection here for instance, or this SO thread).

This post is about the basic usage of virtualenv.

Step 0 (20131029)
Before doing anything, create the file ~/.pip/pip.conf and edit it:

[install]
download-cache = ~/.pip/download_cache

This way each version is downloaded only once.

Update (20140807): Maybe using a cache is not that good idea. It happened to me that a new version came out of a package that I wanted to install but because of the cache, it installed an older version! I didn’t understand why I still had an out-of-date version. So I removed the ~/.pip folder entirely. I don’t recommend using a cache.

Installation
Install virtualenv:

sudo pip install virtualenv

Trying out
Create a directory where you want to store your project:

$ cd ~/python
$ mkdir mystuff
$ cd mystuff

Let’s create here a virtual environment:

$ virtualenv venv

You will get a subdirectory called “venv“, which contains a clean Python virtual environment. Activate it with the following command:

$ . venv/bin/activate

Notice that the prompt changes: now it includes the name of the virtual environment. If you call the command “python” or “pip“, the binaries in the virtual environment will be called.

Running example:

jabba@jabba-uplink:~$ cd python
jabba@jabba-uplink:~/python$ mkdir mystuff
jabba@jabba-uplink:~/python$ cd mystuff/
jabba@jabba-uplink:~/python/mystuff$ ls -al
total 16
drwxrwxr-x   2 jabba jabba  4096 Sep 21 15:58 .
drwxrwxr-x 234 jabba jabba 12288 Sep 21 15:58 ..
jabba@jabba-uplink:~/python/mystuff$ virtualenv venv
...
jabba@jabba-uplink:~/python/mystuff$ ls -al
total 20
drwxrwxr-x   3 jabba jabba  4096 Sep 21 15:58 .
drwxrwxr-x 234 jabba jabba 12288 Sep 21 15:58 ..
drwxrwxr-x   6 jabba jabba  4096 Sep 21 15:58 venv
jabba@jabba-uplink:~/Dropbox/python/mystuff$ . venv/bin/activate
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ which python
/home/jabba/Dropbox/python/mystuff/venv/bin/python                                                     
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ which pip
/home/jabba/Dropbox/python/mystuff/venv/bin/pip                                                               

To stop using a virtual environment, use the command “deactivate“.

Example
Say I have the package “requests” globally installed (with “sudo pip install requests“). When I launch the python shell, this module can be imported.

However, when you activate a clean virtual environment, globally installed 3rd party packages are not available. They must be installed with the virtual environment’s “pip” command, and they will be installed inside the “venv” directory. This is the real advantage of virtualenv: virtual environments are completely isolated from each other and you can install different package versions, there won’t be any conflict.

Running example:

jabba@jabba-uplink:~/Dropbox/python/mystuff$ which python
/usr/bin/python
jabba@jabba-uplink:~/Dropbox/python/mystuff$ python
Python 2.7.4 (default, Apr 19 2013, 18:28:01) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Persistent session history and tab completion are enabled.
>>> import requests
>>> 
jabba@jabba-uplink:~/Dropbox/python/mystuff$ . venv/bin/activate
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ which python
/home/jabba/Dropbox/python/mystuff/venv/bin/python
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ python
Python 2.7.4 (default, Apr 19 2013, 18:28:01) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Persistent session history and tab completion are enabled.
>>> import requests
Traceback (most recent call last):
  File "", line 1, in 
ImportError: No module named requests
>>> 
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ which pip
/home/jabba/Dropbox/python/mystuff/venv/bin/pip
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ pip install requests
Downloading/unpacking requests
  Downloading requests-1.2.3.tar.gz (348kB): 348kB downloaded
  Running setup.py egg_info for package requests
    
Installing collected packages: requests
  Running setup.py install for requests
    
Successfully installed requests
Cleaning up...
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ python
Python 2.7.4 (default, Apr 19 2013, 18:28:01) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Persistent session history and tab completion are enabled.
>>> import requests
>>> 
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$

Where to put the project files
Store your project files (in this example) in the “~/python/mystuff” directory. If the virtual environment is activated, then the modules installed in the virtual environment are available for your project:

(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ ls -al
total 24
drwxrwxr-x   3 jabba jabba  4096 Sep 21 16:16 .
drwxrwxr-x 234 jabba jabba 12288 Sep 21 15:58 ..
-rwxrw-r--   1 jabba jabba    53 Sep 21 16:16 hello.py
drwxrwxr-x   6 jabba jabba  4096 Sep 21 16:13 venv
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ cat hello.py 
#!/usr/bin/env python

import requests
print "hello"
(venv)jabba@jabba-uplink:~/Dropbox/python/mystuff$ ./hello.py 
hello

We installed “requests” in the previous step, and it’s nicely available for the project. If you uninstall requests from the virtual environment with “pip uninstall requests” and try to run hello.py, you will get an import error message.

In Part 2 we will see how to colorize the prompt and how to activate a virtual environment easily.

Categories: python Tags:

Python from command line

September 18, 2013 Leave a comment

Problem
You want to calculate something with Python quickly, from the command line. You might even want to use Python in a bash script to produce some result.

Solution

$ python -c "print 2*3"
6

Storing the result in a variable:

$ X=`python -c "print 2*3"`
$ echo $X
6

Thanks to Tajti A. for the tip.

Update (20170803)
You can also pass a bash variable to the embedded Python:

VAL="cat dog cat"                                                                                                                                                                             
NEW=`python3 -c "print('$VAL'.replace('dog', 'wolf'))"`                                                                                                                                       
echo $NEW

Output: “cat wolf cat”.

Categories: python Tags: , , ,

Anaconda Python distribution

September 15, 2013 Leave a comment

See https://store.continuum.io/cshop/anaconda/.

Completely free enterprise-ready Python distribution for large-scale data processing, predictive analytics, and scientific computing.

It has the advantage that it installs everything in a specified directory. Say you need to use a machine where you don’t have root permission and/or this machine has an old Python version. Just install Anaconda and use the latest Python version it is shipped with.

Update (20160208)
I had Anaconda 2.3 and 2.5 came out a few days ago. How to upgrade to the new distribution?

$ conda update --all
$ conda install anaconda=2.5
$ conda update --all
Categories: python Tags:

Python + Excel

September 13, 2013 Leave a comment

Links

  • DataNitro (cheapest licence is $500, however they have a 30-day free trial)
  • pyxll

Both projects have a short video that demonstrates the basic usage.

Categories: python Tags:

Python APIs

September 9, 2013 Leave a comment
Categories: python Tags: ,