How to deal with Linux/Python dependencies?

Question

Due to lack of support for some libraries I want to use, I moved some Python development from Windows to Linux development. I've spent most of the day messing about getting nowhere with dependencies.

The question

Whenever I pick up Linux, I usually run into some kind of dependency issue, usually with development libraries, whether they're installed via apt-get, easy_install or pip. I can waste days on what should be simple tasks, spending longer on getting libraries to work than writing code. Where can I learn about strategy for dealing with these kind of issues rather than aimlessly googling for someone who's come across the same problem before?

---

An example

Just one example: I wanted to generate some QR codes. So, I thought I'd use github.com/bitly/pyqrencode which is based on pyqrcode.sourceforge.net but supposedly without the Java dependencies. There are others (pyqrnative, github.com/Arachnid/pyqrencode) but that one seemed like the best bet for my needs.

So, I found the package on pypi and thought using that would make life easier:

(I've perhaps made life more difficult for myself by using virtualenv to keep things neat and tidy.)

(myenv3)mat@ubuntu:~/myenv3$ bin/pip install pyqrencode
Downloading/unpacking pyqrencode
  Downloading pyqrencode-0.2.tar.gz
  Running setup.py egg_info for package pyqrencode

Installing collected packages: pyqrencode
  Running setup.py install for pyqrencode
    building 'qrencode' extension
    gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c qrencode.c -o build/temp.linux-i686-2.7/qrencode.o
    gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions build/temp.linux-i686-2.7/qrencode.o -lqrencode -o build/lib.linux-i686-2.7/qrencode.so

Successfully installed pyqrencode
Cleaning up...

(I guess I probably sudo apt-get install libqrencode-dev at some point prior to that too.)

So then I tried to run the test script:

(myenv3)mat@ubuntu:~/myenv3$ python test_qr.py 
Traceback (most recent call last):
  File "test_qr.py", line 1, in <module>
    from qrencode import Encoder
  File "qrencode.pyx", line 1, in init qrencode (qrencode.c:1520)
ImportError: No module named ImageOps

:(

Well, investigations revealed that ImageOps appears to be part of PIL...

(myenv3)mat@ubuntu:~/myenv3$ pip install pil
Downloading/unpacking pil
  Downloading PIL-1.1.7.tar.gz (506Kb): 122Kb downloaded
Operation cancelled by user
Storing complete log in /home/mat/.pip/pip.log
(myenv3)mat@ubuntu:~/myenv3$ bin/pip install pil
Downloading/unpacking pil
  Downloading PIL-1.1.7.tar.gz (506Kb): 506Kb downloaded
  Running setup.py egg_info for package pil
    WARNING: '' not a valid package name; please use only.-separated package names in setup.py

Installing collected packages: pil
  Running setup.py install for pil
    WARNING: '' not a valid package name; please use only.-separated package names in setup.py
    building '_imaging' extension
    gcc ...
    building '_imagingmath' extension
    gcc ...
    --------------------------------------------------------------------
    PIL 1.1.7 SETUP SUMMARY
    --------------------------------------------------------------------
    version       1.1.7
    platform      linux2 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24)
                  [GCC 4.5.2]
    --------------------------------------------------------------------
    *** TKINTER support not available
    *** JPEG support not available
    *** ZLIB (PNG/ZIP) support not available
    *** FREETYPE2 support not available
    *** LITTLECMS support not available
    --------------------------------------------------------------------
    To add a missing option, make sure you have the required
    library, and set the corresponding ROOT variable in the
    setup.py script.

    To check the build, run the selftest.py script.
    ...
Successfully installed pil
Cleaning up...

Hmm, PIL's installed but hasn't picked up the libraries I installed with sudo apt-get install libjpeg62 libjpeg62-dev libpng12-dev zlib1g zlib1g-dev earlier. I'm not sure how to tell pip to feed the library locations to setup.py. Googling suggests a variety of ideas which I've tried, but none of them seem to help much other than to send me round in circles.

Ubuntu 11.04: Installing PIL into a virtualenv with PIP suggests using the pillow package instead, so let's try that:

(myenv3)mat@ubuntu:~/myenv3$ pip install pillow
Downloading/unpacking pillow
  Downloading Pillow-1.7.5.zip (637Kb): 637Kb downloaded
  Running setup.py egg_info for package pillow

    ...
Installing collected packages: pillow
  Running setup.py install for pillow
    building '_imaging' extension
    gcc ...
    --------------------------------------------------------------------
    SETUP SUMMARY (Pillow 1.7.5 / PIL 1.1.7)
    --------------------------------------------------------------------
    version       1.7.5
    platform      linux2 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24)
                  [GCC 4.5.2]
    --------------------------------------------------------------------
    *** TKINTER support not available
    --- JPEG support available
    --- ZLIB (PNG/ZIP) support available
    --- FREETYPE2 support available
    *** LITTLECMS support not available
    --------------------------------------------------------------------
    To add a missing option, make sure you have the required
    library, and set the corresponding ROOT variable in the
    setup.py script.

    To check the build, run the selftest.py script.
    ...
Successfully installed pillow
Cleaning up...

Well, we seem to have the JPEG and PNG support this time, yay!

(myenv3)mat@ubuntu:~/myenv3$ python test_qr.py 
Traceback (most recent call last):
  File "test_qr.py", line 1, in <module>
    from qrencode import Encoder
  File "qrencode.pyx", line 1, in init qrencode (qrencode.c:1520)
ImportError: No module named ImageOps

Still no ImageOps though. Now I'm stumped, is ImageOps missing from pillow, or is it a different problem that was also there with pil.

Answer 1

I see two separate problems here:

1. Keeping track of all the python modules you need for your project.

2. Keeping track of all the dynamic libraries you need for the python modules in your project.

For the first problem, I have found that buildout is good help, althought it takes a litle while to grasp.

In your case, I would start by creating a directory for my new project. I would then go into that directory and download bootstrap.py

wget http://python-distribute.org/bootstrap.py 

I would then create a buildout.cfg file:

[buildout]
parts = qrproject
        python
eggs = pyqrencode

[qrproject]
recipe = z3c.recipe.scripts
eggs = ${buildout:eggs}
entry-points= qrproject=qrprojectmodule:run
extra-paths = ${buildout:directory}

# This is a simple way of creating an interpreter that will have
# access to all the eggs / modules that this project uses.
[python]
recipe = z3c.recipe.scripts
interpreter = python
eggs = ${buildout:eggs}
extra-paths = ${buildout:directory}

In this buildout.cfg I'm referencing the module qrprojectmodule (in entry-points under [qrproject]. This will create a bin/qrproject that runs the function run in the module qrprojectmodule. So I will also create the file qrprojectmodule.py

import qrencode

def run():
    print "Entry point for qrproject. Happily imports qrencode module"

Now it's time to run bootstrap.py with the python binary you want to use:

python bootstrap.py

Then run the generated bin/buildout

bin/buildout

This will create two additional binaries in the bin/ directory - bin/qrproject and bin/python. The former is your project's main binary. It will be created automatically each time you run buildout and will have all the modules and eggs you want loaded. The second is simply a convenient way to get a python prompt where all your modules and eggs are loaded, for easy debugging. The fine thing here is that bin/buildout will automatically install any python eggs that the eggs (in your case pyqrencode) have specified as dependencies.

Actually, you will probably get a compilation error in the step where you run bin/buildout. This is because you need to address problem 2: All dynamic libraries being available on your system. On Linux, it's usually best to get help from your packaging system. I'm going to assume you're using a Debian derivate such as Ubuntu here.

The pyqrencode web site specifies that you need the libqrencode library for pyqrencode to work. So I used my package manager to search for that:

$ apt-cache search libqrencode
libqrencode-dev - QR Code encoding library -- development
libqrencode3 - QR Code encoding library
qrencode - QR Code encoder into PNG image

In this case, I want the -dev package, as that installs linkable libraries and header files required to compile python C-modules. Also, the dependency system in the package manager will make sure that if I install libqrencode-dev, I will also get libqrencode3, as that is required at runtime, i.e. after compilation of the module.

So, I install the package:

sudo apt-get install libqrencode-dev

Once that has completed, rerun bin/buildout and the pyqrencode module will (hopefully) compile and install successfully. Now try to run bin/qrproject

$ bin/qrproject 
Entry point for qrproject. Happily imports qrencode module

Success! :-)

So, in summary:

1. Use buildout to automatically download and install all the python modules/eggs you need for your project.

2. Use your system's package manager to install any dynamic (C) libraries required by the python modules you use.

Be aware that in many cases there are already packaged versions of your python modules available in the package system. For example, pil is available by installing the python-imaging package on Ubuntu. In this case, you don't need to install it via buildout, and you don't need to worry about libraries being available - the package manager will install all dependencies required for the module to run. Doing it via buildout can however make it easier to distribute your project and make it run on other systems.