Howto figure out whether two lists have any elements in common

While working with FragIt I had to figure out whether two lists had some elements in common (it does not matter which!) and I came up with the following piece of code (with the help of Google and Stack Overflow of course)

	def shares_elements(a, b):
	"""Returns True if lists (sets) a and b shares elements. Otherwise false.
	"""
	sa = set(a)
	sb = set(b)
	return len(sa & sb) > 0

view raw shareselements.py hosted with ❤ by GitHub

It takes advantage of the highly underused Sets available in python on which we can do some neat mathematically sane operations. I guess people just go for Lists and work their magic on that.

One way to safe guard imports of external libraries

Say you have some code that is dependent on some 3rd party library, for instance users can download FragIt that requires Open Babel, but what happens if the user 'forgot' to install Open Babel or configure the paths correctly? Importing FragIt under these circumstances will crash your application because it has some explicit import statements to Open Babel. What should you do as a developer to safe guard this behavior?

The best way to deal with the issue (and this it not just the case of using FragIt, but any kind of library) which I learned from the very excellent "The Clean Coder" by Robert C. Martin is to write a wrapper for the FragIt API to suit your specific needs because this you can unit test and know instantly when the API has changed.

In header of this wrapper, you can provide code that looks like

	# always assume that we have openbabel
	has_openbabel = True
	try:
	import openbabel
	except ImportError:
	has_openbabel = False
	def FragIt_GetFragmentationPoints(input):
	""" Obtains the fragmentation points using the FragIt API
	based on the inpu
	"""
	# If the user has no Open Babel installed, inform the user
	if not has_openbabel:
	def FragIt_GetFragmentationPoints(input):
	print "WARNING: FragIt API not available because"
	print " Open Babel is missing / misconfigured."
	return None

view raw safeguard.py hosted with ❤ by GitHub

where we have defined a wrapper to the API to return the fragmentation points we want. However, if Open Babel is not defined, we overwrite that function, letting the user know that something is not right.

There are many ways to deal with it, this is one.

By the way: happy 1 year anniversary for the last blog post!