"from python import lecture" aka "python_intro_2.pdf" is the collection of slides for Muli Ben-Yehuda (IBM Haifa Research Labs) lecture(s?) on basic Python. Ben-Yehuda has >>> fac = lambda n:[1,0][n>0] or fac(n-1)*n Which defines the factorial function in terms of what it does to a dummy variable n. If n=0 then 0! = 1 by definition. Otherwise n! = (n-1)! n, a recursive definition. Now [1,0] is the list with entries 1 and 0. [n>0] is a list of just one item, ether FALSE or TRUE. But in python that's 0 or 1. The catenation [1,0][1] is 0 and [1,0][0] is 1. The second case is just fine, being non-zero, the "or" is 1 (TRUE) regardless of what nonsense comes afterwards. But if n>0, then the first half of the "or" is 0, and it evaluates the second. In C one would write this as n>0 \$ fac(n-1)*n : 1; reading "if n>0 then return fac(n-1)*n, else return 1. The initialization using the list-catenation feature is nerdy, especially since fac(-1) returns 1, but (-1)! is undefined. Nevertheless, the lambda syntax is useful. For example, the squaring function could be defined as >>> squ = lambda x: x*x Exercise. Define the absolute value function. Appendix: The triple-function in python: This is too good to be true [e0,e1,e2,e3,e4][f0,f1,f3][g0] provided f0,f1,f2 are expressions whose values are integers in the range of 0..4, the first juxtaposition picks out the [f0th,f1st,f2nd] and then picks out the g0th, as long as g0 returns a integer in f0..f3. What a language! This makes bool?e1,e0 = [e0,e1][bool] a quintessentially postfix thingy.