indexing - How does the Python conditional operator workaround work?

Question

From what I have read, I found that a built-in ternary operator does not exist (I will be happy to know more about it.).

I found the following code as a substitute:

def val():
    var = float(raw_input("Age:"))
    status = ("Working","Retired")[var>65]
    print "You should be:",status

I couldn't understand how this code works; can anyone explain me how actually the code is working? I am also interested to know why the ternary operator doesn't exist; any references or links about this will be ore useful.

I'm running Python 2.6.4 on Windows Vista.

Answer 1

Python has a construct that is sort of like the ternary operator in C, et al. It works something like this:

my_var = "Retired" if age > 65 else "Working"

and is equivalent to this C code:

my_var = age > 65 ? "Retired" : "Working";

As for how the code you posted works, let's step through it:

("Working","Retired")

creates a 2-tuple (an immutable list) with the element "Working" at index 0, and "Retired" at index 1.

var>65

returns True if var is greater than 65, False if not. When applied to an index, it is converted into 1 (True) or 0 (False). Thus, this boolean value provides an index into the tuple created on the same line.

Why hasn't Python always had a ternary operator? The simple answer is that Guido van Rossum, the author of Python, didn't like/didn't want it, apparently believing that it was an unnecessary construct that could lead to confusing code (and anyone who's seen massively-nested ternary operators in C can probably agree). But for Python 2.5, he relented and added the grammar seen above.