Python - ternary operator or if statement? [closed]

Question

Which of these two alternatives is more readable?

token = map.get(token_str)
if not token:
   token = Course(token_str)
return token

or

token = map.get(token_str)
token if token else Course(token_str)

Answer 1

get() takes an optional second parameter, for a value to use if the key isn't present:

token = map.get(token_str, Course(token_str))

I think that one is clearly the best solution, unless you don't want to create and immediately throw away a Course object. In that case, I would use 'or':

token = map.get(token_str) or Course(token_str)

Both of these are only one line, that one line is quite readable, and there is no unnecessary repetition.

There is one third possible situation, in which you don't want to unnecessarily construct a Course instance, and you also can't rely on the values that are present to evaluate as True. Then 'or' wouldn't work correctly.

Then I would use an 'if' statement:

token = map.get(token_str)
if token is None:
    token = Course(token_str)

but that's personal preference. Something using 'in' is also fine:

token = map[token_str] if token_str in map else Course(token_str)

although of course that looks up token_str in map twice, so if you need to be really time efficient then don't do that.

Answer 2

If this behavior is implemented at a specific consuming location of your map, I favor token = map.get(token_str) or Course(token_str) over both of your alternatives. This, like your original, assumes that no expected entries will have a False value.

If this is a general need, such that for all token_str the map should contain a Course object, you could make this part of the dict instance itself:

class Courses(dict):
    def __missing__(self, key):
        return Course(key)

map = Courses()

# ...
token = map[token_str]

Note that this handles False values differently from your original code; it only instantiates new Course objects when there wasn't an entry in map.

Side note: try to avoid using builtins such as map as local or global variables. It works fine, but throws readers off. In this case I would suggest a name such as courses.

Answer 3

In python, a common style is "easier to ask for forgiveness than permission" (http://docs.python.org/2/glossary.html#term-eafp). I.e.

try:
    return map[token_str]
except KeyError:
    return Course(token_str)