Shorter, more pythonic way of writing an if statements

Question

I have this

bc = 'off'

if c.page == 'blog':
    bc = 'on'

print bc

Is there a more pythonic (and/or shorter) way of writing this in python?

Answer 1

Shortest one should be:

bc = 'on' if c.page=='blog' else 'off'

Generally this might look a bit confusing, so you should only use it when it is clear what it means. Don't use it for big boolean clauses, since it begins to look ugly fast.

Answer 2

Or you could use an inline if statement:

>>> cpage = 'blog'
>>> bc = 'on' if cpage == 'blog' else 'off'
>>> bc
'on'
>>> cpage = 'asdf'
>>> bc = 'on' if cpage == 'blog' else 'off'
>>> bc
'off'

There's a bit of a writeup on that feature at this blog, and the relevant PEP is PEP308. The inline if statement was introduced in Python 2.5.

This one might be a little less pythonic, but you can use and/or in this fashion:

>>> cpage = 'asdf'
>>> bc = (cpage == 'blog') and 'on' or 'off'
>>> bc
'off'
>>> cpage = 'blog'
>>> bc = (cpage == 'blog') and 'on' or 'off'
>>> bc
'on'

This one is used more often in lambda statements than on a line by itself, but the form

 A and B or C

is similar to

   if A:
       return B
   else:
       return C

I was going to write out a little bit longer explanation, but they covered it better at Dive into Python. They also noted a couple caveats that you probably need to know.

Answer 3

Well, not being a python guy please take this with a huge grain of salt, but having written (and, with more difficulty, read) a lot of clever code over the years, I find myself with a strong preference now for readable code. I got the gist of what your original code was doing even though I'm a nobody as a Python guy. To be sure, you could hide it and maybe impress a Python wonk or two, but why?

Answer 4

This is:

1. definitely shorter
2. arguably Pythonic (pre-Python 2.5, which introduced the controversial X if Z else Y syntax)

3. questionably readable. With those caveats in mind, here it goes:

   bc = ("off","on")[c.page=="blog"]
   

EDIT: As per request, the generalized form is:

   result = (on_false, on_true)[condition]

Explanation: condition can be anything that evaluates to a Boolean. It is then treated as an integer since it is used to index the tuple: False == 0, True == 1, which then selects the right item from the tuple.

Answer 5

you can use "a = b if c else d" but if you are using a python version prior to 2.5

bc = c.page == "blog" and "on" or "off"

can do the trick also.

Answer 6

Another possibility is to use a dict if you can compute the values outside of the function that accesses them (i.e. the values are static, which also addresses the evaluation issue in scrible's answer's comments).

want_bc = {True: "on", False: "off"}

...

bc = want_bc[c.page == "blog"]

I prefer this and/or the tuple indexing solutions under the general rubric of preferring computation to testing.