Python equivalent of getOrElse

Question

In languages with optional types, you have a fn called orElse or getOrElse which lets you do this:

None.getOrElse(1) == 1
Some(2).getOrElse(1) == 2

I.e. you can specify a default value. In Python, I find I am writing:

if output_directory:
    path = output_directory
else:
    path = '.'

and it would be cleaner with a getOrElse call. Searching for orElse and getOrElse turns up logical operators. Is there a suitable Python fn?

Edit: getOrElse is not the same as a ternary operator call because that requires you to reference output_directory twice. Which is a particular issue if it's replaced with a complex expression. (Quite normal in a functional style.)

The closest you can get is using a ternary: `path = output_directory if output_directory else '.'` — cs95, Mar 06 '18 at 13:05
Possible duplicate of [Does Python have a ternary conditional operator?](https://stackoverflow.com/questions/394809/does-python-have-a-ternary-conditional-operator) — Keyur Potdar, Mar 06 '18 at 13:09

score 9 · Accepted Answer · answered Mar 06 '18 at 13:05

9

Just use or for equivalent logic:

path = output_directory or '.'

answered Mar 06 '18 at 13:05

Chris_Rands

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score 6 · Answer 2 · answered Feb 11 '19 at 20:17

Note that the above solution does not work precisely like the canonical getOrElse semantics of an FP language such as Scala or Haskell in corner cases where the bool(x) == False. For example

Some(0).getOrElse(1) == 0
Some('').getOrElse('A') == ''

but for either of the two definitions below

def getOrElse1(x, y):
  return x or y

def getOrElse2(x, y):
  return x if x else y

we have

getOrElse(x=0, 1) == 1
getOrElse(x='', 'A') == 'A'

An equivalent semantics is given by the definition:

def getOrElse(x, y):
  return x if x is not None else y

Python equivalent of getOrElse

2 Answers2