Python assignment and test of function using elif

Question

I have seen questions similar to this but not quite the same. I have an expression

if foo == ....
  return -1
elif myFunction(bar) != -1:
  return myFunction(bar)
elif ...

I don't want to calculate myFunction(bar) twice. If it were simply an if, I could do

temp = myFunction(bar)
if temp != -1
  return temp

However doing this with an elif would result in unnecessary calculations of temp if we were to follow the intitial if.

I can see a solution using an

if ...
else
  temp = myFunction(bar)
  if temp != -1:
    return temp
  elif ...

But that now starts to become more ugly. Is there a better way to accomplish this?

Answer 1

If you do this a lot, it might pay to have a memoizing decorator around. The only memoizer that is currently in the standard library (for newer Python 3.x) is lru_cache, which is overkill, but shows the general idea:

>>> def func():
...     print("foo")
...     return 1
... 
>>> if func():
...     print(func())
...     
foo
foo
1

Now memoize func:

>>> from functools import lru_cache
>>> func = lru_cache(1)(func)
>>> if func():
...    print(func())
...     
foo
1

Answer 2

If you do not want to call myFunction(bar) twice then there is no other way than using an intermediate variable to store the result in. People here start proposing complex caching solutions for this. This can be pretty convenient in extreme cases, but before doing so, let's get back to the basics a bit. You should make proper use of the fact that you want to return from within your conditional blocks. In these situations you can save many elses. What follows now basically is the code block from your question, but without dots, with proper indentation, and with names according to PEP8:

if foo == "bar"
    return -1
elif myfunction(bar) != -1:
    return myfunction(bar)
else
    return None

It can easily be replaced with:

if foo == "bar"
    return -1
t = myfunction(bar)
if t != -1:
    return t
return None

As already stated in another answer, you can call your function twice if it does not affect the performance of your code. The result would look as simple as

if foo == "bar"
    return -1
if myfunction(bar) != -1:
    return myfunction(bar)
return None