1

Assume you have a function, that sometimes returns a value, and sometimes doesn't, because there really is nothing you could return in this case, not even a default value or something. Now you want to do something with the result, but of course only when there is one.

Example:

result = function_call(params)
if result:
    print result

Is there a way to write this in a more pythonic way, maybe even in one line?

Like that:

print function_call(params) or #nothing

(Note that I mean it shouldn't print "nothing" or "None". It should actually just not print at all, if the result is None)

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  • 1
    A requirement is no setup, I assume? If setup code is OK, you could make a function for that. Especially easy to integrate in py3 where print is a function itself. Commented Aug 6, 2012 at 11:02
  • Isn't it 3 lines of Python-like code? We can usually replace 3 lines of Java code with about 1/3 of a line of Python :) Commented Aug 6, 2012 at 11:58
  • Yes @gnibbler, if something: lines are a big part of what is very Java-like and can be often written better in python. Commented Aug 6, 2012 at 11:59
  • 1
    @erikb85, porting just 3 lines at a time doesn't give much chance to leverage the power of Python. Commented Aug 6, 2012 at 12:07
  • @gnibbler: I am not porting. And yes, I just want to improve this little detail in my skillset. I do that often, so my python code improves step by step over the years. :) Commented Aug 6, 2012 at 12:09

3 Answers 3

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5

No; in Python, name binding is a statement and so cannot be used as an expression within a statement. Since print is also a statement you're going to require 3 lines; in Python 3 you could write:

result = function_call(params)
print(result) if result else None

This isn't quite true for name binding within a comprehension or generator, where name binding is a syntax item that has statement-like semantics:

[print(result) for result in generator_call(params) if result]

As Kos says, you can abuse this to create a one-element comprehension:

[print(result) for result in (function_call(params), ) if result]

Another syntax item that performs name binding and can similarly be abused is the lambda expression:

(lambda result: print(result) if result else None)(function_call(params))

Note that in both these cases the operation on the return value must be an expression and not a statement.

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4 Comments

+1; you can make that for result in (function_call(params),).
That's more code golf than pythonic refactoring though. The 3 line version is significantly more readable.
At first I thought print(result) if result else None would print None, but I was wrong. It's actually working!
@erikb85, that's because the first part (print(result)) is only evaluated if the check condition (if result) evaluates to True. Otherwise None is evaluated - which, obviously evaluates to None :)
3

I think the more Pythonic version is actually closer to your original:

result = function_call(params)
if result is not None:
    do_something(result)

Checking for is (not) None seems very idiomatic to me - I've used it several times myself and I've also seen it used elsewhere[citation-needed].

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3 Comments

Depends if you want to do_something is result is an empty string/list/etc. or not. +1 for just sticking with the 3 lines
This is very expressive in terms of "what happens here", but not in terms of "what did the author want to do here". It's a completely valid response, yet just not my personal taste for python code.
@erikb85, as I mentioned, I believe the Pythonic standard is to return None when there's no viable result (see dict.get for one example).
-1

From the answers up to now I would do that:

>>> from __future__ import print_function #if Python2.7
>>> def filtered_print(txt):
...    txt and print(txt)
...
>>> filtered_print('hello world')
hello world
>>> filtered_print('None')
None
>>> filtered_print(None)
>>>

If someone else has a better solution in mind, I am still open for alternatives, though!

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2 Comments

Note that someone downvoted this, because he thinks, it is bad style to write code in A and B style instead of 2 lines with an if. I think it's not dangerous and a lot of people will understand this code quite fine. Depending on the people you work with you might choose not to use this solution!
Finally I decided against this, because the marked solution works and looks better.

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