Elegant pythonic solution for a simple loop

Question

The function bars takes a list of Foo objects and returns a list of the bar attributes of those:

 def bars(foos):
      bars = []
      for foo in foos:
          bars.append(foo.bar)
      return bars

Coming from a Java background, this is the way I would have solved this in the past. But I feel that there is a more elegant pythonic way to solve this. How would I do that?

It's "pythonic" - I imagine pythonians are residents of Pythonia or something :-) — roippi, Sep 01 '14 at 20:14
It's generally referred to as *Pythonic* - not *Pythonian* :p — Jon Clements, Sep 01 '14 at 20:15

score 6 · Accepted Answer · answered Sep 01 '14 at 20:13

6

Use list-comprehensions:

[foo.bar for foo in foos]

(You can wrap it with def bars(foos):, but I think it's more readable without it)

answered Sep 01 '14 at 20:13

Korem

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score 1 · Answer 2 · edited May 23 '17 at 10:33

For your example, @Korem's suggestion of a list comprehension is the ideal solution.

But here's an alternative for when you encounter that pattern where you

initialize an empty sequence
append to it in a loop
return the sequence

but maybe need to do some more heavy lifting that you can't simply express in an expression (list comprehensions don't allow statements, only epxressions) - create a generator function:

def bars(foos):
    for foo in foos:
        yield foo.bar

There's just two things to note:

Generator functions don't return a sequence, they return a generator. That generator needs to be consumed by iterating over it. If you would use it like a sequence (indexing its elements for example), you'd need to turn it into a sequence e.g. by doing list(bars(f)):

>>> gen = bars(f)
>>> gen[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'generator' object has no attribute '__getitem__'
>>>
>>> list(gen)[0]
42

Once you iterate over it the generator will be exhausted:

>>> gen = bars(f)
>>> list(gen)
[42, 42, 42]
>>> list(gen)
[]

Elegant pythonic solution for a simple loop

2 Answers2