Python从列表理解中解压缩空输入

Question

When working with a function that returns multiple values with a tuple, I will often find myself using the following idiom to unpack the results from inside a list comprehension.

fiz, buz = zip(*[f(x) for x in input])

Most of the time this works fine, but it throws a ValueError: need more than 0 values to unpack if input is empty. The two ways I can think of to get around this are

fiz = []
buz = []
for x in input:
    a, b = f(x)
    fiz.append(a)
    buz.append(b)

and

if input:
    fiz, buz = zip(*[f(x) for x in input])
else:
    fiz, buz = [], []

but neither of these feels especially Pythonic—the former is overly verbose and the latter doesn't work if input is a generator rather than a list (in addition to requiring an if/else where I feel like one really shouldn't be needed).

Is there a good simple way to do this? I've mostly been working in Python 2.7 recently, but would also be interested in knowing any Python 3 solutions if they are different.

Answer 1

If f = lambda x: (x,x**2) then this works

x,y = zip(*map(f,input)) if len(input) else ((),())

If input=[] , x=() and y=() .

If input=[2] , x=(2,) and y=(4,)

If input=[2,3] , x=(2,3) and y=(4,9)

They're tuples (not lists), but thats thats pretty easy to change.

Answer 2

I would consider using collections.namedtuple() for this sort of thing. I believe the named tuples are deemed more pythonic, and should avoid the need for complicated list comprehensions and zipping / unpacking.

---

From the documentation :

>>> p = Point(11, y=22)     # instantiate with positional or keyword arguments
>>> p[0] + p[1]             # indexable like the plain tuple (11, 22)
33
>>> x, y = p                # unpack like a regular tuple
>>> x, y
(11, 22)
>>> p.x + p.y               # fields also accessible by name
33
>>> p                       # readable __repr__ with a name=value style
Point(x=11, y=22)

Answer 3

You could use:

fiz = []
buz = []
results = [fiz, buz]

for x in input:
    list(map(lambda res, val: res.append(val), results, f(x)))

print(results)

Note about list(map(...)): in Python3, map returns a generator, so we must use it if we want the lambda to be executed.list does it.

(adapted from my answer to Pythonic way to append output of function to several lists , where you could find other ideas.)