return variable outputs in Python function?

Question

I have a function that returns a list. When I know the number of elements in the list I can do something like:

x,y,z=foo()

however, the number of elements in the list can vary. I would like to do something like

x,y*=foo()

where x would contain the first element in the returned list and y* would contain the remainder of however many elements there are left. Obviously , this syntax is not supported in Python. Is there a philosophical reason for this? Am I thinking about this the wrong way?

thanks in advance.

Ned Batchelder · Accepted Answer · 2009-12-12 12:49:30Z

16

There is no philosophical objection to your syntax, in fact, it's supported in Python 3.1:

>>> a, *b = range(10)
>>> a
0
>>> b
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> # Or even more flexibly:
>>> a, *b, c = range(10)
>>> a
0
>>> b
[1, 2, 3, 4, 5, 6, 7, 8]
>>> c
9
>>>

answered Dec 12, 2009 at 12:49

Ned Batchelder

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1 Comment

reckoner Over a year ago

Is this syntax supported in Python 2.6?

Peter Hansen · Accepted Answer · 2009-12-12 18:05:16Z

3

A slightly generalized approach:

>>> def unpack(seq, count):
...   return seq[:count] + [seq[count:]]
>>> a, b = unpack([1, 2, 3], 1)
>>> a
1
>>> b
[2, 3]
>>> a, b = unpack([1, 2], 1)
>>> a
1
>>> b
[2]
>>> a, b = unpack([1], 1)
>>> a
1
>>> b
[]
>>> a, b = unpack([], 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack
>>> a, b, c, d = unpack(range(10), 3)
>>> a
0
>>> b
1
>>> c
2
>>> d
[3, 4, 5, 6, 7, 8, 9]

edited Dec 12, 2009 at 18:05

answered Dec 12, 2009 at 15:20

Peter Hansen

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2 Comments

reckoner Over a year ago

is there a way to somehow figure out what the number of desired outputs is instead of supplying it as an input to the unpack function?

Peter Hansen Over a year ago

Without serious complication, no. You'd have to inspect the bytecode of the caller, which would be well past sanity. Reproducing the link you included above in another comment (where someone does just that): code.activestate.com/recipes/284742 . Doing something like this is similar to using macros in C to define new language constructs: very bad idea. Better to live with a few warts in the language you've chosen than to make up a language with only one programmer in the world who knows what it means.

S.Lott · Accepted Answer · 2009-12-12 12:45:47Z

3

Like this

results = foo()
x= results[0]
y= results[1:]

Or change your function to return a two-tuple

def foo():
    ....
    return someList[0], someList[1:]

answered Dec 12, 2009 at 12:45

S.Lott

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1 Comment

reckoner Over a year ago

this approach is taken more generally in the following: Recipe 284742: Finding out the number of values the caller is expecting code.activestate.com/recipes/284742

user2940756 · Accepted Answer · 2016-12-27 18:18:46Z

0

What you need to do is to return a list

e.g. return [ ... ]

You then receive the list and find its length e.g. abc=f(.....)

len(abc) would give you the number of elements in the list

for iiabc in abc: would allow you to access each of the elements

answered Dec 27, 2016 at 18:18

user2940756

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