What is the good python3 equivalent for auto tuple unpacking in lambda?

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Question :

What is the good python3 equivalent for auto tuple unpacking in lambda?

Consider the following python2 code

In [5]: points = [ (1,2), (2,3)]

In [6]: min(points, key=lambda (x, y): (x*x + y*y))
Out[6]: (1, 2)

This is not supported in python3 and I have to do the following:

>>> min(points, key=lambda p: p[0]*p[0] + p[1]*p[1])
(1, 2)

This is very ugly. If the lambda was a function, I could do

def some_name_to_think_of(p):
  x, y = p
  return x*x + y*y

Removing this feature in python3 forces the code to either do the ugly way(with magic indexes) or create unnecessary functions(The most bothering part is to think of good names for these unnecessary functions)

I think the feature should be added back at least to lambdas alone. Is there a good alternative?

Update: I am using the following helper extending the idea in the answer

def star(f):
  return lambda args: f(*args)

min(points, key=star(lambda x,y: (x*x + y*y))

Update2: A cleaner version for star

import functools

def star(f):
    def f_inner(args):
        return f(*args)
    return f_inner
Asked By: balki


Answer #1:

No, there is no other way. You covered it all. The way to go would be to raise this issue on the Python ideas mailing list, but be prepared to argue a lot over there to gain some traction.

Actually, just not to say “there is no way out”, a third way could be to implement one more level of lambda calling just to unfold the parameters – but that would be at once more inefficient and harder to read than your two suggestions:

min(points, key=lambda p: (lambda x,y: (x*x + y*y))(*p))

update Python 3.8

As of now, Python 3.8 alpha1 is available, and PEP 572- assignment expressions are implemented.

So, if one uses a trick to execute multiple expressions inside a lambda – I usually do that by creating a tuple and just returning the last component of it, it is possible to do:

>>> a = lambda p:(x:=p[0], y:=p[1], x ** 2 + y ** 2)[-1]
>>> a((3,4))

One should keep in mind that this kind of code will seldom be more readable or practical than having a full function. Still, there are possible uses – if there are various one-liners that would operate on this point, it could be worth to have a namedtuple, and use the assignment expression to effectively “cast” the incoming sequence to the namedtuple:

>>> from collections import namedtuple
>>> point = namedtuple("point", "x y")
>>> b = lambda s: (p:=point(*s), p.x ** 2 + p.y ** 2)[-1]
Answered By: balki

Answer #2:

According to http://www.python.org/dev/peps/pep-3113/ tuple unpacking are gone, and 2to3 will translate them like so:

As tuple parameters are used by lambdas because of the single
expression limitation, they must also be supported. This is done by
having the expected sequence argument bound to a single parameter and
then indexing on that parameter:

lambda (x, y): x + y

will be translated into:

lambda x_y: x_y[0] + x_y[1]

Which is quite similar to your implementation.

Answered By: jsbueno

Answer #3:

I don’t know any good general alternatives to the Python 2 arguments unpacking behaviour. Here’s a couple of suggestion that might be useful in some cases:

  • if you can’t think of a name; use the name of the keyword parameter:

    def key(p): # more specific name would be better
        x, y = p
        return x**2 + y**3
    result = min(points, key=key)
  • you could see if a namedtuple makes your code more readable if the list is used in multiple places:

    from collections import namedtuple
    from itertools import starmap
    points = [ (1,2), (2,3)]
    Point = namedtuple('Point', 'x y')
    points = list(starmap(Point, points))
    result = min(points, key=lambda p: p.x**2 + p.y**3)
Answered By: njzk2

Answer #4:

While the destructuring arguments was removed in Python3, it was not removed from comprehensions. It is possible to abuse it to obtain similar behavior in Python 3. In essence, we take advantage of the fact that co-routines allow us to turn functions inside out, and yield is not a statement, and hence is allowed within lambdas.

For example:

points = [(1,2), (2,3)]
print(min(points, key=lambda y: next(x*x + y*y for x,y in (lambda a: (yield a))(y))))

In comparison with the accepted answer of using a wrapper, this solution is able to completely destructure the arguments while the wrapper only destructures the first level. That is,

values = [(('A',1),'a'), (('B',0),'b')]
print(min(values, key=lambda y: next(b for (a,b),c in (lambda x: (yield x))(y))))

In comparison to

values = [(('A',1),'a'), (('B',0),'b')]
print(min(points, key=lambda p: (lambda a,b: (lambda x,y: (y))(*a))(*p)))

Alternatively one can also do

values = [(('A',1),'a'), (('B',0),'b')]
print(min(points, key=lambda y: next(b for (a,b),c in [y])))

Or slightly better

print(min(values, key=lambda y: next(b for ((a,b),c) in (y,))))

This is just to suggest that it can be done, and should not be taken as a recommendation.

Answered By: jfs

Answer #5:

I think the better syntax is x * x + y * y let x, y = point, let keyword should be more carefully chosen.

The double lambda is the closest version.
lambda point: (lambda x, y: x * x + y * y)(*point)

High order function helper would be useful in case we give it a proper name.

def destruct_tuple(f):
  return lambda args: f(*args)

destruct_tuple(lambda x, y: x * x + y * y)
Answered By: rahul

Answer #6:

Based on Cuadue suggestion and your comment on unpacking still being present in comprehensions, you can use, using numpy.argmin :

result = points[numpy.argmin(x*x + y*y for x, y in points)]
Answered By: anthony.hl

Answer #7:

Another option is to write it into a generator producing a tuple where the key is the first element. Tuples are compared starting from beginning to end so the tuple with the smallest first element is returned. You can then index into the result to get the value.

min((x * x + y * y, (x, y)) for x, y in points)[1]
Answered By: njzk2

Answer #8:

Consider whether you need to unpack the tuple in the first place:

min(points, key=lambda p: sum(x**2 for x in p))

or whether you need to supply explicit names when unpacking:

min(points, key=lambda p: abs(complex(*p))
Answered By: daz

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