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balki balki - 7 months ago 36
Python 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))


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))