Why does returning this lambda expression result in a string?

Question

I recently started learning Python 3.5.1, and am currently experimenting with lambda expressions. I tried setting up the simple method below.

def sum_double(a, b):
  return lambda a, b: a+b if a != b else (a+b)*2, a, b

All it is supposed to do is return the sum of a and b, and twice their sum if a is equal to b, but instead I get an output that looks like this.

Code:

print(sum_double(1, 2))
print(sum_double(2, 3))
print(sum_double(2, 2))

Output:

(<function sum_double.<locals>.<lambda> at 0x000001532DC0FA60>, 1, 2)  
(<function sum_double.<locals>.<lambda> at 0x000001532DC0FA60>, 2, 3)
(<function sum_double.<locals>.<lambda> at 0x000001532DC0FA60>, 2, 2)

Am I doing this wrong? Why is this happening, and how would I use a lambda expression to achieve my desired functionality if that is even possible?

Why are you using a lambda here? A lambda is a function, so you're returning a function from your function. — Morgan Thrapp
– Morgan Thrapp, Commented Jul 22, 2016 at 20:32
@MorganThrapp Just seeing what I can and cannot do with lambdas in order to fully understand them and their behaviors. — pianoisland
– pianoisland, Commented Jul 22, 2016 at 20:38

Dimitris Fasarakis Hilliard · Accepted Answer · 2016-07-22 20:32:27Z

2

Well, you're not calling the lambda function and as such the return value is a tuple of the defined lambda function and the values of a and b.

Change it to call the lambda before returning while supplying the arguments to it:

return (lambda a, b: a+b if a != b else (a+b)*2)(a, b)

And it works just fine:

print(sum_double(1, 2))
3

print(sum_double(2, 2))
8

answered Jul 22, 2016 at 20:32

Dimitris Fasarakis Hilliard

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4 Comments

pianoisland Over a year ago

Thanks, this worked! I will accept your answer when the time restriction expires.

Michael Hoff Over a year ago

For future reference: This is not how lambda expressions are supposed to be used. In this specific case, you are wrapping your functionality in two redundant function calls. Either omit the lambda completely, or directly assign the lambda function to the variable sum_double.

Dimitris Fasarakis Hilliard Over a year ago

@MichaelHoff totally agree. I never objected to the use, since the OP did state he was experimenting with lambdas and, while experimenting, you're generally free to try any wacky thing.

Michael Hoff Over a year ago

@Jim: I thought so. I just wanted to make sure that this issue is clear when some beginner comes across this question :)

Michael Hoff · Accepted Answer · 2016-07-22 20:32:10Z

2

sum_double = lambda a, b: a+b if a != b else (a+b)*2

lambda itself defines a function. A def is not required.

In your case, a tuple consisting of the returned function, a and b is printed.

answered Jul 22, 2016 at 20:32

Michael Hoff

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