[Tutor] List comprehension + lambdas - strange behaviour

[Alan Gauld]

>> I found this strange behaviour of lambdas, closures and list
>> comprehensions:
>>
>>   
>>>>> funs = [lambda: x for x in range(5)]
>>>>> [f() for f in funs]
>>>>>         
>> [4, 4, 4, 4, 4]
>>
>> Of course I was expecting the list [0, 1, 2, 3, 4] as the result. The
>> 'x' was bound to the final value of 'range(5)' expression for ALL
>> defined functions. Can you explain this? Is this only counterintuitive
>> example or an error in CPython?

As others have pointed out you are returning a reference not a value.

You can do what you want by defining a lo cal closure using:

funs = [lambda y = x: y for x in range(5)]

Now you can do

for f in funs:
    print f()

and get the answer you expect.

HTH,

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/