"index" attribute - Python

Question

When I was reading this answer https://stackoverflow.com/a/9841401/5175044 to a question on removing duplicates from a string, I couldn't understand what the index attribute meant in

''.join(sorted(set(foo), key=foo.index)) 

I did this example run:

foo = "Wicked long string"
>>> "".join(sorted(set(foo),key = foo.index))
'Wicked longstr'
>>> "".join(sorted(set(foo)))
' Wcdegiklnorst'

which leads me to think that it helps in keeping the order of the characters.

Answer 1

As mentioned by others Pythons sorted() function and the sort() method of a list provides the key parameter to specify a function to be called on each list element prior to making comparisons.

The key thing is that this key parameter is a function object that takes when it is called only one argument and returns exactly one value which is used for the sorting.

A anonymous "lambda" function is often used for this purpose, since its definition does not include a return statement and therefor always contains an one expression which is returned.

For instance

>>> myKey = lambda e: id(e)

creates an (anonymous) function object

>>> type(myKey)
<class 'function'>

which takes one argument and returns a value and would therefore be a valid key for sorting.

If you want to call the myKey function object now you would simply do this:

>>> myKey(42)
503732608

---

To solve your problem you could create a lambda function as key which takes an element and returns its index in the foo string to keep the order of the characters:

>>> foo = "Wicked long string"
>>> "".join(sorted(set(foo), key = lambda e: foo.index(e)))
'Wicked longstr'

On the other hand -- since foo.index is a function object itself which takes one argument and returns one value -- you could pass this object instead to the sorted() function and by-pass the lambda definition:

>>> "".join(sorted(set(foo),key = foo.index))
'Wicked longstr'

Hope this helps :)