Behaviour of Python Sets

Question

s={['list']}

Gives error as below:

Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

s=set(['list'])

However above works fine. Why?

• busybear: The first example should be a SyntaxError since you can't define a set that way. You can use either {} or set() to define a set.
• CDspace: Did you mean s = {['list']}. In python 3.6.0 your code gives a SyntaxError. But this gives the error you posted.
• Shubham S. Naik: It was a typo. Corrected the same

Answer 1

Your first example should be giving you a SyntaxError.

{['list']} is a set containing a list which raises an error because lists are not hashable.
set(['list']) is a set built from an iterable that happens to be a list. The equivalent expression using curly braces would be {'list'}, which works fine because strings are hashable.

• Shubham S. Naik: It was a typo.Corrected the same.

Answer 2

The first example is not valid because lists are unhashable.

{['list']} is read as a set containing a single item of type list, but lists cannot be used as set items or keys in python, so you get an error.

The closest analogue would be to use a tuple {('list')}, since tuples are hashable, but it seems more likely that you just want the string, in which case you should write:

s = {'list'}

The second example is valid python syntax.

It calls the set constructor on a list of items to get a set of those items.

• Shubham S. Naik: It was a typo.Corrected the same.
• Ollin Boer Bohan: Yup! I edited the answer to reflect the edited question.