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I have a list and I am trying to obtain either a set or a list of the unique items in that list. I also need to remove all instances of a specific item from the list, which in this case is 'USD'.

currencies = ['AUD', 'AUD', 'CAD', 'CHF', 'EUR', 'GBp', 'GBp', 'HKD', 'JPY', 'KRW', 'NOK', 'SEK', 'TWD', 'USD', 'USD', 'ZAr']

I initially tried, 

foreign_currencies = set(currencies).discard('USD')

but noticed that the function was returning a 'NoneType'.

In order to get it to work, I had to do it in two steps.

foreign_currencies = set(currencies)
foreign_currencies = foreign_currencies.discard('USD')

Can anyone tell me why this is the case and/or explain what I am not understanding about the order of execution? In the first example, is .discard() being called before the set is constructed? Is it something deeper that I am not getting?

EDIT: Although the responses in "Why does list.append evaluate to false?" answer my question, my question was not a duplicate. The question posed was not the same, the answer is.

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Kapocsi
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2 Answers2

2

Because discard does not return any output.It does an in place removal.

You need to do 

foreign_currencies = set(currencies)
foreign_currencies.discard('USD') #do not assign this as `discard does not return anything.
print foreign_currencies #now you have the currect result
vks
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0

As vks said, discard is an in place operation. This is what your code 'wants' to do:

foreign_currencies = set(currencies)
foreign_currencies.discard('USD')
Noam Kremen
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