Append values to a set in Python

The Question :

415 people think this question is useful

I have a set like this:

keep = set(generic_drugs_mapping[drug] for drug in drug_input)

How do I add values [0,1,2,3,4,5,6,7,8,9,10] into this set?

The Question Comments :

The Answer 1

444 people think this answer is useful
keep.update(yoursequenceofvalues)

e.g, keep.update(xrange(11)) for your specific example. Or, if you have to produce the values in a loop for some other reason,

for ...whatever...:
  onemorevalue = ...whatever...
  keep.add(onemorevalue)

But, of course, doing it in bulk with a single .update call is faster and handier, when otherwise feasible.

The Answer 2

320 people think this answer is useful

Define set

a = set()

Use add to append single values

a.add(1)
a.add(2)

Use update to add elements from tuples, sets, lists or frozen-sets

a.update([3,4])

>> print(a)
{1, 2, 3, 4}

If you want to add a tuple or frozen-set itself, use add

a.add((5, 6))

>> print(a)
{1, 2, 3, 4, (5, 6)}

Note: Since set elements must be hashable, and lists are considered mutable, you cannot add a list to a set. You also cannot add other sets to a set. You can however, add the elements from lists and sets as demonstrated with the “.update” method.

The Answer 3

120 people think this answer is useful

You can also use the | operator to concatenate two sets (union in set theory):

>>> my_set = {1}
>>> my_set = my_set | {2}
>>> my_set
{1, 2}

Or a shorter form using |=:

>>> my_set = {1}
>>> my_set |= {2}
>>> my_set
{1, 2}

Note: In versions prior to Python 2.7, use set([...]) instead of {...}.

The Answer 4

45 people think this answer is useful

Use update like this:

keep.update(newvalues)

The Answer 5

11 people think this answer is useful

This question is the first one that shows up on Google when one looks up “Python how to add elements to set”, so it’s worth noting explicitly that, if you want to add a whole string to a set, it should be added with .add(), not .update().

Say you have a string foo_str whose contents are 'this is a sentence', and you have some set bar_set equal to set().

If you do bar_set.update(foo_str), the contents of your set will be {'t', 'a', ' ', 'e', 's', 'n', 'h', 'c', 'i'}.

If you do bar_set.add(foo_str), the contents of your set will be {'this is a sentence'}.

The Answer 6

2 people think this answer is useful

The way I like to do this is to convert both the original set and the values I’d like to add into lists, add them, and then convert them back into a set, like this:

setMenu = {"Eggs", "Bacon"}
print(setMenu)
> {'Bacon', 'Eggs'}
setMenu = set(list(setMenu) + list({"Spam"}))
print(setMenu)
> {'Bacon', 'Spam', 'Eggs'}
setAdditions = {"Lobster", "Sausage"}
setMenu = set(list(setMenu) + list(setAdditions))
print(setMenu)
> {'Lobster', 'Spam', 'Eggs', 'Sausage', 'Bacon'}

This way I can also easily add multiple sets using the same logic, which gets me an TypeError: unhashable type: 'set' if I try doing it with the .update() method.

The Answer 7

1 people think this answer is useful
keep.update((0,1,2,3,4,5,6,7,8,9,10))

Or

keep.update(np.arange(11))

The Answer 8

0 people think this answer is useful

For me, in Python 3, it’s working simply in this way:

keep = keep.union((0,1,2,3,4,5,6,7,8,9,10))

I don’t know if it may be correct…

Add a Comment