Add list to set

Question

Tested on Python 2 6 interpreter    gt  gt  gt  a set  abcde    gt  gt  gt  a set   a    c    b    e    d     gt  gt  gt  l   f   g    gt  gt  gt  l   f    g    gt  gt  gt  a add l  Traceback  most recent call last     File   lt pyshell 35 gt    line 1  in  lt module gt      a add l  TypeError  list objects are unhashable   I think that I can t add the list to the set because there s no way Python can tell If I have added the same list twice  Is there a workaround   EDIT  I want to add the list itself  not its elements

User · Answer

Please notice the function set update    The documentation says      Update a set with the union of itself and others

User · Answer

Use set update   or      gt  gt  gt  a   set  abc    gt  gt  gt  l     d    e    gt  gt  gt  a update l   gt  gt  gt  a   e    b    c    d    a     gt  gt  gt  l     f    g    gt  gt  gt  a    set l   gt  gt  gt  a   e    b    f    c    d    g    a     edit  If you want to add the list itself and not its members  then you must use a tuple  unfortunately  Set members must be hashable

User · Answer

You ll want to use tuples  which are hashable  you can t hash a mutable object like a list     gt  gt  gt  a   set  abcde    gt  gt  gt  a set   a    c    b    e    d     gt  gt  gt  t     f    g    gt  gt  gt  a add t   gt  gt  gt  a set   a    c    b    e    d     f    g

User · Answer

You can t add a list to a set because lists are mutable  meaning that you can change the contents of the list after adding it to the set   You can however add tuples to the set  because you cannot change the contents of a tuple    gt  gt  gt  a add   f    g     gt  gt  gt  print a set   a    c    b    e    d     f    g         Edit  some explanation  The documentation defines a set as an unordered collection of distinct hashable objects  The objects have to be hashable so that finding  adding and removing elements can be done faster than looking at each individual element every time you perform these operations  The specific algorithms used are explained in the Wikipedia article  Pythons hashing algorithms are explained on effbot org and pythons   hash   function in the python reference   Some facts    Set elements as well as dictionary keys have to be hashable Some unhashable datatypes    list  use tuple instead set  use frozenset instead dict  has no official counterpart  but there are some  recipes  Object instances are hashable by default with each instance having a unique hash  You can  override this behavior as explained in the python reference

User · Answer

list objects are unhashable  you might want to turn them in to tuples though

User · Answer

You want to add a tuple  not a list    gt  gt  gt  a set  abcde    gt  gt  gt  a set   a    c    b    e    d     gt  gt  gt  l   f   g    gt  gt  gt  l   f    g    gt  gt  gt  t   tuple l   gt  gt  gt  t   f    g    gt  gt  gt  a add t   gt  gt  gt  a set   a    c    b    e    d     f    g       If you have a list  you can convert to the tuple  as shown above  A tuple is immutable  so it can be added to the set

User · Answer

Sets can t have mutable  changeable  elements members  A list  being mutable  cannot be a member of a set   As sets are mutable  you cannot have a set of sets  You can have a set of frozensets though    The same kind of  mutability requirement  applies to the keys of a dict    Other answers have already given you code  I hope this gives a bit of insight  I m hoping Alex Martelli will answer with even more details

User · Answer

I found I needed to do something similar today  The algorithm knew when it was creating a new list that needed to added to the set  but not when it would have finished operating on the list   Anyway  the behaviour I wanted was for set to use id rather than hash  As such I found mydict id mylist     mylist instead of myset add mylist  to offer the behaviour I wanted

User · Answer

Here is how I usually do it   def add list to set my list  my set        my set add each  for each in my list  return my set

User · Answer

To add the elements of a list to a set  use update  From https   docs python org 2 library sets html     s update t   return set s with elements added from t   E g    gt  gt  gt  s   set  1  2    gt  gt  gt  l    3  4   gt  gt  gt  s update l   gt  gt  gt  s  1  2  3  4    If you instead want to add the entire list as a single element to the set  you can t because lists aren t hashable   You could instead add a tuple  e g  s add tuple l     See also TypeError  unhashable type    39 list  39  when using built-in set function for more information on that

User · Answer

Try using   unpack  like below   gt  gt  gt  a set  abcde    gt  gt  gt  a   a    d    e    b    c    gt  gt  gt  l   f   g    gt  gt  gt  l   f    g    gt  gt  gt    l   a    a    d    e    f    b    g    c    gt  gt  gt    Non Editor version  a set  abcde   l   f    g   print   l   a    Output    a    d    e    f    b    g    c

User · Answer

Hopefully this helps    gt  gt  gt  seta   set  1234    gt  gt  gt  listb     a   b   c    gt  gt  gt  seta union listb  set   a    c    b    1    3    2    4     gt  gt  gt  seta set   1    3    2    4     gt  gt  gt  seta   seta union listb   gt  gt  gt  seta set   a    c    b    1    3    2    4

[python] Add list to set?

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