TypeError Unhashable type

Question

I am trying to get a list of list of tuples   something like     1 0   2 0   3 0     1 1   2 1   3 1        I used this statement   set   a b for a in range 3  for b in range 3     But it gives me an error  TypeError  unhashable type   list    I have 2 questions  for the Python Guru s   a  When I look at the Python definition of Hashable -      An object is hashable if it has a hash value which never changes during its lifetime  it needs a hash   method     when I used  dir function on the expression above   dir   a b for a in range 3  for b in range 3     it seems to say the   hash   is there  So  why do I get the error     I was able to get    0  0    1  0    2  0      0  1    1  1    2  1      0  2    1  2    2  2    by using the list command    list list  a b  for a in range 3   for bin range 3     b list and set both takes Iterable as parameter  How come one works list  and another doesn t  set

User · Answer

You ll find that instances of list do not provide a   hash   --rather  that attribute of each list is actually None  try print      hash     Thus  list is unhashable   The reason your code works with list and not set is because set constructs a single set of items without duplicates  whereas a list can contain arbitrary data

User · Answer

and so you should do something like this   set tuple   a b  for a in range 3   for b in range 3         and if needed convert back to list

User · Answer

TLDR   - You can t hash a list  a set  nor a dict to put that into sets  - You can hash a tuple to put it into a set   Example     gt  gt  gt   1  2   3  4   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list    gt  gt  gt   1  2   3  4   set  1  2   3  4      Note that hashing is somehow recursive and the above holds true for nested items     gt  gt  gt   1  2  3   4   2  3    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list    Dict keys also are hashable  so the above holds for dict keys too

User · Answer

A list is unhashable because its contents can change over its lifetime  You can update an item contained in the list at any time   A list doesn t use a hash for indexing  so it isn t restricted to hashable items

User · Answer

The real reason because set does not work is the fact  that it uses the hash function to distinguish  different values  This means that sets only allows hashable objects  Why a list is not hashable is already pointed out

User · Answer

You are creating a set via set      call  and set needs hashable items  You can t have set of lists  Because list s arent hashable      a b  for a in range 3   for b in range 3   is a list  It s not a hashable type  The   hash   you saw in dir      isn t a method  it s just None   A list comprehension returns a list  you don t need to explicitly use list there  just use    gt  gt  gt     a b  for a in range 3   for b in range 3      0  0    1  0    2  0      0  1    1  1    2  1      0  2    1  2    2  2      Try those    gt  gt  gt  a    1  2  3   gt  gt  gt  b   1  2  3   gt  gt  gt  type a   lt class  set  gt   gt  gt  gt  type b   lt class  list  gt   gt  gt  gt   1  2      Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list   gt  gt  gt  print      hash    None  gt  gt  gt              list of lists               gt  gt  gt                set of lists Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list

[python] TypeError : Unhashable type

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