What does the Ellipsis object do

Question

While idly surfing the namespace I noticed an odd looking object called Ellipsis  it does not seem to be or do anything special  but it s a globally available builtin    After a search I found that it is used in some obscure variant of the slicing syntax by Numpy and Scipy    but almost nothing else    Was this object added to the language specifically to support Numpy   Scipy  Does Ellipsis have any generic meaning or use at all   D  workspace numpy gt python Python 2 4 4   71  Oct 18 2006  08 34 43   MSC v 1310 32 bit  Intel   on win32 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  Ellipsis Ellipsis

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FastAPI makes use of the Ellipsis for creating required Parameters  https   fastapi tiangolo com tutorial query-params-str-validations

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In Python 3  you can   use the Ellipsis literal     as a    nop    placeholder for code that hasn t been written yet  def will do something             This is not magic  any expression can be used instead of      e g   def will do something        1   Can t use the word    sanctioned     but I can say that this use was not outrightly rejected by Guido       can  not in   must    should

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This is equivalent  l       1 2 3  l  Ellipsis  1 2 3       is a constant defined inside built-in constants   Ellipsis The same as the ellipsis literal            Special value used mostly in conjunction with extended slicing syntax for user-defined container data types

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Summing up what others have said  as of Python 3  Ellipsis is essentially another singleton constant similar to None  but without a particular intended use  Existing uses include    In slice syntax to represent the full slice in remaining dimensions In type hinting to indicate only part of a type Callable      int  or Tuple str        In type stub files to indicate there is a default value without specifying it   Possible uses could include    As a default value for places where None is a valid option As the content for a function you haven t implemented yet

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getitem   minimal     example in a custom class  When the magic syntax     gets passed to    in a custom class    getitem     receives a Ellipsis class object   The class can then do whatever it wants with this Singleton object   Example   class C object       def   getitem   self  k           return k    Single argument is passed directly  assert C   0     0    Multiple indices generate a tuple  assert C   0  1      0  1     Slice notation generates a slice object  assert C   1 2 3     slice 1  2  3     Ellipsis notation generates the Ellipsis class object    Ellipsis is a singleton  so we can compare with  is   assert C        is Ellipsis    Everything mixed up  assert C   1  2 3 4       6      1  slice 2 3 4   Ellipsis  6    The Python built-in list class chooses to give it the semantic of a range  and any sane usage of it should too of course   Personally  I d just stay away from it in my APIs  and create a separate  more explicit method instead   Tested in Python 3 5 2 and 2 7 12

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Its intended use shouldn t be only for these 3rd party modules  It isn t mentioned properly in the Python documentation  or maybe I just couldn t find that  but the ellipsis     is actually used in CPython in at least one place    It is used for representing infinite data structures in Python  I came upon this notation while playing around with lists   See this question for more info

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This came up in another question recently   I ll elaborate on my answer from there   Ellipsis is an object that can appear in slice notation   For example   myList 1 2       0    Its interpretation is purely up to whatever implements the   getitem   function and sees Ellipsis objects there  but its main  and intended  use is in the numpy third-party library  which adds a multidimensional array type   Since there are more than one dimensions  slicing becomes more complex than just a start and stop index  it is useful to be able to slice in multiple dimensions as well  E g   given a 4x4 array  the top left area would be defined by the slice   2  2     gt  gt  gt  a array    1   2   3   4            5   6   7   8            9  10  11  12           13  14  15  16      gt  gt  gt  a  2  2     top left array   1  2           5  6      Extending this further  Ellipsis is used here to indicate a placeholder for the rest of the array dimensions not specified   Think of it as indicating the full slice     for all the dimensions in the gap it is placed  so for a 3d array  a     0  is the same as a     0  and for 4d  a       0   similarly  a 0     0  is a 0     0   with however many colons in the middle make up the full number of dimensions in the array    Interestingly  in python3  the Ellipsis literal       is usable outside the slice syntax  so you can actually write     gt  gt  gt      Ellipsis   Other than the various numeric types  no  I don t think it s used   As far as I m aware  it was added purely for numpy use and has no core support other than providing the object and corresponding syntax   The object being there didn t require this  but the literal       support for slices did

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In typer     is used to create required parameters  The Argument class expects a default value  and if you pass the     it will complain if the user does not pass the particular argument  You could use None for the same if Ellipsis was not there  but this would remove the opportunity to express that None is the default value  in case that made any sense in your program

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You can use Ellipsis yourself  in custom slicing situations like numpy has done  but it has no usage in any builtin class   I don t know if it was added specifically for use in numpy  but I certainly haven t seen it used elsewhere   See also  How do you use the ellipsis slicing syntax in Python

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You can also use the Ellipsis when specifying expected doctest output   class MyClass object          Example of a doctest Ellipsis       gt  gt  gt  thing   MyClass        gt  gt  gt    Match  lt class    main   MyClass  gt  and  lt class    module  MyClass  gt       gt  gt  gt  type thing              doctest  ELLIPSIS      lt class      MyClass  gt              pass

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From the Python documentation      This object is commonly used by slicing  see Slicings   It supports no   special operations  There is exactly one ellipsis object  named   Ellipsis  a built-in name   type Ellipsis    produces the Ellipsis   singleton       It is written as Ellipsis or

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As of Python 3 5 and PEP484  the literal ellipsis is used to denote certain types to a static type checker when using the typing module   Example 1      Arbitrary-length homogeneous tuples can be expressed using one type and ellipsis  for example Tuple int          Example 2      It is possible to declare the return type of a callable without specifying the call signature by substituting a literal ellipsis  three dots  for the list of arguments    def partial func  Callable      str    args  - gt  Callable      str         Body

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As mentioned by  no     z    and  phoenix - You can indeed use it in stub files  e g    class Foo      bar  Any           def   init   self  name  str      - gt  None        More information and examples of how to use this ellipsis can be discovered here https   www python org dev peps pep-0484  stub-files

[python] What does the Ellipsis object do?

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