Ellipsis is a Python Object. It has no Methods. It is a singleton Object i.e. , provides easy access to single instances.
Various Use Cases of Ellipsis (…):
- Default Secondary Prompt in Python interpreter.
- Accessing and slicing multidimensional Arrays/NumPy indexing.
- In type hinting.
- Used as Pass Statement inside Functions.
Default Secondary Prompt in Python interpreter
Ellipsis notation[…] is used as a default secondary prompt in Python interpreter which is seen during multi-line constructs
Example:
Ellipsis in Python3
Accessing and slicing multidimensional Arrays/NumPy indexing
- Accessing: Giving access to a specified range of elements, just omitting out the serial indices.
- Slicing: Important use of Ellipsis is in slicing higher-dimensional data structures.
Example:
Suppose, we have a 4-dimensional matrix of order 2x2x2x2. To select all first row elements(in case of row-major structure) in the 4th dimension, we can simply use the ellipsis notation
Python3
# importing numpyimport numpy as nparray = np.random.rand(2, 2, 2, 2)print(array[..., 0])print(array[Ellipsis, 0]) |
Output:
[[[0.46253663 0.03092289] [0.72723607 0.75953107]] [[0.33160093 0.79259324] [0.76757812 0.21241883]]] [[[0.46253663 0.03092289] [0.72723607 0.75953107]] [[0.33160093 0.79259324] [0.76757812 0.21241883]]]
In the above example, [:, :, :, 0], [ … , 0] and [Ellipsis, 0] are all equivalent.
We can not have multiple ellipsis in a single slicing like a[… ,index, …]
In type hinting
Ellipsis is used in specifying type hints using the typing module (e.g. Callable[…, str]). It can serve in either way:
When the argument(s) of the function allows the type: Any
Actually callable takes the arguments:
Callable "[" parameters_expression, type_expression "]"
(e.g. Callable[…, str])
Example:
Python3
from typing import Callabledef inject(get_next_item: Callable[..., str]) -> None: ...# Argument type is assumed as type: Anydef foo(x: ...) -> None: ... |
Using ‘…’ as parameters_expression signifies a function that returns a string without specifying the call signature.
When the return value of the function is of type: Any
Actually callable returns this way:
Callable "[" parameters_expression, type_expression "]" -> return_type: #body
Example:
Python3
class flow: # (using "value: Any" to allow arbitrary types) def __understand__(self, name: str, value: ...) -> None: ... |
Used as Pass Statement inside Functions
Ellipsis is used instead of pass statement inside functions. ‘pass’ replaced by ‘…’ or ‘Ellipsis’.
Example:
Python3
# style1def foo(): pass# style2def foo(): ...# both the styles are same |
Ellipsis can also be used as a default argument value. Especially when you want to distinguish between not passing in value and passing in None.
Example:
Python3
def foo(x = ...): return xprint(foo) |
Output:
<function foo at 0x7fabdeea6e18>
Error while working with ellipsis
We should always use colons for accessing the other dimensions i.e. ‘ : ‘.
Python3
l[...,1,...] |
Output:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: an index can only have a single ellipsis ('...')
