Type Confusion#

When using the ndindex API, it is important to avoid type confusion. Many types that are used as indices for arrays also have semantic meaning outside of indexing. For example, tuples and arrays mean one thing when they are indices, but they are also used in contexts that have nothing to do with indexing.

ndindex classes have names that are based on the native class names for the index type they represent. One must be careful, however, to not confuse these classes with the classes they represent. Most methods that work on the native classes are not available on the ndindex classes.

Some general types to help avoid type confusion:

Always use the ndindex() function to create ndindex types. When calling ndindex methods or creating Tuple objects, it is not necessary to convert arguments to ndindex types first. Slice literals (using :) are not valid syntax outside of a getitem (square brackets), but you can use the slice built-in object to create slices. slice(a, b, c) is the same as a:b:c.

Right:
```
idx.as_subindex((1,))
```
Wrong:
```
idx.as_subindex(Tuple(Integer(1))) # More verbose than necessary
```
Keep all index objects as ndindex types until performing actual indexing. If all you are doing with an index is indexing it, and not manipulating it or storing it somewhere else, there is no need to use ndindex. But if you are storing ndindex types separately before indexing, or plan to manipulate them in any way, it is best to convert them to ndindex types with ndindex() as early as possible, and store them in that form. Only convert back to a raw index (with .raw, see the next bullet point) once doing an actual index operation. Avoid mixing ndindex and “raw” or non-ndindex types. There are many reasons to avoid this:
- Raw types (such as int, slice, tuple, array, and so on), do not have any of the same methods as ndindex, so your code may fail
- Some raw types, such as slices, arrays, and tuples containing slices or arrays, are not hashable, so if you try to use them as a dictionary key, they will fail. ndindex types are always hashable.
  
  Wrong
```
# Fails with a TypeError: unhashable type: 'slice'
indices = {
    slice(0, 10): 0,
    slice(10, 20): 1
}
```
  Right
```
indices = {
    ndindex[0:10]: 0,
    ndindex[10:20]: 1
}
```
- ndindex does basic type checking on indices that would otherwise not happen until they are actually used as an index. For example, slice(1.0, 2.0) does not fail until you try to index an array with it, but Slice(1.0, 2.0) fails immediately.
  
  Wrong
```
# Typo would not be caught until idx is actually used to index an array
idx = slice(1, 2.)
```
  Right
```
# Typo would be caught right away
idx = ndindex[1:2.]
# OR
idx = Slice(1, 2.)
```
- NumPy arrays and tuples containing NumPy arrays are not easy to compare, since using == on an array does not produce a boolean. == on an ndindex type will always produce a boolean, which compares if the two indices are exactly equal.
  
  Wrong
```
# Fails with ValueError: The truth value of an array with more than one element is ambiguous.
if idx == (slice(0, 2), np.array([0, 0]):
    ...
```
  Right
```
# Note only one side of the == needs to be an ndindex type
if ndindex(idx) == (slice(0, 2), np.array([0, 0]):
    ...
```
Use .raw to convert an ndindex object to an indexable type. With the exception of Integer, it is impossible for custom types to define themselves as indices to NumPy arrays, so it is necessary to use a[idx.raw] rather than a[idx] when idx is an ndindex type. Since native index types do not have a .raw method, it is recommended to always keep any index object that you are using as an ndindex type, and use .raw only when you need to use it as an index. If you get the error “IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices”, it indicates you forgot to use .raw.

Right:
```
a[idx.raw]
```
Wrong:
```
a[idx] # Gives an error
```
Use ndindex classes only for objects that represent indices. Do not use classes like Integer, Tuple, IntegerArray, or BooleanArray unless the object in question is going to be an index to an array. For example, array shapes are always tuples of integers, but they are not indices, so Tuple should not be used to represent an array shape, but rather just a normal tuple. If an object will be used both as an index and an integer/tuple/array in its own right, either make use of .raw when using it in non-index contexts, or store the objects separately (note that IntegerArray and BooleanArray always make a copy of the input argument, so there is no issue with managing it separately).

Right:
```
np.empty((1, 2, 3))
# idx is an ndindex object
idx.newshape((1, 2, 3))
```
Wrong:
```
np.empty(Tuple(1, 2, 3)) # gives an error
# idx is an ndindex object
idx.newshape(Tuple(1, 2, 3)) # gives an error
```
Try to use ndindex methods to manipulate indices. The whole reason ndindex exists is that writing formulas for manipulating indices is hard, and it’s easy to get the corner cases wrong. If you find yourself manipulating index args directly in complex ways, it’s a sign you should probably be using a higher level abstraction. If what you are trying to do doesn’t exist yet, open an issue so we can implement it.

Additionally, some advice for specific types:

Integer#

Integer should not be thought of as an int type. It represents integers as indices. It is not usable in contexts where integers are usable. For example, arithmetic will not work on it. If you need to manipulate the integer index as an integer, use idx.raw.

Right:
```
idx = ndindex(0)
idx.raw + 1
```
Wrong:
```
idx = ndindex(0)
idx + 1 # Produces an error
```
Integer is the only index type that can be used directly as an array index. This is because the __index__ API allows custom integer objects to define themselves as indices. However, this API does not extend to other index types like slices or tuples. It is recommended to always use idx.raw even if idx is an Integer, so that it will also work even if it is another index type. You should not rely on any ndindex function returning a specific index type (unless it states that it does so in its docstring).

Tuple#

Tuple should not be thought of as a tuple. In particular, things like idx[0] and len(idx) will not work if idx is a Tuple. If you need to access the specific term in a Tuple, use Tuple.args if you want the ndindex type, or Tuple.raw if you want the raw type.

Right:
```
idx = ndindex[0, 0:1]
idx.raw[0] # Gives int(0)
idx.args[0] # Gives Integer(0)
```
Wrong:
```
idx = ndindex[0, 0:1]
idx[0] # Produces an error
```

Tuple is defined as Tuple(*args). Tuple(args) gives an error.

Right:

Tuple(0, slice(0, 1))

Better:

ndindex[0, 0:1]

Wrong:

Tuple((0, slice(0, 1))) # Gives an error

ellipsis#

You should almost never use the ndindex ellipsis class directly. Instead, use ... or ndindex[...]. As noted above, all ndindex methods and Tuple will automatically convert ... into the ndindex type.

Right:
```
idx = ndindex(...)
idx.reduce()
```
Wrong:
```
idx = ...
idx.reduce() # Gives an error
```
We recommend preferring ... over the built-in name Ellipsis, as it is more readable. The ... syntax is allowed everywhere that Ellipsis would work.

Right:
```
idx = ndindex[0, ..., 1]
```
Wrong:
```
idx = ndindex[0, Ellipsis, 1] # Less readable
```
If you do use ellipsis beware that it is the class, not the instance, unlike the built-in Ellipsis object. This is done for consistency in the internal ndindex class hierarchy.

Right:
```
idx = ndindex[0, ..., 1]
```
Wrong:
```
idx = ndindex[0, ellipsis, 1] # Gives an error
```
The below do not give errors, but it is easy to confuse them with the above. It is best to just use ..., which is more concise and easier to read.
```
idx = ndindex[0, ellipsis(), 1] # Easy to confuse, less readable
idx.reduce()
```
```
idx = ndindex[0, Ellipsis, 1] # Easy to confuse, less readable
idx.reduce()
```
ellipsis is not singletonized, unlike the built-in .... Aside from singletonization not being necessary for ndindex types, it would be impossible to make ellipsis() is ... return True. If you are using ndindex, you should use == to compare against ..., and avoid using is. Note that as long as you know idx is an ndindex type, this is safe to do, since even the array index types IntegerArray and BooleanArray allow == comparison (unlike NumPy arrays).

Right:
```
if idx == ...:
```
Wrong:
```
if idx is Ellipsis: # Will be False if idx is the ndindex ellipsis type
```
```
if idx is ellipsis(): # Will always be False (ellipsis() creates a new instance)
```

Newaxis#

The advice for Newaxis is almost identical to the advice for ellipsis. Note that np.newaxis is just an alias for None.

You should almost never use the ndindex Newaxis class directly. Instead, use np.newaxis, None, ndindex(np.newaxis), or ndindex(None). As noted above, all ndindex methods and Tuple will automatically convert None into the ndindex type.

Right:
```
idx = ndindex[np.newaxis]
idx.reduce()
```
Wrong:
```
idx = np.newaxis
idx.reduce() # Gives an error
```
If you do use Newaxis beware that it is the class, not the instance, unlike the NumPy np.newaxis object (i.e., None). This is done for consistency in the internal ndindex class hierarchy.

Right:
```
idx = ndindex[0, np.newaxis, 1]
```
```
idx = ndindex[0, None, 1]
```
Wrong:
```
idx = ndindex[0, Newaxis, 1] # Gives an error
```
The below does not give an error, but it is easy to confuse it with the above. It is best to just use np.newaxis or None, which is more concise and easier to read.
```
idx = ndindex[0, Newaxis(), 1] # Easy to confuse
idx.reduce()
```
Newaxis is not singletonized, unlike the built-in None. It would also be impossible to make Newaxis() is np.newaxis or Newaxis() is None return True. If you are using ndindex, you should use == to compare against np.newaxis or None, and avoid using is. Note that as long as you know idx is an ndindex type, this is safe to do, since even the array index types IntegerArray and BooleanArray allow == comparison (unlike NumPy arrays).

Right:
```
if idx == np.newaxis:
```
```
if idx == None:
```
Wrong:
```
if idx is np.newaxis: # Will be False if idx is the ndindex Newaxis type
```
```
if idx is None: # Will be False if idx is the ndindex Newaxis type
```
```
if idx is Newaxis(): # Will be False (Newaxis() creates a new instance)
```

IntegerArray and BooleanArray#

IntegerArray and BooleanArray should not be thought of as arrays. They do not have the methods that numpy.ndarray would have. They also have fixed dtypes (intp and bool_) and are restricted by what is allowed as indices by NumPy. To access the arrays they represent, use idx.array or idx.raw.

Right:
```
idx = IntegerArray(array([0, 1]))
idx.array[0]
```
Wrong:
```
idx = IntegerArray(array([0, 1]))
idx[0] # Gives an error
```
Like all other ndindex types, IntegerArray and BooleanArray are immutable.. The .array object on them is set as read-only to enforce this. To modify an array index, create a new object. All ndindex methods that manipulate indices, like reduce(), return new objects. If you create an IntegerArray or BooleanArray object out of an existing array, the array is copied so that modifications to the original array do not affect the ndindex objects.

Right:
```
idx = IntegerArray(array([0, 1]))
arr = idx.array.copy()
arr[0] = 1
idx2 = IntegerArray(arr)
```
Wrong:
```
idx = IntegerArray(array([0, 1]))
idx.array[0] = 1 # Gives an error
```