jphollowed - 6 months ago 40

Python Question

I am trying to add object instances of

`astropy`

`Angles`

`NumPy`

`ValueError: setting an array element with a sequence.`

the

`Angle`

`<Angle 1.2557346257567 deg>`

If I put them in a normal Python list, then I get:

`s = [<Angle 1.2562500714928306 deg>,`

<Angle 1.2562500714928306 deg>,

<Angle 1.2562500714928306 deg>]

len(s)

>>> 3

len(s[0])

>>> TypeError: 'Angle' object with a scalar value has no len()

So, my first question is, in what way is a Python object a sequence? And, since numpy arrays need to be initialized to specific dimensions, how do I find the "length" of this object so that I can load them into a numpy array?

Answer

I don't have `astrop`

package and don't know the details of the `Angle`

object. But I can make some Python and numpy observations.

`<Angle 1.2557346257567 deg>`

is the string representation of this object, produced by its `__repr__`

method.

The TypeError for `len(s[0])`

means that this object does not a `__len__`

method. It is not a subclass of list, nor a specialized `numpy`

array. Check its docs. It may have a way of yielding a numeric value or values.

You don't say how you are trying to 'add' an Angle to an array, or what kind of array. If the array is numeric, e.g. `dtype=float`

, then doing

```
A[0]= <Angle...>
```

is not going to work, because `Angle`

is not a number, nor does it produce one - at least not without some method. You need to tell us what this target array is supposed to contain. Numbers, angle objects?

You can build an array that contains objects. `np.array(s)`

might work. It does when the list contains dictionary objects.

```
In [67]: ll
Out[67]: [{1: 2}, {}, {'a': 1, 'b': 2}]
In [68]: np.array(ll)
Out[68]: array([{1: 2}, {}, {'b': 2, 'a': 1}], dtype=object)
```

But `np.array([...])`

can be tricky to use, since it's designed to produce a multidimensional array of numbers - if possible.

Or you might have to make `A = np.zeros((3,),dtype=object)`

array, and assign values individually, `A[0]=s[0]`

. But such an object array is just a variant on a list. It's not a 2d array of numbers.

From a previous `astropy`

question:

How to covert np.ndarray into astropy.coordinates.Angle class?

```
Angle(angles_quantity).wrap_at('360d').value # This returns a simple ndarray again.
```

=================

Digging into the docs and astropy github code, I see that `Angle`

is a subclass of `Quantity`

which is a subclass of `ndarray`

. It tries, in effect to be an array (or scalar) with appropriate `unit`

definition.

The `.value`

method returns `self.view(np.ndarray)`

, unless it's shape is 0d, in which case it returns `self.item()`

, the scalar value.

So you should be able to define an `Angle`

with multiple values, e.g.

```
Angle([1.23, 1.24, 1.25])
```

I don't know if you can join multiple `Angles`

into one

```
Angle(s)
```

where `s`

is your list of Angle objects, might work. I don't see `Angle`

versions of `concatenate`

or `stack`

.