Creating structured array from flat list

Question

Suppose I have a flat list L like this:

In [97]: L
Out[97]: [2010.5, 1, 2, 3, 4, 5]

...and I want to get a structured array like this:

array((2010.5, [1, 2, 3, 4, 5]),
      dtype=[('A', '<f4'), ('B', '<u4', (5,))])

How can I most efficiently make this conversion? I can not pass the latter dtype directly to array(L, ...) or to array(tuple(L)):

In [98]: dtp                                                                                                                                                                                 [9/1451]
Out[98]: [('A', '<f4', 1), ('B', '<u4', 5)]

In [99]: array(L, dtp)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-99-32809e0456a7> in <module>()
----> 1 array(L, dtp)

TypeError: expected an object with a buffer interface
In [101]: array(tuple(L), dtp)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-101-4d0c49a9f01d> in <module>()
----> 1 array(tuple(L), dtp)

ValueError: size of tuple must match number of fields.

What does work is to pass a temporary dtype where each field has one entry, then view this with the dtype I actually want:

In [102]: tdtp
Out[102]: 
[('a', numpy.float32),
 ('b', numpy.uint32),
 ('c', numpy.uint32),
 ('d', numpy.uint32),
 ('e', numpy.uint32),
 ('f', numpy.uint32)]

In [103]: array(tuple(L), tdtp).view(dtp)
Out[103]: 
array((2010.5, [1, 2, 3, 4, 5]), 
      dtype=[('A', '<f4'), ('B', '<u4', (5,))])

But to create this temporary dtype is an additional step that I would like to avoid if possible.

Is it possible to go directly from my flat list to my structured dtype, without using the intermediate dtype shown above?

(Note: in my real use case I have a reading routine reading a custom file format and many values per entry; so I would prefer to avoid a situation where I need to construct both the temporary and the actual dtype by hand.)

Warren Weckesser · Accepted Answer · 2014-03-07 20:18:13Z

4

Instead of passing tuple(L) to array, pass an argument with nested values that match the nesting of the dtype. For the example you showed, you could pass in (L[0], L[1:]):

In [28]: L
Out[28]: [2010.5, 1, 2, 3, 4, 5]

In [29]: dtp
Out[29]: [('A', '<f4', 1), ('B', '<u4', 5)]

In [30]: array((L[0], L[1:]), dtype=dtp)
Out[30]: 
array((2010.5, [1L, 2L, 3L, 4L, 5L]), 
      dtype=[('A', '<f4'), ('B', '<u4', (5,))])

answered Mar 7, 2014 at 20:18

Warren Weckesser

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