user40314 - 8 months ago 38

Python Question

I am trying to understand the meaning of

`ndarray.data`

ndarray.data -- Python buffer object pointing to the start of the array’s data

According to this description, I was expecting this to be a pointer to the C-array underlying the instance of ndarray.

Consider

`x = np.arange(5, dtype=np.float64)`

Form

`y`

`x`

`y = x[3:1:-1]`

I was expecting

`x.data`

`0.`

`y.data`

`3.`

`y.data`

`3*x.itemsize`

`x.data`

`>>> import numpy as np`

>>> x = np.arange(5, dtype=np.float64)

>>> y = x[ 3:1:-1]

>>> x.data

<memory at 0x000000F2F5150348>

>>> y.data

<memory at 0x000000F2F5150408>

>>> int('0x000000F2F5150408', 16) - int('0x000000F2F5150348', 16)

192

>>> 3*x.itemsize

24

The

`'data'`

`__array_interface`

`>>> y.__array_interface__['data'][0] - x.__array_interface__['data'][0]`

24

So this begs the question, what does thegive?`ndarray.data`

Thanks in advance.

Answer

Generally the number displayed by `x.data`

isn't meant to be used by you. `x.data`

is the buffer, which can be used in other contexts that expect a buffer.

```
np.frombuffer(x.data,dtype=float)
```

replicates your `x`

.

```
np.frombuffer(x[3:].data,dtype=float)
```

this replicates `x[3:]`

. But from Python you can't take `x.data`

, add 192 bits (3*8*8) to it, and expect to get `x[3:]`

.

I often use the `__array_interface__['data']`

value to check whether two variables share a data buffer, but I don't use that number for any thing. These are informative numbers, not working values.

I recently explored this in