How to make numpy array mutable?

Question

packed_bytes = resources[bv.byteOffset : bv.byteOffset + bv.byteLength]

arr = np.frombuffer(
    packed_bytes,
    dtype=component_type[accessor.componentType].replace("{}", ""),
).reshape((-1, accessor_type[accessor.type]))
arr.flags.writeable = True # this does not work

It gives:

ValueError: cannot set WRITEABLE flag to True of this array

When trying to set writeable flag to the numpy array. Before adding that line of code, it gave:

ValueError: output array is read-only

When trying to write to the array. Any ideas on how to fix this? I have no idea why this is an issue.

The problem may be the buffer. An array made in the usual way with np.array and its own 'fresh' databuffer is writeable. There are too many unknowns in your question for (most of) us to help you. — hpaulj
– hpaulj, Commented May 20, 2022 at 2:02
look like your packed_bytes need to be a mutable object itself for this to work — Copperfield
– Copperfield, Commented May 20, 2022 at 2:45

hpaulj · Accepted Answer · 2022-05-20 03:22:11Z

Using an example from frombuffer:

x=np.frombuffer(b'\x01\x02', dtype=np.uint8)

x
Out[105]: array([1, 2], dtype=uint8)

x.flags
Out[106]: 
  C_CONTIGUOUS : True
  F_CONTIGUOUS : True
  OWNDATA : False
  WRITEABLE : False
  ALIGNED : True
  WRITEBACKIFCOPY : False
  UPDATEIFCOPY : False

x[0]=3
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [107], in <cell line: 1>()
----> 1 x[0]=3

ValueError: assignment destination is read-only

The buffer is a bytestring. Python strings are not mutable, so the resulting array isn't either.

with bytearray

x=np.frombuffer(bytearray(b'\x01\x02'), dtype=np.uint8)

x.flags
Out[112]: 
  C_CONTIGUOUS : True
  F_CONTIGUOUS : True
  OWNDATA : False
  WRITEABLE : True
  ALIGNED : True
  WRITEBACKIFCOPY : False
  UPDATEIFCOPY : False

x[0]=3

x
Out[114]: array([3, 2], dtype=uint8)

interesting, if you do the same with a bytearray instead, it is writable and you can see the changes in the original bytearray

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How to make numpy array mutable?

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