Copying data into a buffer (byte array)

Question

I am trying to copy a blob of data (some bytes) into a larger block at some position. I can do this easily enough in C, but now I am doing it in Python and I am curious as to what is the best/correct way to do this.

The way I did it was:

struct.pack_into("p", buffer, pos, str(data))

Where data and buffer are of type bytearray. Python would not let me copy the data into the buffer without converting it to a string (see the type conversion above), so I was wondering what is the correct way to insert one bytearray into another?

Martijn Pieters · Accepted Answer · 2019-02-06 17:53:29Z

4

bytearray objects are mutable sequences, you can copy the contents of one into another at a given position by assigning to a slice:

buffer[pos:pos + len(data)] = data

There is no need or use for struct.pack_into() here. Note that data can be any iterable of integers, provided they fall in the range 0-255; it doesn't have to be a bytes or bytearray object.

Demo:

>>> buffer = bytearray(10)
>>> data = bytes.fromhex('deadbeef')
>>> pos = 3
>>> buffer[pos:pos + len(data)] = data
>>> buffer
bytearray(b'\x00\x00\x00\xde\xad\xbe\xef\x00\x00\x00')

edited Feb 6, 2019 at 17:53

answered Feb 6, 2019 at 17:48

Martijn Pieters

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1 Comment

Xofo Over a year ago

Thanks! Good answer - The slice of bytearray buffer from pos to pos + len(data) is replaced by the contents of the iterable bytearray data.

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