Create binary file in python

Question

I have a very large binary file called file1.bin and I want to create a file, file2.bin, that holds only the first 32kb of file1.bin.

So I'm reading file1 as follows:

myArr = bytearray()

with open(r"C:\Users\User\file1.bin", "rb") as f:
byte = f.read(1)
for i in range(32,678):
    myArr.extend(byte)
    byte = f.read(1)

My question is: How do I proceed from here to create the file2 binary file out of myArr?

I tried

with open(r"C:\Users\User\file2.bin", "w") as f:
f.write(myArr)

but this results in:

f.write(myArr)
TypeError: must be string or pinned buffer, not bytearray

Why not read in larger chunks? And have you seen the shutil module? Last but not least, do use the ironpython tag as it might be relevant. — Martijn Pieters
– Martijn Pieters, Commented Feb 20, 2014 at 14:04
Just so you know, 0x32000 bytes is about 200 kB. Do you mean to write 32000 bytes instead? — Kevin
– Kevin, Commented Feb 20, 2014 at 14:05
You already knew to open the file for reading with 'rb', why did you open the file to write to in text mode instead? 'wb' would solve the exception you have in one go. — Martijn Pieters
– Martijn Pieters, Commented Feb 20, 2014 at 14:07
range(32,768) is [32, 33, 34, 35, ... 766, 767]. You really don't want that comma. — Hugh Bothwell
– Hugh Bothwell, Commented Feb 20, 2014 at 14:26

Joel Cornett · Accepted Answer · 2014-02-20 14:18:16Z

10

You need to open the file in binary write mode (wb).

with open('file2.bin', 'wb') as f:
    f.write(myArr)

Also, the way you are reading from the input file is pretty inefficient. f.read() allows you to read more than one byte at a time:

with open('file1.bin', 'rb') as f:
    myArr = bytearray(f.read(32678))

Will do exactly what you want.

answered Feb 20, 2014 at 14:18

Joel Cornett

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J. Katzwinkel · Accepted Answer · 2014-02-20 14:24:27Z

0

Open files with appropriate flags, then read from one in blocks of 1024 bytes and write to other, if less than 1024 bytes are remaining, copy byte-wise.

fin = open('file1.bin', 'rb')
fout = open('file2.bin', 'w+b')
while True:
  b=fin.read(1024)
  if b:
    n = fout.write(b)
  else:
    while True:
      b=fin.read(1)
      if b:
        n=fout.write(b)
      else:
        break
    break

answered Feb 20, 2014 at 14:24

J. Katzwinkel

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