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Python Writing Binary Files in Python 3.x

  1. Dec 14, 2015 #1

    I've been trying to write a simple binary file in Python 3 and have not yet been able to find a clear answer / solution online.

    Could you please suggest a simple way to read / write binary files in Python 3?

    My Code:

    Code (Text):

    # Create Binary File

    def dec_base(k,b=2):
        """ Default base is 2 """
        num = ""
        while k:
            num +=str(k%b)
            k = k//2
        return int(num)

    test_list = []
    for i in range(10):
    with open("test.bin", "wb") as f:
        for i in test_list:

    Any help is appreciated.
  2. jcsd
  3. Dec 14, 2015 #2
    You have misunderstood the builtin bin function. Print its output to the console to see what it does. If you must write ints to a binary file, you wil need to choose an encoding. Use the struct module.
  4. Dec 15, 2015 #3


    Staff: Mentor

    The following code creates and initializes a list object (numList), and then packs it into a struct named buf. The code writes the struct to a file opened in binary mode, and closes the file.

    After that, the code reopens the file, reads the contents, and unpacks the contents into a list. After printing the list, the code closes the file again.

    In the pack and unpack operations, the "10i" string is a format string used to pack or unpack the 10 integer values.
    Code (Python):

    import struct

    numList = []
    with open("test.bin", "wb") as f:
       for i in range(10):

       buf = struct.pack( "10i",  *numList)

    with open("test.bin", "rb") as f:
       list = struct.unpack("10i", buf)
    Last edited: Dec 15, 2015
  5. Dec 15, 2015 #4
    Note that calling your list 'list' will mask the builtin type. Also note that explicit closing of the file is unnecessary within the with context manager. The file object's __exit__ method will close the file.
  6. Dec 15, 2015 #5


    Staff: Mentor

    Which I didn't intend to do. I have edited my code to use a different variable name.
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