[python] Python, how to read bytes from file and save it?

I want to read bytes from a file and then write those bytes to another file, and save that file.

How do I do this?

Use the open function to open the file. The open function returns a file object, which you can use the read and write to files:

file_input = open('input.txt') #opens a file in reading mode
file_output = open('output.txt') #opens a file in writing mode

data = file_input.read(1024) #read 1024 bytes from the input file
file_output.write(data) #write the data to the output file

with open("input", "rb") as input:
    with open("output", "wb") as output:
        while True:
            data = input.read(1024)
            if data == "":
                break
            output.write(data)

The above will read 1 kilobyte at a time, and write it. You can support incredibly large files this way, as you won't need to read the entire file into memory.

In my examples I use the 'b' flag ('wb', 'rb') when opening the files because you said you wanted to read bytes. The 'b' flag tells Python not to interpret end-of-line characters which can differ between operating systems. If you are reading text, then omit the 'b' and use 'w' and 'r' respectively.

This reads the entire file in one chunk using the "simplest" Python code. The problem with this approach is that you could run out memory when reading a large file:

ifile = open(input_filename,'rb')
ofile = open(output_filename, 'wb')
ofile.write(ifile.read())
ofile.close()
ifile.close()

This example is refined to read 1MB chunks to ensure it works for files of any size without running out of memory:

ifile = open(input_filename,'rb')
ofile = open(output_filename, 'wb')
data = ifile.read(1024*1024)
while data:
    ofile.write(data)
    data = ifile.read(1024*1024)
ofile.close()
ifile.close()

This example is the same as above but leverages using with to create a context. The advantage of this approach is that the file is automatically closed when exiting the context:

with open(input_filename,'rb') as ifile:
    with open(output_filename, 'wb') as ofile:
        data = ifile.read(1024*1024)
        while data:
            ofile.write(data)
            data = ifile.read(1024*1024)

See the following:

• Python open(): http://docs.python.org/library/functions.html#open
• Python read(): http://docs.python.org/library/stdtypes.html#file.read
• Python write(): http://docs.python.org/library/stdtypes.html#file.write
• Dive into Python File Object Tutorial: http://diveintopython.net/file_handling/file_objects.html