Recipe2.12.Sending Binary Data to Standard Output Under Windows

Recipe 2.12. Sending Binary Data to Standard Output Under Windows

Credit: Hamish Lawson

Problem

You want to send binary data (e.g., an image) to stdout under Windows.

Solution

That's what the setmode function, in the platform-dependent (Windows-only) msvcrt module in the Python Standard Library, is for:

import sys if sys.platform == "win32":     import os, msvcrt     msvcrt.setmode(sys.stdout.fileno( ), os.O_BINARY)

You can now call sys.stdout.write with any bytestring as the argument, and the bytestring will go unmodified to standard output.

Discussion

While Unix doesn't make (or need) a distinction between text and binary modes, if you are reading or writing binary data, such as an image, under Windows, the file must be opened in binary mode. This is a problem for programs that write binary data to standard output (as a CGI script, for example, could be expected to do), because Python opens the sys.stdout file object on your behalf, normally in text mode.

You can have stdout opened in binary mode instead by supplying the -u command-line option to the Python interpreter. For example, if you know your CGI script will be running under the Apache web server, as the first line of your script, you can use something like:

#! c:/python23/python.exe -u

assuming you're running under Python 2.3 with a standard installation. Unfortunately, you may not always be able to control the command line under which your script will be started. The approach taken in this recipe's "Solution" offers a workable alternative. The setmode function provided by the Windows-specific msvcrt module lets you change the mode of stdout's underlying file descriptor. By using this function, you can ensure from within your program that sys.stdout gets set to binary mode.

See Also

Documentation for the msvcrt module in the Library Reference and Python in a Nutshell.