what is the difference between os.open and os.fdopen in python

You are confusing the built-in open() function with os.open() provided by the os module. They are quite different; os.open(filename, "w") is not valid Python (os.open accepts integer flags as its second argument), open(filename, "w") is.

In short, open() creates new file objects, os.open() creates OS-level file descriptors, and os.fdopen() creates a file object out of a file descriptor.

File descriptors are a low-level facility for working with files directly provided by the operating system kernel. A file descriptor is a small integer that identifies the open file in a table of open files kept by the kernel for each process. A number of system calls accept file descriptors, but they are not convenient to work with, typically requiring fixed-width buffers, multiple retries in certain conditions, and manual error handling.

File objects are Python classes that wrap file descriptors to make working with files more convenient and less error-prone. They provide, for example, error-handling, buffering, line-by-line reading, charset conversions, and are closed when garbage collected.

To recapitulate:

• Built-in open() takes a file name and returns a new Python file object. This is what you need in the majority of cases.

• os.open() takes a file name and returns a new file descriptor. This file descriptor can be passed to other low-level functions, such as os.read() and os.write(), or to os.fdopen(), as described below. You only need this when writing code that depends on operating-system-dependent APIs, such as using the O_EXCL flag to open(2).

• os.fdopen() takes an existing file descriptor — typically produced by Unix system calls such as pipe() or dup(), and builds a Python file object around it. Effectively it converts a file descriptor to a full file object, which is useful when interfacing with C code or with APIs that only create low-level file descriptors.

Built-in open can be emulated by combining os.open() (to create a file descriptor) and os.fdopen() (to wrap it in a file object):

# functionally equivalent to open(filename, "r")
f = os.fdopen(os.open(filename, os.O_RDONLY))