python sys.stdout and C++ iostreams::cout

问题

I was assuming that sys.stdout would be referencing the same physical stream as iostreams::cout running in the same process, but this doesn't seem to be the case. The following code, which makes a call to a C++ function with a python wrapper called "write", that writes to cout:

from cStringIO import StringIO 
import sys 
orig_stdout = sys.stdout 
sys.stdout = stringout = StringIO() 
write("cout") # wrapped C++ function that writes to cout 
print "-" * 40 
print "stdout" 
sys.stdout = orig_stdout 
print stringout.getvalue() 

immediately writes "cout" to the console, then the separator "---...", and finally, as the return value of stringout.getvalue(), the string "stdout". My intention was to capture in stringout also the string written to cout from C++. Does anyone know what is going on, and if so, how I can capture what is written to cout in a python string?

Thanks in advance.

回答1:

sys.stdout is a Python object that writes to standard output. It is not actually the standard output file handle; it wraps that file handle. Altering the object that sys.stdout points to in Python-land does not in any way affect the stdout handle or the std::cout stream object in C++.

回答2:

With help from comp.lang.python and after some searching on this site: As cdhowie pointed out, the standard output file handle has to be accessed at a lower level. In fact, its file descriptor can be obtained as sys.stdout.fileno() (which should be 1), and then os.dup and os.dup2 can be used.

I found this answer to a similar question very helpful.

What I really wanted was to capture the output in a string, not a file. The python StringIO class however doesn't have a file descriptor and cannot be used in place of an actual file, so I came up with the not fully satisfactory workaround in which a temporary file is written and subsequently read.

回答3:

It cannot possibly be the same stream, as Python is written in C, and not C++, and has no access to std::cout. Whether it uses stdout or implements its own stream based on fd 1, I don't know, but in any case, you'd be advised to flush between writes using the two objects (Python and C++).

来源：https://stackoverflow.com/questions/14382772/python-sys-stdout-and-c-iostreamscout