When the sys module is initialized in _PySys_Init(), it
creates file objects for stdin, stdout and stderr with
PyFile_FromFile() which calls fill_file_fields() to
initialize the file object. At the end of
initialization dircheck() is called, to ensure that the
file is not a directory. If the underlaying file
descriptors for stdin, stdout, stderr (0, 1, 2) are
closed by the parent process, dircheck() fails at least
on AIX. I don't know exactly why this fails but the
following expression in dircheck() 

if (fstat(fileno(f->f_fp), &buf) == 0 &&
S_ISDIR(buf.st_mode))

evaluates to True for stdin which results in setting an
exception and returning NULL to _PySys_Init().

The applied patch changes dircheck() to return without
further checks if the file object is for stdin, stdout
or stderr.

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This patch is not acceptable in this form. What you describe
seems to point to a clear bug in AIX, and, as other AIX
users have not reported that as a problem, appears to be
specific to your installation. Maybe there is a problem with
the compiler installation somehow?

If you want to investigate this further, please try to
reproduce the stat/ISDIR problem in a small stand-alone C
program. If that succeeds, please generate an
strace/ktrace/truss output for that binary.

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Closing due to lack of response and improbability of validity.