Standard input is simply data going into a program that the program can read when it runs.

Standard output is the output of a program, written there by the program as it is running.

Sys.stdin is a readable file object.

In [3]: sys.stdin
Out[3]: <open file '<stdin>', mode 'r' at 0x14020>

sys.stdout is a writable file object. And just as sys.stdin has the same type as a readable file object, so sys.stdout has the same type as a writable file object