I don't think this is in scope for Click. You would check in whatever way you would check in Python in general, by examining some theoretical property of the file object. In most cases, the name of the file will be the special value <stdin>, but I can't say that's guaranteed given anything can replace sys.stdin, such as PDB, PyDev, pytest, IDLE, etc. All you get from File is an open stream, and I'd suggest that that it shouldn't matter where it's pointing at that point.

Just chiming in to add that another property of a file that you can inspect to check whether it probably corresponds to the process’s original stdin is its .fileno() value. By convention, the three file descriptors 0, 1, and 2 represent standard input, standard output, and standard error, respectively. I think this should work even if something like pdb, pytest, etc. reassigns the corresponding sys attributes. However, it will not work if something dup’s and then closes the original stdin, but maybe that’s a feature and not a bug depending on what you’re trying to do.