Builtin open() in Python 3 (and io.open() in Python 2.7) accept unicode strings, byte strings and integers as the first arguments (general path-like objects also are supported in Python 3.6, but it doesn't matter). bool is a subtype of int, and False is equal to integer 0. 0 is a file descriptor of sys.stdin.

>>> import sys
>>> sys.stdin.fileno()
0

Thus open(False) is equivalent to open(sys.stdin.fileno()). It creates a file object that wraps the file descriptor 0. This is legitimate operation.