Why doesn't os.normpath collapse a leading double slash?

Question

Under Unix, os.path.normpath collapses multiple slashes into single ones except when exactly two slashes appear that the start of the path. Why the exception?

To illustrate, I get the following transformations:

//double/slash/stays -> //double/slash/stays
/double/slash//gone// -> /double/slash/gone/
double//slash//gone/ -> double/slash/gone
///triple/slash/gone -> /triple/slash/gone
////quad/slash/gone -> /quad/slash/gone

This seems strange to me. I can vaguely imagine this is useful for SMB mounts or URLS, but I don't think I care about those. Is there any hidden wisdom to Python's behaviour, or should I just collapse the leading // myself?

[update] In view of the answer below, it looks like the best thing is not to collapse the //, but to either just accept it, or to treat it as an error.

Answer 1

Because POSIX allows treating a path beginning with two slashes in an implementation-defined manner. In other words, //foo does not necessarily mean the same thing as /foo on all POSIX systems.

From IEEE Std 1003.1:

A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.

See also this bug report (which was closed as invalid).