This is an issue with Python 2 too which I hope can be fixed too. The original logic in the code was committed around 16 years back : https://github.com/python/cpython/commit/bbc0568a5c7d3849a22c78d545823a4b952c0933 and tests are also around 10 years old too.

➜  cpython git:(2bea771609) ✗ ./python.exe
Python 2.7.15+ (remotes/upstream/2.7:2bea771609, Jul 30 2018, 18:07:51)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
➜  cpython git:(2bea771609) ✗ ./python.exe bpo34276.py
file://root/a [SplitResult(scheme='file', netloc='root', path='/a', query='', fragment='')]
file://root/a
ROUNDTRIP: True

file:///root/a [SplitResult(scheme='file', netloc='', path='/root/a', query='', fragment='')]
file:///root/a
ROUNDTRIP: True

file:////root/a [SplitResult(scheme='file', netloc='', path='//root/a', query='', fragment='')]
file://root/a
ROUNDTRIP: False

file://///root/a [SplitResult(scheme='file', netloc='', path='///root/a', query='', fragment='')]
file:///root/a
ROUNDTRIP: False


Thanks