# Buggy code
def validate_specifiers(instance, attr_, value):
    if value == "":
        return True
    try:
        SpecifierSet(value)
    except (InvalidMarker, InvalidSpecifier):
        raise ValueError("Invalid Specifiers {0}".format(value))


# Traceback (EOFError)
Courtesy Notice: Pipenv found itself running within a virtual environment, so it will automatically use that environment, instead of creating its own for any project. You can set PIPENV_IGNORE_VIRTUALENVS=1 to force pipenv to ignore that environment and create its own instead.
Running $ pipenv lock then $ pipenv sync.
Locking [dev-packages] dependencies…
Using pip: -i https://pypi.python.org/simple --extra-index-url https://${PYPI_AUTH}@pypi.domain.io/pypi

ROUND 1
Current constraints:

Finding the best candidates:

Finding secondary dependencies:
------------------------------------------------------------
Result of round 1: stable, done

Locking [packages] dependencies…
Using pip: -i https://pypi.python.org/simple --extra-index-url https://${PYPI_AUTH}@pypi.domain.io/pypi

ROUND 1
Current constraints:
privatepackage

Finding the best candidates:
User for pypi.domain.io:
INFO:pip9._vendor.requests.packages.urllib3.connectionpool:Starting new HTTPS connection (1): pypi.org
INFO:pip9._vendor.requests.packages.urllib3.connectionpool:Starting new HTTPS connection (1): pypi.domain.io
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/pipenv/resolver.py", line 85, in <module>
main()
File "/usr/local/lib/python3.6/site-packages/pipenv/resolver.py", line 74, in main
system=system,
File "/usr/local/lib/python3.6/site-packages/pipenv/resolver.py", line 65, in resolve
allow_global=system,
File "/usr/local/lib/python3.6/site-packages/pipenv/utils.py", line 442, in resolve_deps
pre,
File "/usr/local/lib/python3.6/site-packages/pipenv/utils.py", line 352, in actually_resolve_reps
resolved_tree.update(resolver.resolve(max_rounds=PIPENV_MAX_ROUNDS))
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/resolver.py", line 102, in resolve
has_changed, best_matches = self._resolve_one_round()
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/resolver.py", line 193, in _resolve_one_round
best_matches = {self.get_best_match(ireq) for ireq in constraints}
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/resolver.py", line 193, in <setcomp>
best_matches = {self.get_best_match(ireq) for ireq in constraints}
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/resolver.py", line 257, in get_best_match
best_match = self.repository.find_best_match(ireq, prereleases=self.prereleases)
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/repositories/pypi.py", line 157, in find_best_match
all_candidates = self.find_all_candidates(ireq.name)
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/piptools/repositories/pypi.py", line 142, in find_all_candidates
candidates = self.finder.find_all_candidates(req_name)
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/notpip/index.py", line 456, in find_all_candidates
for page in self._get_pages(url_locations, project_name):
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/notpip/index.py", line 606, in _get_pages
page = self._get_page(location)
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/notpip/index.py", line 723, in _get_page
return HTMLPage.get_page(link, session=self.session)
File "/usr/local/lib/python3.6/site-packages/pipenv/patched/notpip/index.py", line 832, in get_page
"Cache-Control": "max-age=600",
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/_vendor/requests/sessions.py", line 488, in get
return self.request('GET', url, **kwargs)
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/download.py", line 386, in request
return super(PipSession, self).request(method, url, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/_vendor/requests/sessions.py", line 475, in request
resp = self.send(prep, **send_kwargs)
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/_vendor/requests/sessions.py", line 602, in send
r = dispatch_hook('response', hooks, r, **kwargs)
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/_vendor/requests/hooks.py", line 31, in dispatch_hook
_hook_data = hook(hook_data, **kwargs)
File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/pip9/download.py", line 181, in handle_401
username = six.moves.input("User for %s: " % parsed.netloc)
EOFError: EOF when reading a line

# Fixed code
def validate_specifiers(instance, attr_, value):
    from packaging.specifiers import SpecifierSet, InvalidSpecifier

    if value == "":
        return True
    try:
        SpecifierSet(value)
    except (InvalidMarker, InvalidSpecifier):
        raise ValueError("Invalid Specifiers {0}".format(value))