ProxySchemeUnknown: Not supported proxy scheme localhost

[blueyed]

Reported in 2014 already, new issue due to stale bot.. \o/

Using --proxy localhost:3128/ crashes due to the trailing slash and no scheme:

% pip --proxy localhost:3128/ install example
Looking in indexes: http://localhost:3141/root/pypi/+simple/
Collecting example
ERROR: Exception:
Traceback (most recent call last):
  File "…/Vcs/pip/src/pip/_internal/cli/base_command.py", line 193, in main
    status = self.run(options, args)
  File "…/Vcs/pip/src/pip/_internal/commands/install.py", line 348, in run
    resolver.resolve(requirement_set)
  File "…/Vcs/pip/src/pip/_internal/legacy_resolve.py", line 191, in resolve
    self._resolve_one(requirement_set, req)
  File "…/Vcs/pip/src/pip/_internal/legacy_resolve.py", line 354, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "…/Vcs/pip/src/pip/_internal/legacy_resolve.py", line 302, in _get_abstract_dist_for
    self.require_hashes
  File "…/Vcs/pip/src/pip/_internal/operations/prepare.py", line 282, in prepare_linked_requirement
    req.populate_link(finder, upgrade_allowed, require_hashes)
  File "…/Vcs/pip/src/pip/_internal/req/req_install.py", line 198, in populate_link
    self.link = finder.find_requirement(self, upgrade)
  File "…/Vcs/pip/src/pip/_internal/index.py", line 978, in find_requirement
    candidates = self.find_candidates(req.name, req.specifier)
  File "…/Vcs/pip/src/pip/_internal/index.py", line 965, in find_candidates
    candidates = self.find_all_candidates(project_name)
  File "…/Vcs/pip/src/pip/_internal/index.py", line 930, in find_all_candidates
    for page in self._get_pages(url_locations, project_name):
  File "…/Vcs/pip/src/pip/_internal/index.py", line 1058, in _get_pages
    page = _get_html_page(location, session=self.session)
  File "…/Vcs/pip/src/pip/_internal/index.py", line 231, in _get_html_page
    resp = _get_html_response(url, session=session)
  File "…/Vcs/pip/src/pip/_internal/index.py", line 179, in _get_html_response
    "Cache-Control": "max-age=0",
  File "…/Vcs/pip/src/pip/_vendor/requests/sessions.py", line 546, in get
    return self.request('GET', url, **kwargs)
  File "…/Vcs/pip/src/pip/_internal/download.py", line 607, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "…/Vcs/pip/src/pip/_vendor/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "…/Vcs/pip/src/pip/_vendor/requests/sessions.py", line 646, in send
    r = adapter.send(request, **kwargs)
  File "…/Vcs/pip/src/pip/_vendor/requests/adapters.py", line 412, in send
    conn = self.get_connection(request.url, proxies)
  File "…/Vcs/pip/src/pip/_vendor/requests/adapters.py", line 309, in get_connection
    proxy_manager = self.proxy_manager_for(proxy)
  File "…/Vcs/pip/src/pip/_vendor/requests/adapters.py", line 199, in proxy_manager_for
    **proxy_kwargs)
  File "…/Vcs/pip/src/pip/_vendor/urllib3/poolmanager.py", line 450, in proxy_from_url
    return ProxyManager(proxy_url=url, **kw)
  File "…/Vcs/pip/src/pip/_vendor/urllib3/poolmanager.py", line 401, in __init__
    raise ProxySchemeUnknown(proxy.scheme)
pip._vendor.urllib3.exceptions.ProxySchemeUnknown: Not supported proxy scheme localhost

Environment

• pip version: pip 19.1.1 (master)
• Python version: 3.7
• OS: Linux

#1740 has more details.

[blueyed]

/cc @dstufft since you closed it two times in #1740.

[deveshks]

I can reproduce this issue with pip 20.1.dev0

$ pip --proxy localhost:3128/ install example
ERROR: Exception:
Traceback (most recent call last):
  File "/Users/devesh/pip/src/pip/_internal/cli/base_command.py", line 199, in _main
    status = self.run(options, args)
  File "/Users/devesh/pip/src/pip/_internal/cli/req_command.py", line 185, in wrapper
    return func(self, options, args)
  File "/Users/devesh/pip/src/pip/_internal/commands/install.py", line 332, in run
    requirement_set = resolver.resolve(
  File "/Users/devesh/pip/src/pip/_internal/resolution/legacy/resolver.py", line 179, in resolve
    discovered_reqs.extend(self._resolve_one(requirement_set, req))
  File "/Users/devesh/pip/src/pip/_internal/resolution/legacy/resolver.py", line 362, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/Users/devesh/pip/src/pip/_internal/resolution/legacy/resolver.py", line 313, in _get_abstract_dist_for
    self._populate_link(req)
  File "/Users/devesh/pip/src/pip/_internal/resolution/legacy/resolver.py", line 279, in _populate_link
    req.link = self.finder.find_requirement(req, upgrade)
  File "/Users/devesh/pip/src/pip/_internal/index/package_finder.py", line 900, in find_requirement
    best_candidate_result = self.find_best_candidate(
  File "/Users/devesh/pip/src/pip/_internal/index/package_finder.py", line 883, in find_best_candidate
    candidates = self.find_all_candidates(project_name)
  File "/Users/devesh/pip/src/pip/_internal/index/package_finder.py", line 827, in find_all_candidates
    package_links = self.process_project_url(
  File "/Users/devesh/pip/src/pip/_internal/index/package_finder.py", line 792, in process_project_url
    html_page = self._link_collector.fetch_page(project_url)
  File "/Users/devesh/pip/src/pip/_internal/index/collector.py", line 612, in fetch_page
    return _get_html_page(location, session=self.session)
  File "/Users/devesh/pip/src/pip/_internal/index/collector.py", line 451, in _get_html_page
    resp = _get_html_response(url, session=session)
  File "/Users/devesh/pip/src/pip/_internal/index/collector.py", line 149, in _get_html_response
    resp = session.get(
  File "/Users/devesh/pip/src/pip/_vendor/requests/sessions.py", line 543, in get
    return self.request('GET', url, **kwargs)
  File "/Users/devesh/pip/src/pip/_internal/network/session.py", line 421, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "/Users/devesh/pip/src/pip/_vendor/requests/sessions.py", line 530, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/devesh/pip/src/pip/_vendor/requests/sessions.py", line 643, in send
    r = adapter.send(request, **kwargs)
  File "/Users/devesh/pip/src/pip/_vendor/cachecontrol/adapter.py", line 53, in send
    resp = super(CacheControlAdapter, self).send(request, **kw)
  File "/Users/devesh/pip/src/pip/_vendor/requests/adapters.py", line 412, in send
    conn = self.get_connection(request.url, proxies)
  File "/Users/devesh/pip/src/pip/_vendor/requests/adapters.py", line 309, in get_connection
    proxy_manager = self.proxy_manager_for(proxy)
  File "/Users/devesh/pip/src/pip/_vendor/requests/adapters.py", line 193, in proxy_manager_for
    manager = self.proxy_manager[proxy] = proxy_from_url(
  File "/Users/devesh/pip/src/pip/_vendor/urllib3/poolmanager.py", line 470, in proxy_from_url
    return ProxyManager(proxy_url=url, **kw)
  File "/Users/devesh/pip/src/pip/_vendor/urllib3/poolmanager.py", line 420, in __init__
    raise ProxySchemeUnknown(proxy.scheme)
pip._vendor.urllib3.exceptions.ProxySchemeUnknown: Not supported proxy scheme localhost

Also the requests doc on proxies states that Note that proxy URLs must include the scheme.

So although the URL localhost:3128/ has an interpreted scheme localhost

>>> u = urllib3.util.parse_url('localhost:3128/')
>>> u
Url(scheme='localhost', auth=None, host=None, port=None, path='/3128/', query=None, fragment=None)

It is not a valid scheme.

https://github.com/urllib3/urllib3/blob/1f0825bfe29d50f361e849a49ad1e394886b7773/src/urllib3/poolmanager.py#L451-L454

Considering these facts, we can close this issue since localhost actually represents a host, and can never be considered as a valid scheme.