Using pip fails with the same error messages independent of version an user. Also su or sudo -H doesn't change anything: How to Reproduce install python 3.x on your Mac and try pip After a quite long search I found that it is a problem with the proxy config. You need to remove all settings in network settings/proxy even those defining which domains shouldn't use a proxy. Proxy settings work fine with everything except python. xcode-select install ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" brew install pyenv pyenv install 3.7.3 pip list --outdated or pip install something Exception xxx-MBP:~ xxx$ pip list --outdated Exception: Traceback (most recent call last): File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 179, in main status = self.run(options, args) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/commands/list.py", line 144, in run packages = self.get_outdated(packages, options) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/commands/list.py", line 152, in get_outdated dist for dist in self.iter_packages_latest_infos(packages, options) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/commands/list.py", line 152, in <listcomp> dist for dist in self.iter_packages_latest_infos(packages, options) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/commands/list.py", line 179, in iter_packages_latest_infos all_candidates = finder.find_all_candidates(dist.key) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/index.py", line 610, in find_all_candidates for page in self._get_pages(url_locations, project_name): File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/index.py", line 743, in _get_pages page = _get_html_page(location, session=self.session) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/index.py", line 229, in _get_html_page resp = _get_html_response(url, session=session) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/index.py", line 177, in _get_html_response "Cache-Control": "max-age=0", File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_vendor/requests/sessions.py", line 546, in get return self.request('GET', url, **kwargs) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_internal/download.py", line 403, in request return super(PipSession, self).request(method, url, *args, **kwargs) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_vendor/requests/sessions.py", line 524, in request prep.url, proxies, stream, verify, cert File "/Users/xxxx.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_vendor/requests/sessions.py", line 700, in merge_environment_settings env_proxies = get_environ_proxies(url, no_proxy=no_proxy) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_vendor/requests/utils.py", line 761, in get_environ_proxies if should_bypass_proxies(url, no_proxy=no_proxy): File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip/_vendor/requests/utils.py", line 745, in should_bypass_proxies bypass = proxy_bypass(parsed.hostname) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/urllib/request.py", line 2610, in proxy_bypass return proxy_bypass_macosx_sysconf(host) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/urllib/request.py", line 2587, in proxy_bypass_macosx_sysconf return _proxy_bypass_macosx_sysconf(host, proxy_settings) File "/Users/xxx/.pyenv/versions/3.7.3/lib/python3.7/urllib/request.py", line 2573, in _proxy_bypass_macosx_sysconf if (hostIP >> mask) == (base >> mask): ValueError: negative shift count

I don't quite understand what the issue is. * What proxy settings do you have? * Is the problem reproducible without using pip? - By using the requests API to fetch a web page - By using the stdlib http.client API to do the same

The error is due to invalid proxy settings on the system. I can reproduce the error by enabling a HTTP proxy in system settings, while adding "172.19.0.0/40" to the proxy exclude list in the HTTP proxy settings.

I've tested the same settings with Safari, and that browser ignores excludes with an invalid prefix length. I've created a PR that does the same for urllib.request.

 New changeset 93a1ccabdede416425473329b8c718d507c55e29 by Ronald Oussoren in branch 'master': bpo-41471: Ignore invalid prefix lengths in system proxy settings on macOS (GH-22762) https://github.com/python/cpython/commit/93a1ccabdede416425473329b8c718d507c55e29 

 New changeset aa7b03b31bf09b21a012b24e21977538e66b0172 by Miss Skeleton (bot) in branch '3.9': bpo-41471: Ignore invalid prefix lengths in system proxy settings on macOS (GH-22762) (GH-22773) https://github.com/python/cpython/commit/aa7b03b31bf09b21a012b24e21977538e66b0172 

 New changeset e7c5a43984f82ef9634cb0b2b8c4750b2fd431a0 by Miss Skeleton (bot) in branch '3.8': bpo-41471: Ignore invalid prefix lengths in system proxy settings on macOS (GH-22762) (GH-22774) https://github.com/python/cpython/commit/e7c5a43984f82ef9634cb0b2b8c4750b2fd431a0