# Buggy code
def install_req_from_req_string(
    req_string,  # type: str
    comes_from=None,  # type: Optional[InstallRequirement]
    isolated=False,  # type: bool
    wheel_cache=None,  # type: Optional[WheelCache]
    use_pep517=None,  # type: Optional[bool]
):
    # type: (...) -> InstallRequirement
    try:
        req = Requirement(req_string)
    except InvalidRequirement:
        raise InstallationError("Invalid requirement: '%s'" % req)

    domains_not_allowed = [
        PyPI.file_storage_domain,
        TestPyPI.file_storage_domain,
    ]
    if req.url and comes_from.link.netloc in domains_not_allowed:
        # Explicitly disallow pypi packages that depend on external urls
        raise InstallationError(
            "Packages installed from PyPI cannot depend on packages "
            "which are not also hosted on PyPI.\n"
            "%s depends on %s " % (comes_from.name, req)
        )

    return InstallRequirement(
        req,
        comes_from,
        isolated=isolated,
        wheel_cache=wheel_cache,
        use_pep517=use_pep517,
    )


# Traceback (AttributeError)
$ pip install -e .
Obtaining file:///nas/home/broot/Programs/tools/catutils
req: shapely
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
req: netCDF4
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
req: aershp>=0.10
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
req: numpy!=1.10,!=1.11.0
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
req: scipy
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
req: GDAL
comes_from: catutils==0.15.dev1 from file:///nas/home/broot/Programs/tools/catutils
Requirement already satisfied: shapely in /rd22/scratch/broot/miniconda/envs/py3k/lib/python3.6/site-packages (from catutils==0.15.dev1) (1.6.4.post1)
Requirement already satisfied: netCDF4 in /rd22/scratch/broot/miniconda/envs/py3k/lib/python3.6/site-packages (from catutils==0.15.dev1) (1.4.1)
req: numpy>=1.7
comes_from: netCDF4 in /rd22/scratch/broot/miniconda/envs/py3k/lib/python3.6/site-packages (from catutils==0.15.dev1)
req: cftime
comes_from: netCDF4 in /rd22/scratch/broot/miniconda/envs/py3k/lib/python3.6/site-packages (from catutils==0.15.dev1)
Requirement already satisfied: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1) (0.16.dev1)
req: nose
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: numpy
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: matplotlib<2.2.0
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: Shapely
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: GDAL
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: pyyaml
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: jinja2
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: scripttest
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: netCDF4
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: six
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
req: atomic_context@ git+ssh://git@***********.com/common/atomic_context.git@release
comes_from: aershp>=0.10 in /nas/home/broot/Programs/tools/aershp/lib (from catutils==0.15.dev1)
Exception:
Traceback (most recent call last):
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 143, in main
status = self.run(options, args)
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 318, in run
resolver.resolve(requirement_set)
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/resolve.py", line 102, in resolve
self._resolve_one(requirement_set, req)
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/resolve.py", line 318, in _resolve_one
add_req(subreq, extras_requested=available_requested)
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/resolve.py", line 275, in add_req
wheel_cache=self.wheel_cache,
File "/home/broot/scratch/miniconda/envs/py3k/lib/python3.6/site-packages/pip/_internal/req/constructors.py", line 290, in install_req_from_req
if req.url and comes_from.link.netloc in domains_not_allowed:
AttributeError: 'NoneType' object has no attribute 'netloc'

# Fixed code
def install_req_from_req_string(
    req_string,  # type: str
    comes_from=None,  # type: Optional[InstallRequirement]
    isolated=False,  # type: bool
    wheel_cache=None,  # type: Optional[WheelCache]
    use_pep517=None,  # type: Optional[bool]
):
    # type: (...) -> InstallRequirement
    try:
        req = Requirement(req_string)
    except InvalidRequirement:
        raise InstallationError("Invalid requirement: '%s'" % req)

    domains_not_allowed = [
        PyPI.file_storage_domain,
        TestPyPI.file_storage_domain,
    ]
    if (
        req.url
        and comes_from
        and comes_from.link
        and comes_from.link.netloc in domains_not_allowed
    ):
        # Explicitly disallow pypi packages that depend on external urls
        raise InstallationError(
            "Packages installed from PyPI cannot depend on packages "
            "which are not also hosted on PyPI.\n"
            "%s depends on %s " % (comes_from.name, req)
        )

    return InstallRequirement(
        req,
        comes_from,
        isolated=isolated,
        wheel_cache=wheel_cache,
        use_pep517=use_pep517,
    )