Improve the error message when reading a bad requirements file.

news/6527.bugfix

@@ -0,0 +1,2 @@
+Add the line number and file location to the error message when reading an
+invalid requirements file in certain situations.

src/pip/_internal/req/constructors.py

@@ -205,11 +205,15 @@ def install_req_from_line(
     isolated=False,  # type: bool
     options=None,  # type: Optional[Dict[str, Any]]
     wheel_cache=None,  # type: Optional[WheelCache]
-    constraint=False  # type: bool
+    constraint=False,  # type: bool
+    line_source=None,  # type: Optional[str]
 ):
     # type: (...) -> InstallRequirement
     """Creates an InstallRequirement from a name, which might be a
     requirement, directory containing 'setup.py', filename, or URL.
+
+    :param line_source: An optional string describing where the line is from,
+        for logging purposes in case of an error.
     """
     if is_url(name):
         marker_sep = '; '
@@ -289,10 +293,17 @@ def install_req_from_line(
                   not any(op in req_as_string for op in operators)):
                 add_msg = "= is not a valid operator. Did you mean == ?"
             else:
-                add_msg = ""
-            raise InstallationError(
-                "Invalid requirement: '%s'\n%s" % (req_as_string, add_msg)
+                add_msg = ''
+            if line_source is None:
+                source = ''
+            else:
+                source = ' (from {})'.format(line_source)
+            msg = (
+                'Invalid requirement: {!r}{}'.format(req_as_string, source)
             )
+            if add_msg:
+                msg += '\nHint: {}'.format(add_msg)
+            raise InstallationError(msg)
     else:
         req = None
 

src/pip/_internal/req/req_file.py

@@ -138,7 +138,7 @@ def process_line(
     session=None,  # type: Optional[PipSession]
     wheel_cache=None,  # type: Optional[WheelCache]
     use_pep517=None,  # type: Optional[bool]
-    constraint=False  # type: bool
+    constraint=False,  # type: bool
 ):
     # type: (...) -> Iterator[InstallRequirement]
     """Process a single requirements line; This can result in creating/yielding
@@ -187,10 +187,16 @@ def process_line(
         for dest in SUPPORTED_OPTIONS_REQ_DEST:
             if dest in opts.__dict__ and opts.__dict__[dest]:
                 req_options[dest] = opts.__dict__[dest]
+        line_source = 'line {} of {}'.format(line_number, filename)
         yield install_req_from_line(
-            args_str, line_comes_from, constraint=constraint,
+            args_str,
+            comes_from=line_comes_from,
             use_pep517=use_pep517,
-            isolated=isolated, options=req_options, wheel_cache=wheel_cache
+            isolated=isolated,
+            options=req_options,
+            wheel_cache=wheel_cache,
+            constraint=constraint,
+            line_source=line_source,
         )
 
     # yield an editable requirement

tests/unit/test_req.py

@@ -530,7 +530,7 @@ def test_unidentifiable_name(self):
         with pytest.raises(InstallationError) as e:
             install_req_from_line(test_name)
         err_msg = e.value.args[0]
-        assert ("Invalid requirement: '%s'\n" % test_name) == err_msg
+        assert "Invalid requirement: '{}'".format(test_name) == err_msg
 
     def test_requirement_file(self):
         req_file_path = os.path.join(self.tempdir, 'test.txt')

tests/unit/test_req_file.py

@@ -190,6 +190,26 @@ def test_only_one_req_per_line(self):
         with pytest.raises(InstallationError):
             list(process_line("req1 req2", "file", 1))
 
+    def test_error_message(self):
+        """
+        Test the error message if a parsing error occurs (all of path,
+        line number, and hint).
+        """
+        iterator = process_line(
+            'my-package=1.0',
+            filename='path/requirements.txt',
+            line_number=3
+        )
+        with pytest.raises(InstallationError) as exc:
+            list(iterator)
+
+        expected = (
+            "Invalid requirement: 'my-package=1.0' "
+            '(from line 3 of path/requirements.txt)\n'
+            'Hint: = is not a valid operator. Did you mean == ?'
+        )
+        assert str(exc.value) == expected
+
     def test_yield_line_requirement(self):
         line = 'SomeProject'
         filename = 'filename'