Fix #1655 by using platform-correct path handling for relative imports

This fixes an issue which showed up on Windows in 0.17.x where
the VSCode Python extension passes paths which have their drive
letter normalised to lower case. Since paths are (mostly) case
insensitive on Windows, that shouldn't matter. However Jedi's
handling was `==` on `str`s, meaning that completions based on
relative imports on windows didn't work.

On master this same issue had started appearing due to the partial
migration to use `Path` instead of `str` (which will avoid such
issues entirely once complete), as `Path` instances were being
compared to `str` instances without first converting.

We fix both by clarifying which is used here and then doing the
proper conversions.

jedi/inference/imports.py

@@ -10,6 +10,7 @@
 """
 import os
 from pathlib import Path
+from typing import List, Optional, Tuple
 
 from parso.python import tree
 from parso.tree import search_ancestor
@@ -123,31 +124,35 @@ def _add_error(value, name, message):
         debug.warning('ImportError without origin: ' + message)
 
 
-def _level_to_base_import_path(project_path, directory, level):
+def _level_to_base_import_path(
+    project_path: Path,
+    directory: Path,
+    level: int,
+) -> Tuple[Optional[List[str]], Optional[str]]:
     """
     In case the level is outside of the currently known package (something like
     import .....foo), we can still try our best to help the user for
     completions.
     """
     for i in range(level - 1):
         old = directory
-        directory = os.path.dirname(directory)
+        directory = directory.parent
         if old == directory:
             return None, None
 
     d = directory
-    level_import_paths = []
+    level_import_paths: List[str] = []
     # Now that we are on the level that the user wants to be, calculate the
     # import path for it.
     while True:
         if d == project_path:
-            return level_import_paths, d
+            return level_import_paths, str(d)
         dir_name = os.path.basename(d)
         if dir_name:
             level_import_paths.insert(0, dir_name)
-            d = os.path.dirname(d)
+            d = d.parent
         else:
-            return None, directory
+            return None, str(directory)
 
 
 class Importer:
@@ -202,7 +207,7 @@ def __init__(self, inference_state, import_path, module_context, level=0):
                     directory = os.path.dirname(path)
 
                 base_import_path, base_directory = _level_to_base_import_path(
-                    project_path, directory, level,
+                    project_path, Path(directory), level,
                 )
                 if base_directory is None:
                     # Everything is lost, the relative import does point

test/examples/issue1655/subdir/file3.py

@@ -0,0 +1,2 @@
+def do_nothing_file3():
+    pass

test/test_inference/test_imports.py

@@ -393,6 +393,124 @@ def test_relative_imports_without_path(Script):
     assert [c.name for c in script.complete()] == ['api', 'import', 'whatever']
 
 
+def test_relative_import_using_paths(Script):
+    project_path = get_example_dir('issue1655')
+    project = Project(project_path, sys_path=[], smart_sys_path=False)
+    file_path = project_path / 'subdir' / 'other.py'
+
+    script = Script(
+        "from . ",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == ['file3', 'import']
+
+    script = Script(
+        "from . import ",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == [
+        'file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+    script = Script(
+        "from . import file3\n\nfile3.",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == [
+        'do_nothing_file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+
+def test_relative_import_using_strs(Script):
+    project_path = get_example_dir('issue1655')
+    project = Project(str(project_path), sys_path=[], smart_sys_path=False)
+    file_path = project_path / 'subdir' / 'other.py'
+
+    script = Script(
+        "from . ",
+        project=project,
+        path=str(file_path),
+    )
+    assert [c.name for c in script.complete()] == ['file3', 'import']
+
+    script = Script(
+        "from . import ",
+        project=project,
+        path=str(file_path),
+    )
+    assert [c.name for c in script.complete()] == [
+        'file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+    script = Script(
+        "from . import file3\n\nfile3.",
+        project=project,
+        path=str(file_path),
+    )
+    assert [c.name for c in script.complete()] == [
+        'do_nothing_file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+
+@pytest.mark.skipif("sys.platform!='win32'")
+def test_relative_import_case_insensitive_windows(Script):
+    project_path = get_example_dir('issue1655')
+    project = Project(str(project_path).lower(), sys_path=[], smart_sys_path=False)
+    file_path = project_path / 'subdir' / 'other.py'
+
+    script = Script(
+        "from . ",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == ['file3', 'import']
+
+    script = Script(
+        "from . import ",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == [
+        'file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+    script = Script(
+        "from . import file3\n\nfile3.",
+        project=project,
+        path=file_path,
+    )
+    assert [c.name for c in script.complete()] == [
+        'do_nothing_file3',
+        '__doc__',
+        '__file__',
+        '__name__',
+        '__package__',
+    ]
+
+
 def test_relative_import_out_of_file_system(Script):
     code = "from " + '.' * 100
     assert not Script(code).complete()