fix: expose markdown source as __file__

Author: tgolob

README.md

@@ -120,6 +120,20 @@ assert captured.out == "hello\n"
 
 As you can see above, the fixture value will be injected as a global. For `autouse=True` fixtures, the value is only injected as a global if it's explicitly added using a `fixture:<name>` marker.
 
+### The source file path
+
+Code fences receive a `__file__` global containing the path of the Markdown or
+Python file where the fence was found. This makes it possible to load fixtures
+stored next to a document:
+
+````markdown
+```python
+from pathlib import Path
+
+fixture = Path(__file__).with_name("fixture.json").read_text()
+```
+````
+
 ### Async Fixtures (pytest-asyncio)
 
 If you have [pytest-asyncio](https://pypi.org/project/pytest-asyncio/) installed, you can use async fixtures with your markdown tests. The async fixture will be executed and its resolved value will be injected into the test code:

src/pytest_markdown_docs/plugin.py

@@ -95,6 +95,7 @@ def runtest(self):
 
         mod = types.ModuleType("fence")  # dummy module
         all_globals = mod.__dict__
+        all_globals["__file__"] = str(self.test_definition.source_path)
         for global_set in global_sets:
             all_globals.update(global_set)
 

tests/plugin_test.py

@@ -85,6 +85,24 @@ def pytest_markdown_docs_globals():
     result.assert_outcomes(passed=1, failed=2)
 
 
+def test_file_global_points_to_markdown_source(testdir):
+    testdir.makefile(".txt", neighbor="from neighbor\n")
+    testdir.makefile(
+        ".md",
+        test_file="""
+```python
+from pathlib import Path
+
+assert Path(__file__).name == "test_file.md"
+assert Path(__file__).with_name("neighbor.txt").read_text().strip() == "from neighbor"
+```
+""",
+    )
+
+    result = testdir.runpytest("--markdown-docs")
+    result.assert_outcomes(passed=1)
+
+
 def test_continuation(testdir):
     testdir.makefile(
         ".md",