Add a coverage plugin that handles ipykernel cells

.gitignore

@@ -12,5 +12,6 @@ __pycache__/
 .ipynb_checkpoints/
 .pytest_cache/
 .coverage
+coverage.json
 
 /.python-version

Changes.md

@@ -2,6 +2,12 @@
 
 Note: development is tracked on the [`develop` branch](https://github.com/chmp/ipytest/tree/develop).
 
+## `0.14.1`
+
+- Add a [Coverage.py](https://coverage.readthedocs.io/en/latest/index.html)
+  plugin (`ipytest.cov`) to support collecting coverage information in
+  notebooks. See the Readme for usage notes.
+
 ## `0.14.0`
 
 - Removed support for Python 3.7 after it reached its end of life

Readme.md

@@ -96,6 +96,7 @@ this case, using [`ipytest.run()`][ipytest.run] and
 | [`clean`][ipytest.clean]
 | [`force_reload`][ipytest.force_reload]
 | [`Error`][ipytest.Error]
+| [`ipytest.cov`](#ipytestcov)
 
 <!-- minidoc "function": "ipytest.autoconfig", "header_depth": 3 -->
 ### `ipytest.autoconfig(rewrite_asserts=<default>, magics=<default>, clean=<default>, addopts=<default>, run_in_thread=<default>, defopts=<default>, display_columns=<default>, raise_on_error=<default>)`
@@ -304,6 +305,36 @@ Error raised by ipytest on test failure
 
 <!-- minidoc -->
 
+<!-- minidoc "module": "ipytest.cov", "header_depth": 3 -->
+### `ipytest.cov`
+
+A coverage.py plugin to support coverage in Jupyter notebooks
+
+The plugin must be enabled in a `.coveragerc` next to the current notebook or
+the `pyproject.toml` file. See the [coverage.py docs][coverage-py-config-docs]
+for details. In case of a `.coveragerc` file, the minimal configuration reads:
+
+```ini
+[run]
+plugins =
+    ipytest.cov
+```
+
+With this config file, the coverage can be collected using
+[pytest-cov][ipytest-cov-pytest-cov] with
+
+```pyhton
+%%ipytest --cov
+
+def test():
+    ...
+```
+
+[coverage-py-config-docs]: https://coverage.readthedocs.io/en/latest/config.html
+[ipytest-cov-pytest-cov]: https://pytest-cov.readthedocs.io/en/latest/config.html
+
+<!-- minidoc -->
+
 ## Development
 
 Setup a Python 3.10 virtual environment and install the requirements via
@@ -345,9 +376,11 @@ previous runs. These packages include:
 While PyTest itself is generally supported, support for PyTest plugins depends
 very much on the plugin. The following plugins are known to not work:
 
-- [pytest-cov](https://github.com/chmp/ipytest/issues/88)
 - [pytest-xdist](https://github.com/chmp/ipytest/issues/90)
 
+See [`ipytest.cov`](#ipytestcov) on how to use `ipytest` with
+[pytest-cov](https://pytest-cov.readthedocs.io/en/latest/config.html).
+
 Please create an issue, if I missed a packaged or mischaracterized any package.
 
 ## License

ipytest/_impl.py

@@ -274,7 +274,7 @@ def _fmt(arg):
     if defopts == "auto":
         defopts = eval_defopts_auto(all_args, arg_mapping)
 
-    return [*all_args, *([filename] if defopts else [])]
+    return [*all_args, *(["--", filename] if defopts else [])]
 
 
 class ArgMapping(dict):

ipytest/cov.py

@@ -0,0 +1,106 @@
+"""A coverage.py plugin to support coverage in Jupyter notebooks
+
+The plugin must be enabled in a `.coveragerc` next to the current notebook or
+the `pyproject.toml` file. See the [coverage.py docs][coverage-py-config-docs]
+for details. In case of a `.coveragerc` file, the minimal configuration reads:
+
+```ini
+[run]
+plugins =
+    ipytest.cov
+```
+
+With this config file, the coverage can be collected using
+[pytest-cov][ipytest-cov-pytest-cov] with
+
+```pyhton
+%%ipytest --cov
+
+def test():
+    ...
+```
+
+[coverage-py-config-docs]: https://coverage.readthedocs.io/en/latest/config.html
+[ipytest-cov-pytest-cov]: https://pytest-cov.readthedocs.io/en/latest/config.html
+"""
+import linecache
+import os
+import os.path
+import re
+
+import coverage.parser
+import coverage.plugin
+import coverage.python
+
+# prevent the definitions from being documented in the readme
+__all__ = []
+
+
+def coverage_init(reg, options):
+    reg.add_file_tracer(IPythonPlugin())
+
+
+class IPythonPlugin(coverage.plugin.CoveragePlugin):
+    def __init__(self):
+        self._filename_pattern = self._build_filename_pattern()
+
+    @classmethod
+    def _build_filename_pattern(self):
+        try:
+            import ipykernel.compiler
+
+        except ImportError:
+            return None
+
+        else:
+            return re.compile(
+                r"^"
+                + re.escape(ipykernel.compiler.get_tmp_directory())
+                + re.escape(os.sep)
+                + r"\d+.py"
+            )
+
+    def file_tracer(self, filename):
+        if not self._is_ipython_cell_file(filename):
+            return None
+
+        return IPythonFileTracer(filename)
+
+    def file_reporter(self, filename):
+        return IPythonFileReporter(filename)
+
+    def _is_ipython_cell_file(self, filename: str):
+        if self._filename_pattern is None:
+            return False
+
+        if os.path.exists(filename):
+            return False
+
+        if self._filename_pattern.match(filename) is None:
+            return False
+
+        return filename in linecache.cache
+
+
+class IPythonFileTracer(coverage.plugin.FileTracer):
+    def __init__(self, filename):
+        self._filename = filename
+
+    def source_filename(self):
+        return self._filename
+
+
+class IPythonFileReporter(coverage.python.PythonFileReporter):
+    @property
+    def parser(self):
+        if self._parser is None:
+            self._parser = coverage.parser.PythonParser(text=self.source())
+            self._parser.parse_source()
+
+        return self._parser
+
+    def source(self):
+        if self.filename not in linecache.cache:
+            raise ValueError()
+
+        return "".join(linecache.cache[self.filename][2])