Mark git filter required on installation

By [default](https://git-scm.com/docs/gitattributes#_filter), "A missing filter driver definition in the config, or a filter driver that exits with a non-zero status, is not an error but makes the filter a no-op passthru." i.e. if the `nbstripout` is misconfigured or fails, the filter is ignored.

Declare the filter to be `required` when installing, such that it *must* succeed.

Closes #191

README.md

@@ -100,11 +100,14 @@ Show help and usage instructions:
 
 ### Using as a Git filter
 
-Set up the git filter and attributes as described in the manual installation
-instructions below:
+Set up the [git filter](https://git-scm.com/docs/gitattributes#_filter) and
+attributes as described in the manual installation instructions below:
 
     nbstripout --install
 
+Note: The filter is declared as `required`, meaning the filter *must* succeed.
+Failures and misconfigurations will not simply cause the filter to be ignored.
+
 Set up the git filter using `.gitattributes`:
 
     nbstripout --install --attributes .gitattributes

nbstripout/_nbstripout.py

@@ -201,6 +201,7 @@ def install(git_config, install_location=INSTALL_LOCATION_LOCAL, python=None, at
         filepath = f'"{PureWindowsPath(python or sys.executable).as_posix()}" -m nbstripout'
         check_call(git_config + ['filter.nbstripout.clean', filepath])
         check_call(git_config + ['filter.nbstripout.smudge', 'cat'])
+        check_call(git_config + ['filter.nbstripout.required', 'true'])
         check_call(git_config + ['diff.ipynb.textconv', filepath + ' -t'])
         attrfile = _get_attrfile(git_config, install_location, attrfile)
     except FileNotFoundError:
@@ -251,6 +252,7 @@ def uninstall(git_config, install_location=INSTALL_LOCATION_LOCAL, attrfile=None
     try:
         call(git_config + ['--unset', 'filter.nbstripout.clean'], stdout=open(devnull, 'w'), stderr=STDOUT)
         call(git_config + ['--unset', 'filter.nbstripout.smudge'], stdout=open(devnull, 'w'), stderr=STDOUT)
+        call(git_config + ['--unset', 'filter.nbstripout.required'], stdout=open(devnull, 'w'), stderr=STDOUT)
         call(git_config + ['--remove-section', 'diff.ipynb'], stdout=open(devnull, 'w'), stderr=STDOUT)
         attrfile = _get_attrfile(git_config, install_location, attrfile)
     except FileNotFoundError:

tests/test_git_integration.py

@@ -24,6 +24,7 @@ def test_install(pytester: pytest.Pytester):
     config = ConfigParser()
     config.read(".git/config")
     assert re.match(r".*python.* -m nbstripout", config['filter "nbstripout"']["clean"])
+    assert config['filter "nbstripout"']["required"] == "true"
     assert config['filter "nbstripout"']["smudge"] == "cat"
     assert re.match(r".*python.* -m nbstripout -t", config['diff "ipynb"']["textconv"])
 
@@ -40,6 +41,7 @@ def test_install_different_python(pytester: pytest.Pytester):
         r".*DIFFERENTPYTHON.* -m nbstripout", config['filter "nbstripout"']["clean"]
     )
     assert sys.executable not in config['filter "nbstripout"']["clean"]
+    assert config['filter "nbstripout"']["required"] == "true"
     assert config['filter "nbstripout"']["smudge"] == "cat"
     assert re.match(
         r".*DIFFERENTPYTHON.* -m nbstripout -t", config['diff "ipynb"']["textconv"]