Improve testing of missing toml parser

Add a new constant (in a dummy module) which can be used to explicitly
disable TOML parsing, solely for the purpose of testing. Comments on
the variable explain why it is necessary.

Also cleanup tox config a little. Remove the depends listing in favor
of ordering environments in the env_list in a sensible way. This
allows for parallel invocation only when the coverage environments are
absent, but this is still possible. For example,
`tox r -e cov_clean; tox p -m ci; tox r -e cov` would imitate CI
usage.

Author: sirosen

.github/workflows/build.yaml

@@ -60,7 +60,9 @@ jobs:
           key: tox-os-${{ runner.os }}-py${{ matrix.py }}-weeknum-${{ steps.get-week-number.outputs.cachedate }}-job-${{ github.job }}-hash-${{ hashFiles('tox.ini', 'setup.cfg') }}
 
       - name: test
-        run: python -m tox run -m ci
+        run: |
+          python -m tox run-parallel -m ci
+          python -m tox run -e cov
       - name: twine-check
         run: python -m tox run -e twine-check
 

src/check_jsonschema/_testing.py

@@ -0,0 +1,20 @@
+import os
+import sys
+
+PYTEST_LOADED = "pytest" in sys.modules
+
+# this constant exists in service of the testsuite being able to forcibly disable the TOML
+# loader
+# the reason for this is that
+#   - toml loading is enabled on py3.11+ with 'tomllib'
+#   - on python < 3.11, toml loading is enabled with 'tomli'
+#   - on python < 3.11, *coverage requires tomli*
+#
+# the end result is that it is not possible without special trickery to measure coverage
+# in the disabled TOML state
+# in order to measure coverage, we need 'tomli', so we will include this "magical" test
+# constant in order to "forcibly" disable the 'tomli' usage
+FORCE_TOML_DISABLED = False
+
+if PYTEST_LOADED and os.getenv("FORCE_TOML_DISABLED") == "1":
+    FORCE_TOML_DISABLED = True

src/check_jsonschema/parsers/toml.py

@@ -4,12 +4,14 @@
 import sys
 import typing as t
 
+from check_jsonschema._testing import FORCE_TOML_DISABLED
+
 if sys.version_info < (3, 11):
     try:
         import tomli as toml_implementation
 
         has_toml = True
-    except ImportError:
+    except ImportError:  # pragma: no cover
         has_toml = False
 else:
     has_toml = True
@@ -54,10 +56,10 @@ def _normalize(data: t.Any) -> t.Any:
 
 
 # present a bool for detecting that it's enabled
-ENABLED = has_toml
+ENABLED = has_toml and not FORCE_TOML_DISABLED
 
 
-if has_toml:
+if ENABLED:
     ParseError: type[Exception] = toml_implementation.TOMLDecodeError
 
     def load(stream: t.BinaryIO) -> t.Any:

tox.ini

@@ -1,21 +1,17 @@
 [tox]
-# desired label:
-#   ci = py{,-json5,-pyjson5,-tomli}{,-format}"
-#
-# a tox issue will be filed to ask how this could be better supported
-# for now, 'ci' is an expanded matrix:
-labels =
-  ci = py, py-format, py-json5, py-json5-format, py-pyjson5, py-pyjson5-format, py-tomli, py-tomli-format, coverage
-
 envlist =
+    mypy
     cov_clean
     py{311,310,39,38,37}
-    py{37,310}-tomli{,-format}
+    py310-{notoml,tomli-format}
     py{37,311}-{json5,pyjson5}
     cov
 skip_missing_interpreters = true
 minversion = 4.0.0
 
+labels =
+    ci = py-notoml, py-tomli-format, py-json5, py-pyjson5
+
 [testenv]
 description = "run tests with pytest"
 usedevelop = true
@@ -25,10 +21,9 @@ deps =
     pyjson5: pyjson5
     tomli: tomli
     format: jsonschema[format]
+set_env =
+    notoml: FORCE_TOML_DISABLED=1
 commands = coverage run -m pytest {posargs}
-depends =
-    py{,37,38,39,310,311}{,-json5,-pyjson5,-tomli}: cov_clean
-    cov: py{,37,38,39,310,311}{,-json5,-pyjson5,-tomli}
 
 [testenv:cov_clean]
 description = "erase coverage data to prepare for a new run"
@@ -40,7 +35,7 @@ commands = coverage erase
 description = "combine and report coverage data"
 deps = coverage
 skip_install = true
-commands_pre = coverage combine
+commands_pre = - coverage combine
 commands = coverage report --skip-covered
 
 [testenv:mypy]