Skip tests that depend on lxml if not installed (#12813)

Detect if lxml is importable in the test suite, if it is not, then skip the report tests which 
depend on it.

This is useful for enabling CI on new Python versions that may not have lxml wheels yet.

Closes #11662.

mypy/test/testcheck.py

@@ -21,6 +21,12 @@
 from mypy.errors import CompileError
 from mypy.semanal_main import core_modules
 
+try:
+    import lxml  # type: ignore
+except ImportError:
+    lxml = None
+
+import pytest
 
 # List of files that contain test case descriptions.
 typecheck_files = [
@@ -117,6 +123,8 @@ class TypeCheckSuite(DataSuite):
     files = typecheck_files
 
     def run_case(self, testcase: DataDrivenTestCase) -> None:
+        if lxml is None and os.path.basename(testcase.file) == 'check-reports.test':
+            pytest.skip("Cannot import lxml. Is it installed?")
         incremental = ('incremental' in testcase.name.lower()
                        or 'incremental' in testcase.file
                        or 'serialize' in testcase.file)

mypy/test/testcmdline.py

@@ -18,6 +18,13 @@
     assert_string_arrays_equal, normalize_error_messages, check_test_output_files
 )
 
+try:
+    import lxml  # type: ignore
+except ImportError:
+    lxml = None
+
+import pytest
+
 # Path to Python 3 interpreter
 python3_path = sys.executable
 
@@ -35,6 +42,8 @@ class PythonCmdlineSuite(DataSuite):
     native_sep = True
 
     def run_case(self, testcase: DataDrivenTestCase) -> None:
+        if lxml is None and os.path.basename(testcase.file) == 'reports.test':
+            pytest.skip("Cannot import lxml. Is it installed?")
         for step in [1] + sorted(testcase.output2):
             test_python_cmdline(testcase, step)
 

mypy/test/testreports.py

@@ -5,11 +5,21 @@
 from mypy.report import CoberturaPackage, get_line_rate
 
 
+try:
+    import lxml  # type: ignore
+except ImportError:
+    lxml = None
+
+import pytest
+
+
 class CoberturaReportSuite(Suite):
+    @pytest.mark.skipif(lxml is None, reason="Cannot import lxml. Is it installed?")
     def test_get_line_rate(self) -> None:
         assert_equal('1.0', get_line_rate(0, 0))
         assert_equal('0.3333', get_line_rate(1, 3))
 
+    @pytest.mark.skipif(lxml is None, reason="Cannot import lxml. Is it installed?")
     def test_as_xml(self) -> None:
         import lxml.etree as etree  # type: ignore