pytester: LineMatcher: assert Sequence when matching in order

This can be helpful when passing a set accidentally.

changelog/4931.feature.rst

@@ -0,0 +1 @@
+pytester's ``LineMatcher`` asserts that the passed lines are a sequence.

src/_pytest/pytester.py

@@ -4,6 +4,7 @@
 from __future__ import print_function
 
 import codecs
+import collections
 import gc
 import os
 import platform
@@ -1325,6 +1326,7 @@ def _match_lines(self, lines2, match_func, match_nickname):
             will be logged to stdout when a match occurs
 
         """
+        assert isinstance(lines2, collections.abc.Sequence)
         lines2 = self._getlines(lines2)
         lines1 = self.lines[:]
         nextline = None

testing/test_pytester.py

@@ -17,6 +17,7 @@
 from _pytest.main import EXIT_TESTSFAILED
 from _pytest.pytester import CwdSnapshot
 from _pytest.pytester import HookRecorder
+from _pytest.pytester import LineMatcher
 from _pytest.pytester import SysModulesSnapshot
 from _pytest.pytester import SysPathsSnapshot
 
@@ -453,3 +454,15 @@ def test_timeout():
     )
     with pytest.raises(testdir.TimeoutExpired):
         testdir.runpytest_subprocess(testfile, timeout=1)
+
+
+def test_linematcher_with_set():
+    """Test LineMatcher with regard to passing in a set (accidentally)."""
+    lm = LineMatcher([])
+
+    with pytest.raises(AssertionError):
+        lm.fnmatch_lines({})
+    lm.fnmatch_lines([])
+    lm.fnmatch_lines(())
+
+    assert lm._getlines({}) == {}