pytester: reset log output in _match_lines (#70)

This is necessary for when using e.g. `no_fnmatch_line` after it.
Factor it out into `_fail`.

(cherry picked from commit aade7ed0045ba32557ef8565cbab28a2c91053a7)

Ref: https://github.com/pytest-dev/pytest/pull/5914#issuecomment-549182242

changelog/5914.bugfix.rst

@@ -0,0 +1 @@
+pytester: fix ``no_fnmatch_line`` when used after positive matching.

src/_pytest/pytester.py

@@ -1438,8 +1438,10 @@ class LineMatcher:
                     self._log("{:>{width}}".format("and:", width=wnick), repr(nextline))
                 extralines.append(nextline)
             else:
-                self._log("remains unmatched: {!r}".format(line))
-                pytest.fail(self._log_text.lstrip())
+                msg = "remains unmatched: {!r}".format(line)
+                self._log(msg)
+                self._fail(msg)
+        self._log_output = []
 
     def no_fnmatch_line(self, pat):
         """Ensure captured lines do not match the given pattern, using ``fnmatch.fnmatch``.
@@ -1465,18 +1467,20 @@ class LineMatcher:
         __tracebackhide__ = True
         nomatch_printed = False
         wnick = len(match_nickname) + 1
-        try:
-            for line in self.lines:
-                if match_func(line, pat):
-                    self._log("%s:" % match_nickname, repr(pat))
-                    self._log("{:>{width}}".format("with:", width=wnick), repr(line))
-                    pytest.fail(self._log_text.lstrip())
-                else:
-                    if not nomatch_printed:
-                        self._log(
-                            "{:>{width}}".format("nomatch:", width=wnick), repr(pat)
-                        )
-                        nomatch_printed = True
-                    self._log("{:>{width}}".format("and:", width=wnick), repr(line))
-        finally:
-            self._log_output = []
+        for line in self.lines:
+            if match_func(line, pat):
+                msg = "{}: {!r}".format(match_nickname, pat)
+                self._log(msg)
+                self._log("{:>{width}}".format("with:", width=wnick), repr(line))
+                self._fail(msg)
+            else:
+                if not nomatch_printed:
+                    self._log("{:>{width}}".format("nomatch:", width=wnick), repr(pat))
+                    nomatch_printed = True
+                self._log("{:>{width}}".format("and:", width=wnick), repr(line))
+        self._log_output = []
+
+    def _fail(self, msg):
+        log_text = self._log_text
+        self._log_output = []
+        pytest.fail(log_text)

testing/test_pytester.py

@@ -530,7 +530,7 @@ def test_no_matching(function):
             ]
         else:
             assert obtained == [
-                "nomatch: '{}'".format(good_pattern),
+                " nomatch: '{}'".format(good_pattern),
                 "     and: 'cachedir: .pytest_cache'",
                 "     and: 'collecting ... collected 1 item'",
                 "     and: ''",
@@ -542,6 +542,14 @@ def test_no_matching(function):
     func(bad_pattern)  # bad pattern does not match any line: passes
 
 
+def test_no_matching_after_match():
+    lm = LineMatcher(["1", "2", "3"])
+    lm.fnmatch_lines(["1", "3"])
+    with pytest.raises(pytest.fail.Exception) as e:
+        lm.no_fnmatch_line("*")
+    assert str(e.value).splitlines() == ["fnmatch: '*'", "   with: '1'"]
+
+
 def test_pytester_addopts(request, monkeypatch):
     monkeypatch.setenv("PYTEST_ADDOPTS", "--orig-unused")