Close opened resource. (#235)

* Close opened resource.

* Update pytest_html/plugin.py

Co-Authored-By: Bruno Rocha <rochacbruno@users.noreply.github.com>

* Use `f""` instead of `.format()` call.

* Add validation for raised warnings to tests.

pytest_html/plugin.py

@@ -68,7 +68,8 @@ def pytest_configure(config):
     htmlpath = config.getoption("htmlpath")
     if htmlpath:
         for csspath in config.getoption("css"):
-            open(csspath)
+            if not os.path.exists(csspath):
+                raise IOError(f"No such file or directory: '{csspath}'")
         if not hasattr(config, "slaveinput"):
             # prevent opening htmlpath on slave nodes (xdist)
             config._html = HTMLReport(htmlpath, config)

testing/test_pytest_html.py

@@ -759,7 +759,7 @@ def test_pass(): pass
         assert re.search(regex_error, html) is not None
 
     @pytest.mark.parametrize("colors", [(["red"]), (["green", "blue"])])
-    def test_css(self, testdir, colors):
+    def test_css(self, testdir, recwarn, colors):
         testdir.makepyfile("def test_pass(): pass")
         css = {}
         cssargs = []
@@ -770,14 +770,16 @@ def test_css(self, testdir, colors):
             cssargs.extend(["--css", path])
         result, html = run(testdir, "report.html", "--self-contained-html", *cssargs)
         assert result.ret == 0
+        assert len(recwarn) == 0
         for k, v in css.items():
             assert str(v["path"]) in html
             assert v["style"] in html
 
-    def test_css_invalid(self, testdir):
+    def test_css_invalid(self, testdir, recwarn):
         testdir.makepyfile("def test_pass(): pass")
         result = testdir.runpytest("--html", "report.html", "--css", "style.css")
         assert result.ret
+        assert len(recwarn) == 0
         assert "No such file or directory: 'style.css'" in result.stderr.str()
 
     def test_css_invalid_no_html(self, testdir):