Reporting methods shouldn't permanently change the configuration

4 files changed, 56 insertions, 19 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 5d0ff126..ec79353f 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -28,6 +28,10 @@ Unreleased
   "numbits."  The :mod:`coverage.numbits` module provides functions for working
   with them.
 
+- The reporting methods used to permanently apply their arguments to the
+  configuration of the Coverage object.  Now they no longer do.  The arguments
+  affect the operation of the method, but do not persist.
+
 - A class named "test_something" no longer confuses the ``test_function``
   dynamic context setting.  Fixes `issue 829`_.
 
diff --git a/coverage/config.py b/coverage/config.py
index 516fe1b9..85493df1 100644
--- a/coverage/config.py
+++ b/coverage/config.py
@@ -4,6 +4,7 @@
 """Config file for coverage.py"""
 
 import collections
+import copy
 import os
 import re
 
@@ -215,7 +216,7 @@ class CoverageConfig(object):
         self.xml_output = "coverage.xml"
         self.xml_package_depth = 99
 
-        # Defaults for [JSON]
+        # Defaults for [json]
         self.json_output = "coverage.json"
         self.json_pretty_print = False
         self.json_show_contexts = False
@@ -318,6 +319,10 @@ class CoverageConfig(object):
 
         return used
 
+    def copy(self):
+        """Return a copy of the configuration."""
+        return copy.deepcopy(self)
+
     CONFIG_FILE_OPTIONS = [
         # These are *args for _set_attr_from_config_option:
         #   (attr, where, type_="")
diff --git a/coverage/control.py b/coverage/control.py
index d5988456..c342eca4 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -4,6 +4,7 @@
 """Core control stuff for coverage.py."""
 
 import atexit
+import contextlib
 import os
 import os.path
 import platform
@@ -41,6 +42,21 @@ except ImportError:                                         # pragma: only jytho
 
 os = isolate_module(os)
 
+@contextlib.contextmanager
+def override_config(cov, **kwargs):
+    """Temporarily tweak the configuration of `cov`.
+
+    The arguments are applied to `cov.config` with the `from_args` method.
+    At the end of the with-statement, the old configuration is restored.
+    """
+    original_config = cov.config
+    cov.config = cov.config.copy()
+    try:
+        cov.config.from_args(**kwargs)
+        yield
+    finally:
+        cov.config = original_config
+
 
 class Coverage(object):
     """Programmatic access to coverage.py.
@@ -781,13 +797,14 @@ class Coverage(object):
         Returns a float, the total percentage covered.
 
         """
-        self.config.from_args(
+        with override_config(
+            self,
             ignore_errors=ignore_errors, report_omit=omit, report_include=include,
             show_missing=show_missing, skip_covered=skip_covered,
             report_contexts=contexts,
-            )
-        reporter = SummaryReporter(self)
-        return reporter.report(morfs, outfile=file)
+        ):
+            reporter = SummaryReporter(self)
+            return reporter.report(morfs, outfile=file)
 
     def annotate(
         self, morfs=None, directory=None, ignore_errors=None,
@@ -803,12 +820,12 @@ class Coverage(object):
         See :meth:`report` for other arguments.
 
         """
-        self.config.from_args(
+        with override_config(self,
             ignore_errors=ignore_errors, report_omit=omit,
             report_include=include, report_contexts=contexts,
-            )
-        reporter = AnnotateReporter(self)
-        reporter.report(morfs, directory=directory)
+        ):
+            reporter = AnnotateReporter(self)
+            reporter.report(morfs, directory=directory)
 
     def html_report(self, morfs=None, directory=None, ignore_errors=None,
                     omit=None, include=None, extra_css=None, title=None,
@@ -836,13 +853,13 @@ class Coverage(object):
             changing the files in the report folder.
 
         """
-        self.config.from_args(
+        with override_config(self,
             ignore_errors=ignore_errors, report_omit=omit, report_include=include,
             html_dir=directory, extra_css=extra_css, html_title=title,
             skip_covered=skip_covered, show_contexts=show_contexts, report_contexts=contexts,
-            )
-        reporter = HtmlReporter(self)
-        return reporter.report(morfs)
+        ):
+            reporter = HtmlReporter(self)
+            return reporter.report(morfs)
 
     def xml_report(
         self, morfs=None, outfile=None, ignore_errors=None,
@@ -860,11 +877,11 @@ class Coverage(object):
         Returns a float, the total percentage covered.
 
         """
-        self.config.from_args(
+        with override_config(self,
             ignore_errors=ignore_errors, report_omit=omit, report_include=include,
             xml_output=outfile, report_contexts=contexts,
-            )
-        return render_report(self.config.xml_output, XmlReporter(self), morfs)
+        ):
+            return render_report(self.config.xml_output, XmlReporter(self), morfs)
 
     def json_report(
         self, morfs=None, outfile=None, ignore_errors=None,
@@ -881,12 +898,12 @@ class Coverage(object):
         Returns a float, the total percentage covered.
 
         """
-        self.config.from_args(
+        with override_config(self,
             ignore_errors=ignore_errors, report_omit=omit, report_include=include,
             json_output=outfile, report_contexts=contexts, json_pretty_print=pretty_print,
             json_show_contexts=show_contexts
-        )
-        return render_report(self.config.json_output, JsonReporter(self), morfs)
+        ):
+            return render_report(self.config.json_output, JsonReporter(self), morfs)
 
     def sys_info(self):
         """Return a list of (key, value) pairs showing internal information."""
diff --git a/tests/test_api.py b/tests/test_api.py
index 301257dc..4f248441 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -918,3 +918,14 @@ class TestRunnerPluginTest(CoverageTest):
 
     def test_pytestcov_parallel_append(self):
         self.pretend_to_be_pytestcov(append=True)
+
+
+class ImmutableConfigTest(CoverageTest):
+    """Check that reporting methods don't permanently change the configuration."""
+    def test_config_doesnt_change(self):
+        self.make_file("simple.py", "a = 1")
+        cov = coverage.Coverage()
+        self.start_import_stop(cov, "simple")
+        self.assertEqual(cov.get_option("report:show_missing"), False)
+        cov.report(show_missing=True)
+        self.assertEqual(cov.get_option("report:show_missing"), False)