Make monkeypatch differentiate ImportError sources

Previously `monkeypatch` assumed that any `ImportError` was caused by
a mistake in the specified import path. However this assumption is false 
in case the import target itself causes an `ImportError`.

Fixes: #900

_pytest/monkeypatch.py

@@ -1,8 +1,14 @@
 """ monkeypatching and mocking functionality.  """
 
 import os, sys
+import re
+
 from py.builtin import _basestring
 
+
+RE_IMPORT_ERROR_NAME = re.compile("^No module named (.*)$")
+
+
 def pytest_funcarg__monkeypatch(request):
     """The returned ``monkeypatch`` funcarg provides these
     helper methods to modify objects, dictionaries or os.environ::
@@ -34,14 +40,28 @@ def derive_importpath(import_path, raising):
                         (import_path,))
     rest = []
     target = import_path
+    target_parts = set(target.split("."))
     while target:
         try:
             obj = __import__(target, None, None, "__doc__")
-        except ImportError:
+        except ImportError as ex:
+            if hasattr(ex, 'name'):
+                # Python >= 3.3
+                failed_name = ex.name
+            else:
+                match = RE_IMPORT_ERROR_NAME.match(ex.args[0])
+                assert match
+                failed_name = match.group(1)
+
             if "." not in target:
                 __tracebackhide__ = True
                 pytest.fail("could not import any sub part: %s" %
                             import_path)
+            elif failed_name != target \
+                    and not any(p == failed_name for p in target_parts):
+                # target is importable but causes ImportError itself
+                __tracebackhide__ = True
+                pytest.fail("import error in %s: %s" % (target, ex.args[0]))
             target, name = target.rsplit(".", 1)
             rest.append(name)
         else:

testing/test_monkeypatch.py

@@ -1,4 +1,6 @@
 import os, sys
+import textwrap
+
 import pytest
 from _pytest.monkeypatch import monkeypatch as MonkeyPatch
 
@@ -245,6 +247,21 @@ def test_issue185_time_breaks(testdir):
         *1 passed*
     """)
 
+def test_importerror(testdir):
+    p = testdir.mkpydir("package")
+    p.join("a.py").write(textwrap.dedent("""\
+        import doesnotexist
+
+        x = 1
+    """))
+    testdir.tmpdir.join("test_importerror.py").write(textwrap.dedent("""\
+        def test_importerror(monkeypatch):
+            monkeypatch.setattr('package.a.x', 2)
+    """))
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines("""
+        *import error in package.a.x: No module named {0}doesnotexist{0}*
+    """.format("'" if sys.version_info > (3, 0) else ""))
 
 
 class SampleNew(object):