Fix history: do not nuke on errors, fix encoding for py3 (#4)

* write_history_file: do not nuke history on errors

* Pick write_history_file encoding fix from PyPy (py3.6)

pyrepl/readline.py

@@ -308,15 +308,21 @@ def read_history_file(self, filename='~/.history'):
     def write_history_file(self, filename='~/.history'):
         maxlength = self.saved_history_length
         history = self.get_reader().get_trimmed_history(maxlength)
-        f = open(os.path.expanduser(filename), 'w')
+        entries = ''
         for entry in history:
-            if isinstance(entry, unicode):
-                try:
-                    entry = entry.encode(ENCODING)
-                except UnicodeEncodeError:   # bah, silently fall back...
-                    entry = entry.encode('utf-8')
+            # if we are on py3k, we don't need to encode strings before
+            # writing it to a file
+            if isinstance(entry, unicode) and sys.version_info < (3,):
+                entry = entry.encode('utf-8')
             entry = entry.replace('\n', '\r\n')   # multiline history support
-            f.write(entry + '\n')
+            entries += entry + '\n'
+
+        fname = os.path.expanduser(filename)
+        if PY3:
+            f = open(fname, 'w', encoding='utf-8')
+        else:
+            f = open(fname, 'w')
+        f.write(entries)
         f.close()
 
     def clear_history(self):

testing/test_readline.py

@@ -66,3 +66,40 @@ def test_read_history_file(readline_wrapper, tmp_path):
     histfile.write_bytes(b"foo\nbar\n")
     readline_wrapper.read_history_file(str(histfile))
     assert readline_wrapper.reader.history == ["foo", "bar"]
+
+
+def test_write_history_file(readline_wrapper, tmp_path):
+    histfile = tmp_path / "history"
+
+    reader = readline_wrapper.get_reader()
+    history = reader.history
+    assert history == []
+    history.extend(["foo", "bar"])
+
+    readline_wrapper.write_history_file(str(histfile))
+
+    assert open(str(histfile), "r").readlines() == ["foo\n", "bar\n"]
+
+
+def test_write_history_file_with_exception(readline_wrapper, tmp_path):
+    """The history file should not get nuked on inner exceptions.
+
+    This was the case with unicode decoding previously."""
+    histfile = tmp_path / "history"
+    histfile.write_bytes(b"foo\nbar\n")
+
+    class BadEntryException(Exception):
+        pass
+
+    class BadEntry(object):
+        @classmethod
+        def replace(cls, *args):
+            raise BadEntryException
+
+    history = readline_wrapper.get_reader().history
+    history.extend([BadEntry])
+
+    with pytest.raises(BadEntryException):
+        readline_wrapper.write_history_file(str(histfile))
+
+    assert open(str(histfile), "r").readlines() == ["foo\n", "bar\n"]