add _maybe_copy and _should_copy

to `notebook_compat._compat.nbextensions`, as the logger kwarg was added after notebook 4.1

Author: jcb91

README.md

@@ -16,6 +16,12 @@ Common utilities for jupyter-contrib projects. Includes:
 Changes
 =======
 
+0.3.0
+-----
+
+* Add `_maybe_copy` and `_should_copy` to nbextensions private API, since the
+  `logger` keyword arg was only added after notebook 4.1
+
 0.2.0
 -----
 

src/jupyter_contrib_core/notebook_compat/_compat/nbextensions.py

@@ -33,7 +33,7 @@
     jupyter_config_dir, jupyter_data_dir,
 )
 from notebook.nbextensions import (
-    ArgumentConflict, __version__, _maybe_copy, _safe_is_tarfile,
+    ArgumentConflict, __version__, _safe_is_tarfile,
 )
 from tornado.log import LogFormatter
 from traitlets import Bool
@@ -413,6 +413,29 @@ def _log_format_default(self):
 # -----------------------------------------------------------------------------
 
 
+def _should_copy(src, dest, logger=None):
+    """Return whether a file should be copied."""
+    if not os.path.exists(dest):
+        return True
+    if os.stat(src).st_mtime - os.stat(dest).st_mtime > 1e-6:
+        # we add a fudge factor to work around a bug in python 2.x
+        # that was fixed in python 3.x: http://bugs.python.org/issue12904
+        if logger:
+            logger.warn("Out of date: %s" % dest)
+        return True
+    if logger:
+        logger.info("Up to date: %s" % dest)
+    return False
+
+
+def _maybe_copy(src, dest, logger=None):
+    """Copy a file if it needs updating."""
+    if _should_copy(src, dest, logger=logger):
+        if logger:
+            logger.info("Copying: %s -> %s" % (src, dest))
+        shutil.copy2(src, dest)
+
+
 def _get_nbextension_dir(user=False, sys_prefix=False, prefix=None,
                          nbextensions_dir=None):
     """Return the nbextension directory specified."""