Add support for removing empty cells, closes #131

README.rst

@@ -191,6 +191,14 @@ This is a potentially slower but simpler invocation using ``--tree-filter``: ::
 
     git filter-branch -f --tree-filter 'find . -name "*.ipynb" -exec nbstripout "{}" +'
 
+Removing empty cells
+++++++++++++++++++++
+
+Strip empty cells i.e. cells where ``source`` is either empty or only contains
+whitespace ::
+
+    nbstripout --strip-empty-cells
+
 Keeping some output
 +++++++++++++++++++
 

nbstripout/_nbstripout.py

@@ -279,6 +279,8 @@ def main():
                         help='Do not strip output', default=None)
     parser.add_argument('--extra-keys', default='',
                         help='Extra keys to strip from metadata, e.g. metadata.foo cell.metadata.bar')
+    parser.add_argument('--strip-empty-cells', action='store_true',
+                        help='Remove cells where `source` is empty or contains only whitepace')
     parser.add_argument('--attributes', metavar='FILEPATH',
                         help='Attributes file to add the filter to (in '
                         'combination with --install/--uninstall), '
@@ -339,7 +341,7 @@ def main():
                 with warnings.catch_warnings():
                     warnings.simplefilter("ignore", category=UserWarning)
                     nb = read(f, as_version=NO_CONVERT)
-            nb = strip_output(nb, args.keep_output, args.keep_count, extra_keys)
+            nb = strip_output(nb, args.keep_output, args.keep_count, extra_keys, args.strip_empty_cells)
             if args.dry_run:
                 output_stream.write('Dry run: would have stripped {}\n'.format(
                     filename))
@@ -370,7 +372,7 @@ def main():
             with warnings.catch_warnings():
                 warnings.simplefilter("ignore", category=UserWarning)
                 nb = read(input_stream, as_version=NO_CONVERT)
-            nb = strip_output(nb, args.keep_output, args.keep_count, extra_keys)
+            nb = strip_output(nb, args.keep_output, args.keep_count, extra_keys, args.strip_empty_cells)
             if args.dry_run:
                 output_stream.write('Dry run: would have stripped input from '
                                     'stdin\n')

nbstripout/_utils.py

@@ -30,13 +30,17 @@ def pop_recursive(d, key, default=None):
     return current.pop(nested[-1], default)
 
 
-def _cells(nb):
-    """Yield all cells in an nbformat-insensitive manner"""
+def _cells(nb, conditional=None):
+    """Remove cells not satisfying conditional and yield all other cells."""
     if nb.nbformat < 4:
         for ws in nb.worksheets:
+            if conditional:
+                ws.cells = list(filter(conditional, ws.cells))
             for cell in ws.cells:
                 yield cell
     else:
+        if conditional:
+            nb.cells = list(filter(conditional, nb.cells))
         for cell in nb.cells:
             yield cell
 
@@ -68,7 +72,7 @@ def determine_keep_output(cell, default):
     return default
 
 
-def strip_output(nb, keep_output, keep_count, extra_keys=''):
+def strip_output(nb, keep_output, keep_count, extra_keys='', strip_empty_cells=False):
     """
     Strip the outputs, execution count/prompt number and miscellaneous
     metadata from a notebook object, unless specified to keep either the outputs
@@ -97,7 +101,15 @@ def strip_output(nb, keep_output, keep_count, extra_keys=''):
     for field in keys['metadata']:
         pop_recursive(nb.metadata, field)
 
-    for cell in _cells(nb):
+    # Keep cells if they have any `source` line that contains non-whitespace
+    if strip_empty_cells:
+        def conditional(cell):
+            return any(line.strip() for line in cell.get('source', []))
+    # Keep all cells
+    else:
+        conditional = None
+
+    for cell in _cells(nb, conditional):
         keep_output_this_cell = determine_keep_output(cell, keep_output)
 
         # Remove the outputs, unless directed otherwise