Issue AttributeError: 'list' object has no attribute 'split' nbconvert/filters/strings.py", line 201, in comment_lines return prefix + ("\n" + prefix).join(text.split("\n"))

[ShlomoStept]

An error occurs when processing IPython notebook files with 'nbformat': 4 and 'nbformat_minor': 2 (and possibly other versions). The traceback is as follows:

Traceback (most recent call last):

• python_version, _ = exporter.from_notebook_node(note_book)

File : "../lib/python3.9/site-packages/nbconvert/exporters/templateexporter.py", line 421, in from_notebook_node
output = self.template.render(nb=nb_copy, resources=resources)

File ".../jinja2/environment.py", line 1301, in render

• self.environment.handle_exception()
  File ".../jinja2/environment.py", line 936, in handle_exception
• raise rewrite_traceback_stack(source=source)
  File ".../jupyter/nbconvert/templates/python/index.py.j2", line 1, in top-level template code
• {%- extends 'null.j2' -%}
  File ".../share/jupyter/nbconvert/templates/base/null.j2", line 26, in top-level template code
• {%- block body -%}
  File ".../share/jupyter/nbconvert/templates/base/null.j2", line 29, in block 'body'
• {%- block body_loop -%}
  File ".../share/jupyter/nbconvert/templates/base/null.j2", line 31, in block 'body_loop'
• {%- block any_cell scoped -%}
  File "...share/jupyter/nbconvert/templates/base/null.j2", line 87, in block 'any_cell'
• {%- block markdowncell scoped-%} {%- endblock markdowncell -%}
  File ".../share/jupyter/nbconvert/templates/python/index.py.j2", line 19, in block 'markdowncell'
• {{ cell.source | comment_lines }}

File ".../lib/python3.9/site-packages/nbconvert/filters/strings.py", line 201, in comment_lines

• return prefix + ("\n" + prefix).join(text.split("\n"))
  AttributeError: 'list' object has no attribute 'split'

NOTE: The issue arises because cell.source is a list of strings instead of a single string. As a result, the text.split("\n") operation in the comment_lines function (line 201 in nbconvert/filters/strings.py) fails.

One potential solution to this problem could be to add a function before the output = self.template.render(nb=nb_copy, resources=resources) line in the from_notebook_node method (line 421). This function would take nb_copy as an input and modify it as follows:

for cell in nb_copy.cells:
    if isinstance(cell.source, list):
        cell.source = ''.join(cell.source)

Alternatively, a try-except block could be added that only runs this code if an exception is raised.

just for reference the function i used that triggered this error is

    # Step 0 - Convert the file from bytes to string
    file_as_string = file_as_string.decode('utf-8')
    
    # Step 1 - Get the version of the ipynb file, (a) convert it to JSON format, (b) grab the version
    ipynb_as_json = json.loads(file_as_string)

    # Get the version of the notebook format
    ipynb_version = ipynb_as_json["nbformat"]
    
    # Normalize the notebook
    n_changes, new_notebook = nbformat.validator.normalize(ipynb_as_json, version=ipynb_version)
    
    # Step 2 - Load the notebook from a string
    note_book = nbformat.from_dict(new_notebook)
    
    # Step 3 - Convert the notebook to a Python script
    exporter = PythonExporter()
    
    python_version, _ = exporter.from_notebook_node(note_book)
...