# Buggy code
def persist_tree(self, verbosity=0):
    """Persist changes to the given path.

    We use the file_mask to do a safe merge, avoiding any templated
    sections. First we need to detect where there have been changes
    between the fixed and templated versions.

    We use difflib.SequenceMatcher.get_opcodes
    See: https://docs.python.org/3.7/library/difflib.html#difflib.SequenceMatcher.get_opcodes
    It returns a list of tuples ('equal|replace', ia1, ia2, ib1, ib2).

    """
    write_buff = self.fix_string(verbosity=verbosity)

    # Actually write the file.
    with open(self.path, "w") as f:
        f.write(write_buff)

    # TODO: Make return value of persist_changes() a more interesting result and then format it
    # click.echo(format_linting_fixes(result, verbose=verbose), color=color)
    return True


# Traceback (TypeError)
$ sqlfluff fix --rules L001 forecastservice\sql\get_phasing_v2.sql
==== finding violations ====
== [forecastservice\sql\get_phasing_v2.sql] FAIL
L:   0 | P:   0 | ???? | Failure in Jinja templating: 'first_mon' is undefined. Have you configured your variables?
L:   7 | P:   1 | ???? | Unable to lex characters: ''{% if regi'...'
==== fixing violations ====
2 violations found
Are you sure you wish to attempt to fix these? [Y/n] ...
Attempting fixes...
Persisting Changes...
Traceback (most recent call last):
File "C:\Users\alan\dev\tails-analytics-forecastservice\env\Scripts\sqlfluff-script.py", line 11, in <module>
load_entry_point('sqlfluff==0.2.4', 'console_scripts', 'sqlfluff')()
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\click\core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\click\core.py", line 717, in main
rv = self.invoke(ctx)
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\click\core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\click\core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\click\core.py", line 555, in invoke
return callback(*args, **kwargs)
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\cli\commands.py", line 197, in fix
result.persist_changes(verbosity=verbose)
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\linter.py", line 327, in persist_changes
return self.combine_dicts(*[path.persist_changes(verbosity=verbosity) for path in self.paths])
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\linter.py", line 327, in <listcomp>
return self.combine_dicts(*[path.persist_changes(verbosity=verbosity) for path in self.paths])
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\linter.py", line 253, in persist_changes
return {file.path: file.persist_tree(verbosity=verbosity) for file in self.files}
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\linter.py", line 253, in <dictcomp>
return {file.path: file.persist_tree(verbosity=verbosity) for file in self.files}
File "c:\users\alan\dev\tails-analytics-forecastservice\env\lib\site-packages\sqlfluff\linter.py", line 61, in persist_tree
diff_templ_codes = diff_templ.get_opcodes()
File "C:\Users\alan\AppData\Local\Programs\Python\Python36\lib\difflib.py", line 550, in get_opcodes
for ai, bj, size in self.get_matching_blocks():
File "C:\Users\alan\AppData\Local\Programs\Python\Python36\lib\difflib.py", line 467, in get_matching_blocks
la, lb = len(self.a), len(self.b)
TypeError: object of type 'NoneType' has no len()

# Fixed code
def persist_tree(self, verbosity=0):
    """Persist changes to the given path.

    We use the file_mask to do a safe merge, avoiding any templated
    sections. First we need to detect where there have been changes
    between the fixed and templated versions.

    We use difflib.SequenceMatcher.get_opcodes
    See: https://docs.python.org/3.7/library/difflib.html#difflib.SequenceMatcher.get_opcodes
    It returns a list of tuples ('equal|replace', ia1, ia2, ib1, ib2).

    """
    write_buff, success = self.fix_string(verbosity=verbosity)

    if success:
        # Actually write the file.
        with open(self.path, "w") as f:
            f.write(write_buff)

    # TODO: Make return value of persist_changes() a more interesting result and then format it
    # click.echo(format_linting_fixes(result, verbose=verbose), color=color)
    return success