# Buggy code
def validate_segments(self, text="constructing"):
    """Check the elements of the `segments` attribute are all themselves segments."""
    for elem in self.segments:
        if not isinstance(elem, BaseSegment):
            raise TypeError(
                "In {0} {1}, found an element of the segments tuple which"
                " isn't a segment. Instead found element of type {2}.\nFound: {3}\nFull segments:{4}".format(
                    text, type(self), type(elem), elem, self.segments
                )
            )


# 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 validate_segments(self, text="constructing", validate=True):
    """Validate the current set of segments.

    Check the elements of the `segments` attribute are all
    themselves segments, and that the positions match up.

    `validate` confirms whether we should check contigiousness.
    """
    # Placeholder variables for positions
    start_pos = None
    end_pos = None
    prev_seg = None
    for elem in self.segments:
        if not isinstance(elem, BaseSegment):
            raise TypeError(
                "In {0} {1}, found an element of the segments tuple which"
                " isn't a segment. Instead found element of type {2}.\nFound: {3}\nFull segments:{4}".format(
                    text, type(self), type(elem), elem, self.segments
                )
            )
        # While applying fixes, we shouldn't validate here, because it will fail.
        if validate:
            # If we have a comparison point, validate that
            if end_pos and elem.get_start_pos_marker() != end_pos:
                raise TypeError(
                    "In {0} {1}, found an element of the segments tuple which"
                    " isn't contigious with previous: {2} > {3}".format(
                        text, type(self), prev_seg, elem
                    )
                )
            start_pos = elem.get_start_pos_marker()
            end_pos = elem.get_end_pos_marker()
            prev_seg = elem
            if start_pos.advance_by(elem.raw) != end_pos:
                raise TypeError(
                    "In {0} {1}, found an element of the segments tuple which"
                    " isn't self consistent: {2}".format(text, type(self), elem)
                )