If this is unnecessary, please remove it. If it will be uncommented in the future, please add a note explaining why it's turned off.

This is all vestigial, right?

Remove this line.

liua6_codeReview isn't something portable, is it?

This is all vestigial, right?

No spaces between parentheses and function parameters.

Remove this line.

Remove this.

Python makes it really easy to iterate like this:

for line in lines:
    if not re.match("^\s*$", line):
        clearcase_diff_lines.append(line)

This reads better as "if not match:"

No parentheses around the while condition.  You can also use the for ... in ... syntax to make this a lot more readable.

Remove this.

Indentation here looks like tabs. All indentation should be 4 spaces.

Remove this.

Please add spaces around operators.

Remove this.

This is just leftover from the SVN tool, no?

Lots of little things that should be cleaned up. Some large things, but also a lot of code that can be simplified with some Pythonisms.

I am wondering, though, why this converts from a Clearcase diff into a standard GNU diff instead of implementing a diff parser. It seems you should be adding a DiffParser subclass that does the work necessary to parse.

What are the differences between Clearcase diffs and standard diffs? We may be able to simplify this code drastically.

We can't put code in that's specific to a single platform and requires modification to make it work. What's the reason this can't be used on Windows?

How do these names work? I'm a little concerned about having a hard-coded name here.

The file handle returned from open() must be close()'d or there will be a leak.

Should re-use stre.

Why 100?

Only a single blank line between these functions.

Should just return in each case instead of assigning to r.

An empty filename here is confusing. I'm not even sure why this exception is here?

Should be one blank line.

"gun" should probably be "gnu"

This is better done as:

return DiffParser("\n".join(self.ccdiff_to_gnudiff(data)))

Can you document exactly what this function is doing? It's kind of confusing.

No need for the outer parens.

Indentation looks to be using tabs instead of spaces here.

Do we know for sure it's always \r\n?

This could be better written as:

clearcase_diff_lines = [
    line for line in lines
    if not re.match("^\s*$", line)
]

The first part of the if statement should always be true, since we've verified it through the condition in the while loop.

When doing multi-line if statements, it's best to do:

if (.... and
    .... and
    ....):

Indentation is wrong for all this. It should be indented 4 spaces from the if statement.

No need for these parens.

Indentation seems wrong.

These should use \t before the (revision ...) instead of spaces.

Also, the hard-coded "(revision 100)" seems very bad to me. We need to use actual revisions, but made-up ones.

Blank line before this if statement.

These might be better as:

newline = "@@ -%s,0 +%s,%s @@" % (old_start + 1, new_start, new_line_num)

Same with the other ones.

I would do something like:

for i in range(i + 1, i + new_line_num):
    gnu_diff_lines.append("+" + clearcase_diff_lines[i][2:])

Similar to above:

for i in range(i + 1, i + old_line_num):
    gnu_diff_lines.append("-" + clearcase_diff_lines[i][2:])

for i in range(i + 1, i + new_line_num):
    gnu_diff_lines.append("+" + clearcase_diff_lines[i][2:])