Is three a way to do three dot diff between two branches?

[iwccsbfb]

In command line, I can do git diff master..branch and also git diff master...branch. However, in gitPython, it seems the commit.diff(commit2) is only doing two dot diff. And I couldn't find a way to do three dot diff directly (I have searched through the document and googled around).
I am thinking to find the commit where we branch off, and then do a two way diff - this would be the same as the three way diff of two branches. But it does not work exactly as expected:

# this commit is not the root commit, but the child of the root commit...
commit = list(repo.iter_commits('origin/master..HEAD'))[-1]
diff = commit.diff(repo.head.commit)

Would you have any recommendation?
Thank you.

[iwccsbfb]

I believe I have worked around in following way:

def three_dot_diff_with_master(repo):
    # might be slow when the commit tree is very big
    master_commits = repo.iter_commits('origin/master')
    local_commits = set(repo.iter_commits('HEAD'))
    for commit in master_commits :
        if commit in local_commits:
            break
    diffs = commit.diff(repo.head.commit)

I believe this issue can now be closed. But I'll leave the decision to the project admin.
Thank you