How to get last commit for specified file with python(dulwich)?

I need author name and last commit time for a specified file with python. Currentrly, I'm trying to use dulwich.

There're plenty of apis to retrieve objects for a specific SHA like:

repo = Repo("myrepo")
head = repo.head()
object = repo.get_object(head)
author = object.author
time = object.commit_time

But, how do i know the recent commit for the specific file? Is there a way to retrieve it like:

repo = Repo("myrepo")
commit = repo.get_commit('a.txt')
author = commit.author
time = commit.commit_time

or

repo = Repo("myrepo")
sha = repo.get_sha_for('a.txt')
object = repo.get_object(sha)
author = object.author
time = object.commit_time

Thank you.

Something like this seems to work:

from dulwich import repo, diff_tree

fn = 'a.txt'
r = repo.Repo('.')
prev = None
walker = r.get_graph_walker()
cset = walker.next()
while cset is not None:

    commit = r.get_object(cset)
    if prev is None:
        prev = commit.tree
        cset = walker.next()
        continue


    res = None
    delta = diff_tree.tree_changes(r, prev, commit.tree)
    for x in diff_tree.tree_changes(r, prev, commit.tree):
        if x.new.path == fn:
            res = cset
            break

    if res:
        break

    prev = commit.tree
    cset = walker.next()

print fn, res