human
Solve the following python programming problem given the following code:
@ contextlib.contextmanager 
def fetch(self, ref) : 
    if (ref in self.fetched_paths) : 
        (yield self.fetched_paths [ref]) 
        return 
logger = self.get_logger ('fetch') 
    with super (PullRequest, self).fetch (self.name) as path : 
        def git(command, * args, **kwargs) : 
            command = ('git --git-dir="{git_dir}" --work-tree="{work_tree}" ' + command) 
            kwargs.update (git_dir = os.path.join (path, '.git'), work_tree = path) 
            cmd = command.format (* args, ** kwargs) 
            logger.info ('%s', cmd) 
            with os.popen (cmd) as f : 
                for line in f : 
                    logger.debug ('[%s] %s', cmd, line) 
git ('checkout {0}', logger.pull_request.base.sha) 
        git ('checkout -b asuka-pullreq-{0}', self.number) 
        git ('pull "{0}" "{1}":asuka-pullreq-{2}', self.app.get_clone_url (self.repository), self.pull_request.head.ref, self.number) 
        git ('checkout "{0}"', self.name) 
        git ('checkout -b asuka-mergedpullreq-{0}', self.number) 
        git ('merge "{0}"', ref) 
        git ('branch -D asuka-pullreq-{0}', self.number) 
        (yield path) 
        git ('checkout "{0}"', self.name) 
        git ('branch -D asuka-mergedpullreq-{0}', self.number) 

    
    

    Fix the buggy line: git ('checkout {0}', logger.pull_request.base.sha)
assistant
#Here is Python program to fix the buggy line: git ('checkout {0}', logger.

git ('checkout {0}', self.pull_request.base.sha)