import svn.core    
import svn.client    
pool = svn.core.svn_pool_create(None)   
ctx = svn.client.svn_client_create_context(pool)    
revision = svn.core.svn_opt_revision_t()
revision.kind = svn.core.svn_opt_revision_head
svn.client.svn_client_checkout("https://a", ".", revision, True, ctx, pool)