import zipfile

def isdir(z, name):
    return any(x.startswith("%s/" % name.rstrip("/")) for x in z.namelist())

f = zipfile.ZipFile("sample.zip", "r")
print isdir(f, "a")
print isdir(f, "a/b")
print isdir(f, "a/X")

any(x.startswith("%s/" % name.rstrip("/")) for x in z.namelist())

$ mkdir -p a/b/c/d
$ touch a/X
$ zip -r sample.zip a
adding: a/ (stored 0%)
adding: a/X (stored 0%)
adding: a/b/ (stored 0%)
adding: a/b/c/ (stored 0%)
adding: a/b/c/d/ (stored 0%)

$ python z.py
True
True
False

import os, zipfile
dir = "some/directory/"

z = zipfile.ZipFile("myfile.zip")
if dir in z.namelist():
    print "Found %s!" % dir

import zipfile
z = zipfile.ZipFile("file.zip")

if "DirName/" in [member.filename for member in z.infolist()]:
    print("Directory exists in archive")

def is_dir(self):
    """Return True if this archive member is a directory."""
    return self.filename[-1] == '/'

def is_dir(zipinfo):
    return "filemode='d" in zipinfo.__repr__()

def is_dir(fileinfo):
    hi = fileinfo.external_attr >> 16
    return (hi & 0x4000) > 0