python tokenize_model_AttributeError:“module”对象没有属性“tokenize”

我已经在我的virtualenv中安装了nltk。当我做pip freeze时，我可以看到nltk==3.2.1。在

我使用的是Ubuntu14.04和Python2.7。在激活virtualenv之后，当我试图从pythonshell导入nltk时，我得到以下错误。在>>> import nltk

Traceback (most recent call last): File "", line 1, in File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/nltk/__init__.py", line 128, in

from nltk.chunk import * File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/nltk/chunk/__init__.py", line 157, in

from nltk.chunk.api import ChunkParserI File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/nltk/chunk/api.py", line 13, in

from nltk.parse import ParserI File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/nltk/parse/__init__.py", line 79, in

from nltk.parse.transitionparser import TransitionParser File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/nltk/parse/transitionparser.py", line 21, in

from sklearn.datasets import load_svmlight_file File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/__init__.py", line 57, in

from .base import clone File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/base.py", line 11, in

from .utils.fixes import signature File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/utils/__init__.py", line 17, in

from ..externals.joblib import cpu_count File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/externals/joblib/__init__.py", line 127, in

from .parallel import Parallel File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/externals/joblib/parallel.py", line 30, in

from .format_stack import format_exc, format_outer_frames File "/home/kenden/den/codes/myenv/lib/python2.7/site-packages/sklearn/externals/joblib/format_stack.py", line 35, in

generate_tokens = tokenize.tokenize AttributeError: 'module' object has no attribute 'tokenize'

我怎样才能解决这个问题？在