ImportError: No module named _collections

[fansiawang]

Under the "gnmt-mater" directory, I executed the following command to train model:
python -m nmt.nmt --hparams_path=./nmt/standard_hparams/iwslt15.json --src=vi --tgt=en --vocab_prefix=./nmt/nmt_data/iwslt15/vocab --train_prefix=./nmt/nmt_data/iwslt15/train --dev_prefix=./nmt/nmt_data/iwslt15/tst2012 --test_prefix=./nmt/nmt_data/iwslt15/tst2013 --out_dir=./nmt/nmt_attention_model/iwslt152
But it reported an error:

Traceback (most recent call last):
  File "/disk1/wangfang/anaconda2/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/disk1/wangfang/anaconda2/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/disk1/wangfang/gnmt-master/nmt/nmt.py", line 19, in <module>
    import argparse
  File "/disk1/wangfang/anaconda2/lib/python2.7/argparse.py", line 85, in <module>
    import collections as _collections
  File "/disk1/wangfang/anaconda2/lib/python2.7/collections.py", line 20, in <module>
    from _collections import deque, defaultdict
ImportError: No module named _collections

I executed training command successfully two days ago!!! I don't know what happened....Does anybody know how to solve this problem?