MemoryError query

I am using PyYaml with libyaml.   When trying to read a 100Mb YAML file, I get the following error.  This is on a machine with 4G RAM.   How can this happen?
Keith

kbriggs@gold> time python germany.py

yaml_loader_01: germany.pkl does not exist; will build it Traceback (most recent call last):
  File "germany.py", line 21, in <module>
    data=Germany()
  File "germany.py", line 9, in __init__
    s.d=yaml_safe_load('germany.yaml')
  File "/home/kbriggs/yaml_loader_01.py", line 54, in yaml_safe_load
    d=load(fy,Loader=Loader)
  File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 71, in load
    return loader.get_single_data()
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 37, in get_single_data
    node = self.get_single_node()
  File "_yaml.pyx", line 707, in _yaml.CParser.get_single_node (ext/_yaml.c:7695)
  File "_yaml.pyx", line 725, in _yaml.CParser._compose_document (ext/_yaml.c:7948)
  File "_yaml.pyx", line 776, in _yaml.CParser._compose_node (ext/_yaml.c:8638)
  File "_yaml.pyx", line 890, in _yaml.CParser._compose_mapping_node (ext/_yaml.c:10186)
  File "_yaml.pyx", line 774, in _yaml.CParser._compose_node (ext/_yaml.c:8613)
  File "_yaml.pyx", line 774, in _yaml.CParser._compose_node (ext/_yaml.c:8613)
  File "_yaml.pyx", line 772, in _yaml.CParser._compose_node (ext/_yaml.c:8588)
  File "/usr/local/lib/python2.7/site-packages/yaml/resolver.py", line 148, in resolve
    if regexp.match(value):
MemoryError

real	4m17.797s
user	3m59.176s
sys	0m12.225s




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