Python with语句上下文管理器两种实现方法分析

logger = open("log.txt", "w")
try:
　　logger.write('Hello ')
　　logger.write('World')
finally:
　　logger.close()
print logger.closed

with open("log.txt", "w") as logger:
　　logger.write('Hello ')
　　logger.write('World')
print logger.closed

class File(object):
 def __init__(self, file_name, method):
  self.file_obj = open(file_name, method)
 def __enter__(self):
  return self.file_obj
 def __exit__(self, type, value, traceback):
  self.file_obj.close()
with File('demo.txt', 'w') as opened_file:
 opened_file.write('Hola!')

#异常抛出，_exit__返回的是True以外的任何东西，那么这个异常将被with语句抛出
class File(object):
 def __init__(self, file_name, method):
  self.file_obj = open(file_name, method)
 def __enter__(self):
  return self.file_obj
 def __exit__(self, type, value, traceback):
  self.file_obj.close()
  print "type:",type
  print "value:",value
  print "traceback:",traceback
with File('demo.txt', 'w') as opened_file:
 opened_file.undefined_function('Hola!')
#output================================================
# type: <type 'exceptions.AttributeError'>
# value: 'file' object has no attribute 'undefined_function'
# traceback: <traceback object at 0x000000000262D9C8>
#  opened_file.undefined_function('Hola!')
# AttributeError: 'file' object has no attribute 'undefined_function'

#异常忽略，__exit__返回的是True，那么这个异常就被忽略。
class File(object):
 def __init__(self, file_name, method):
  self.file_obj = open(file_name, method)
 def __enter__(self):
  return self.file_obj
 def __exit__(self, exception_type, exception_value, traceback):
  print("Exception has been handled")
  self.file_obj.close()
  return True
with File('demo.txt', 'w') as opened_file:
 opened_file.undefined_function('Hola!')
# output==================================
# Exception has been handled

# coding:utf-8
import contextlib
@contextlib.contextmanager
def myopen(filename, mode):
 f = open(filename, mode)
 try:
  yield f.readlines()
 except Exception as e:
  print e
 finally:
  f.close()
if __name__ == '__main__':
 with myopen(r'c:\ip2.txt', 'r') as f:
  for line in f:
   print line

class File(object):
 def __init__(self, file_name, method):
  self.file_obj = open(file_name, method)
 def __enter__(self):
  return self.file_obj
 def __exit__(self, exception_type, exception_value, traceback):
  self.file_obj.close()
  return True
with File('demo.txt', 'w') as f1,File('demo.txt','w') as f2:
 print f1,f2
# Output============================# <open file 'demo.txt', mode 'w' at 0x000000000263D150> <open file 'demo.txt', mode 'w' at 0x000000000263D1E0>