With statements in Python

with-statement.py

# Java style of file handling
# Good example of sandwich code

filename='test.txt'
try:
	file = open(filename)
	print(file.readlines())
except Exception as e:
	print(e.args[0])
finally:
	file.close()
	
# Output  
# ['This \n', ' is \n', ' temp\n']
#-----------------------------------------------------------------------------
# Using with statement to read text from file
# file closing will be done automatically

with open('/Users/prasanna/test.txt') as file:
  print(file.readlines())
  
# --------------------------------------------------------------------
# Custom implementation of open

class MyOpen:
	def __init__(self, filename):
		self.filename = filename

# Gaurd methods
# whatever returns here can be used inside the with block (eg: file)
	def __enter__(self):
		self.file = open(self.filename)
		return self.file
	
# This magic method will be called once the execution is complete
# if it returns true, then exceptions will be swallowed
	def __exit__(self, type, value, traceback):
		self.file.close()

# MyOpen can be used in same way as open and we know what is under the hood.
with MyOpen('text.txt') as file:
	print(file.readlines())
	
# http://effbot.org/zone/python-with-statement.htm