问题
with open("hello.txt", "wb") as f:
f.write("Hello Python!\n")
seems to be the same as
f = open("hello.txt", "wb")
f.write("Hello Python!\n")
f.close()
What's the advantage of using open .. as instead of f = ? Is it just syntactic sugar? Just saving one line of code?
回答1:
In order to be equivalent to the with statement version, the code you wrote should look instead like this:
f = open("hello.txt", "wb")
try:
f.write("Hello Python!\n")
finally:
f.close()
While this might seem like syntactic sugar, it ensures that you release resources. Generally the world is more complex than these contrived examples and if you forget a try.. except... or fail to handle an extreme case, you have resource leaks on your hands.
The with statement saves you from those leaks, making it easier to write clean code. For a complete explanation, look at PEP 343, it has plenty of examples.
回答2:
If f.write throws an exception, f.close() is called when you use with and not called in the second case. Also f has a smaller scope and the code is cleaner when using with.
回答3:
The former still closes f if an exception occurs during the f.write().
来源：https://stackoverflow.com/questions/2738365/whats-the-advantage-of-using-with-as-statement-in-python