以...作为声明依赖python是一种好习惯

Question

I'm curious if it is considered safe or good practice to depend on python's with...as statement. For example when opening a file:

with open("myfile","w") as myFile:
    #do something

So in this example I neglected to explicitly call myFile.close() however I can assume it was called when python exited the with...as statement by calling the objects __exit__() method. Is it good practice/safe to depend upon this or would it be better to always explicitly call file.close()

Answer 1

This is what context managers are for , to rely on them to close the file for you. Context managers are called even if there was an exception.

The alternative is to use an finally block instead:

myFile = open("myfile","w")
try:
    # do something with myFile
finally:
    myFile.close()

but because the block inside of the try: might be long, by the time you get to the finally statement you have forgotten what you were setting this up for.

Context managers are more powerful still. Because the __exit__ method is informed of any exceptions, they can act as exception handlers as well (ignore the exception, raise another, etc.).

Answer 2

Yes, the with statement is better way. Since Python 2.5, the file object has been equipped with __enter__() and __exit__() methods. The __exit__() method closes the file object.

Python guarantees that it will call the __exit__() method, but there is not guarantee that the __exit__() method will close the resource, especially with 3rd party code. You need to manually verify that.