May 9, 2023
In this article, we'll look at context managers and how they can be used with Python's "with" statements and how to create our own custom co
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In this article, we'll look at context managers and how they can be used with Python's "with" statements and how to create our own custom co
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Python Tutorial: Context Managers - Efficiently Managing Resources #python In this Python Programming Tutorial, we will be learning how to use context managers to properly manage resources.
Context managers inside coroutines
I suspected that context managers inside coroutines should have their __exit__ methods invoked when a coroutine generator is closed. But I wanted to be sure, so I wrote a couple simple programs to test this theory.
def coroutine(fout): with fout: while True: value = (yield) fout.write(value) if __name__ == '__main__': fout = open('test.txt', 'w') gen = coroutine(fout) print 'fout is closed:', fout.closed gen.next() # prime the generator gen.send('hello\n') gen.send('world\n') gen.send('radical\n') gen.close() print 'fout is closed:', fout.closed
This works exactly as expected. The output is:
fout is closed: False fout is closed: True
But what about context managers that you create on your own?
from contextlib import contextmanager @contextmanager def tag(name): print "<%s>" % name try: yield finally: print "</%s>" % name def coroutine(name): with tag(name): while True: value = (yield) print value if __name__ == '__main__': # Normal usage: with tag('h1'): print 'foo ' print 'bar ' print 'basilisk ' # Using with a coroutine: gen = coroutine('h2') gen.next() # prime the generator gen.send('hello') gen.send('world') gen.send('gorgonzola') gen.close()
Again, this works as expected. The output is:
<h1> foo bar basilisk </h1> <h2> hello world gorgonzola </h2>
The key is to make sure that you have a try-finally block in your context manager function. Without it, the closing tag will NOT appear in the output when you close the generator.