Am 15.03.2012 12:48 schrieb Kiuhnm:
On 3/15/2012 12:14, Thomas Rachel wrote:
Am 15.03.2012 11:44 schrieb Kiuhnm:
Let's try that.
Show me an example of "list comprehensions" and "with" (whatever they
are).
with open("filename", "w") as f:
f.write(stuff)
Here f is created before executing the block and destroyed right after
leaving the block. f's destructor will probably close the file handle.
No, that is the point here: with calls __enter__ on entry and __exit__
on, well, exit.
In the case of files, __enter__ doesn't probably do anything special,
but returns the object again in order to be assigned to f. In __exit__,
the file is closed.
with open("/tmp/filename", "w") as f:
print f
print f
<open file '/tmp/filename', mode 'w' at 0xb74e6d30>
<closed file '/tmp/filename', mode 'w' at 0xb74e6d30>
So after the with clause, f is actually closed, but still present as object.
with lock:
do_something_exclusively()
It's clear what it does, but I don't know if that's special syntax.
If you call "with" special syntax, it is.
Maybe objects can have two special methods that are called respect. on
entering and leaving the with-block.
Exactly, see above.
Here, on entry __enter__ is called which acquires the lock.
__exit__ releases it again.
Or, more likely, lock creates an object which keeps the lock "acquired".
The lock is released when we leave the block.
So we could inspect the lock with
with lock as l:
inspect l...
do_some.....
Or just inspect l - I don't know if a lock's __enter__ methos returns it
again for assignment with "as"...
Thomas
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