You may use as many ‘helper_tables’ as you wish. create table t(helper1 varchar2(50) , helper2 varchar2(50) , dataElement varchar2(50) ); insert into t(helper1, helper2, dataelement) with de as(select level lvl from dual connect by level <10) ,h1 as (select lvl, lvl/1.5 hp from de) ,h2 as (select lvl, lvl/2 hp2 from de) select h1.hp , … Read more
The biggest danger of with, outside of pathological conditions like “with A, B, C, D” is that your code can silently change meaning with no notice to you. Consider this example: with TFoo.Create try Bar := Baz; DoSomething(); finally Free; end; You write this code knowing that Bar is a property of TFoo, and Baz … Read more
The new Python 3 contextlib.ExitStack class was added as a replacement for contextlib.nested() (see issue 13585). It is coded in such a way you can use it in Python 2 directly: import sys from collections import deque class ExitStack(object): “””Context manager for dynamic management of a stack of exit callbacks For example: with ExitStack() as … Read more
Like this: import sys class Context(object): def __enter__(self): try: raise Exception(“Oops in __enter__”) except: # Swallow exception if __exit__ returns a True value if self.__exit__(*sys.exc_info()): pass else: raise def __exit__(self, e_typ, e_val, trcbak): print “Now it’s running” with Context(): pass To let the program continue on its merry way without executing the context block you … Read more
Just use a threading.RLock which is re-entrant meaning it can be acquired multiple times by the same thread. http://docs.python.org/library/threading.html#rlock-objects For clarity, the RLock is used in the with statements, just like in your sample code: lock = threading.RLock() def func1(): with lock: func2() def func2(): with lock: # this does not block even though the … Read more
Is it possible to repeat the code within a with statement? No. As pointed out earlier in that mailing list thread, you can reduce a bit of duplication by making the decorator call the passed function: def do_work(): … # This is not ideal! @transaction(retries=3) def _perform_in_transaction(): # Atomic DB statements … # called implicitly … Read more
There are several differences you appear to have missed: Context manager get a chance to provide a new object just for the block you are executing. Some context managers just return self there (like file objects do), but, as an example, database connection objects could return a cursor object tied to the current transaction. Context … Read more
exc_type is the exception’s class. exc_val is the exception instance. exc_tb is a traceback object, of which there is a reference in types.TracebackType. In general it should be the case that type(exc_val) is exc_type exc_val.__traceback__ is exc_tb Note that __exit__ is still invoked when there was no exception raised by the code under a context … Read more
In Python 2.x and 3.0, 3.1 and 3.2, multiprocessing.Pool() objects are not context managers. You cannot use them in a with statement. Only in Python 3.3 and up can you use them as such. From the Python 3 multiprocessing.Pool() documentation: New in version 3.3: Pool objects now support the context management protocol – see Context … Read more
tl;dr RAII is not possible, you mix it up with scoping in general and when you miss those extra scopes you’re probably writing bad code. Perhaps I don’t get your question(s), or you don’t get some very essential things about Python… First off, deterministic object destruction tied to scope is impossible in a garbage collected … Read more