On Sat, Mar 14, 2009 at 9:58 PM, Nick Coghlan wrote:

I missed the discussion about potentially adding syntactic support for multiple context managers in the with statement, but figured I should mention a real limitation of contextlib.nested that *would* be fixed by adding dedicated syntactic support.

There's a genuine semantic difference between this:

with cmA(): with cmB(): # whatever

and this:

with nested(cmA(), cmB()): # whatever

The latter is actually more accurately translated as:

mgr1, mgr2 = cmA(), cmB(): with mgr1: with mgr2: # whatever

That is, when using nested() the later context managers are created outside the scope of the earlier context managers.

So, to use Christian's example from the previous discussion:

with lock: with open(infile) as fin: with open(outfile, 'w') as fout: fout.write(fin.read())

Using contextlib.nested for that would be outright broken:

with nested(lock, open(infile), open(outfile) as (_, fin, fout): fout.write(fin.read())

1. The files are opened without acquiring the lock first 2. If an IOError is raised while opening "outfile", then "infile" doesn't get closed immediately

I created issue 5491 [1] to point out that the contextlib.nested docs could do with being tweaked to make this limitation clearer.

Dedicated syntax (such as the form that Christian proposed) would fix this problem:

with lock, (open(infile) as fin), (open(outfile, 'w') as fout): fout.write(fin.read())

Of course, a custom context manager doesn't suffer any problems either:

@contextmanager def synced_io(lock, infile, outfile): with lock: with open(infile) as fin: with open(outfile) as fout: yield fin, fout

with synced_io(lock, infile, outfile) as (fin, fout): fout.write(fin.read())

fwiw, I believe you could write a version of nested that generates the above code based on the parameters but I believe it'd be disgustingly slow... @contextmanager def slow_nested(*args): code_lines = [] vars = [] code_lines.append('@contextmanager') code_lines.append('def _nested(*args):') for idx in xrange(len(args)): vars.append('c%d' % idx) code_lines.append('%swith args[%d] as %s:' % (' '*(idx+1), idx, vars[-1])) code_lines.append('%syield %s' % (' '*(len(args)+1), ','.join(vars))) code = '\n'.join(code_lines) print 'CODE:\n', code exec(code) yield _nested(*args)

Cheers, Nick.

[1] http://bugs.python.org/issue5491

-- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia --------------------------------------------------------------- _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

Oh, your right! +1 for adding the new syntax. Nick Coghlan wrote:

I missed the discussion about potentially adding syntactic support for multiple context managers in the with statement, but figured I should mention a real limitation of contextlib.nested that *would* be fixed by adding dedicated syntactic support.

There's a genuine semantic difference between this:

with cmA(): with cmB(): # whatever

and this:

with nested(cmA(), cmB()): # whatever ...