| msg163095 - (view) |
Author: John Firestone (johnf) |
Date: 2012-06-18 10:59 |
| exec(source, Dict()) doesn't call Dict().__getitem__ or Dict().__missing__ if the source string contains a function and the function references an undefined global. class Dict1(dict): def __getitem__(self, key): print ' __getitem__', repr(key) if key == 's': return None return dict.__getitem__(self, key) class Dict2(dict): def __missing__(self, key): print ' __missing__', repr(key) return None source = """if 1: print ' 1' s def f(): print ' 2' s print ' 3' f()""" print 'Dict1.__getitem__' try: exec(source, Dict1()) except NameError as exc_value: print ' %s: %s' % (exc_value.__class__.__name__, exc_value) print 'Dict2.__missing__' try: exec(source, Dict2()) except NameError as exc_value: print ' %s: %s' % (exc_value.__class__.__name__, exc_value) Python 2.7.3 (v2.7.3:70274d53c1dd, Apr 9 2012, 20:32:06) [GCC 4.0.1 (Apple Inc. build 5493)] on darwin >>> import curiosity Dict1.__getitem__ 1 __getitem__ 's' __getitem__ 'f' 2 NameError: global name 's' is not defined Dict2.__missing__ 1 __missing__ 's' 2 NameError: global name 's' is not defined >>> |
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| msg163097 - (view) |
Author: Mark Dickinson (mark.dickinson) *  |
Date: 2012-06-18 12:12 |
| This looks like a documentation issue: it's well documented that in the exec statement, the globals dictionary must be a dict. What's not so clear from the documentation (AFAICT) is that it must actually have *type* dict, rather than merely being an instance of dict. (Or, from experimentation, it *can* be an instance of a dict subclass, but the underlying C-implemented dict methods are called directly, so overloads for __getitem__ and the like don't have any effect.) |
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| msg163098 - (view) |
Author: John Firestone (johnf) |
Date: 2012-06-18 12:34 |
| I find the behavior inconsistent. As you can see from this example, the exec'uted code *does* call the instance's overloaded __getitem__ and __missing__ methods when outside a function, but doesn't when inside. |
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| msg163099 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2012-06-18 12:50 |
| > As you can see from this example, the exec'uted code *does* call the > instance's overloaded __getitem__ and __missing__ methods when outside a > function, but doesn't when inside. Yep; that's because the 's' and 'f' lookups at top level are *local* lookups, and the 's' lookup from inside the body of 'f' is done as a *global* lookup (as explained in the docs here: [1]). In the exec statement, the locals can be any mapping-like object. The behaviour's a bit clearer if you pass separate globals and locals dictionaries: >>> source = """\ ... print s ... def f(): ... print s ... f() ... """ >>> locals = {'s': 1729} >>> globals = {'s': 31415} >>> exec source in globals, locals 1729 31415 [1] http://docs.python.org/reference/executionmodel.html#interaction-with-dynamic-features |
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| msg163100 - (view) |
Author: Amaury Forgeot d'Arc (amaury.forgeotdarc) * |
Date: 2012-06-18 13:48 |
| With Python3 though, __getitem__ seems called though. OTOH the 'print' symbol is not found, even though the Dict1 got a '__builtins__' entry added. |
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| msg163110 - (view) |
Author: John Firestone (johnf) |
Date: 2012-06-18 17:45 |
| Thank you all for the quick and interesting responses! Here is another example, this time showing a simple s sometimes behaves like globals()['s'] and sometimes doesn't. class Dict(dict): def __getitem__(self, key): if key == 's': return 'got s' return dict.__getitem__(self, key) dct = Dict() dct['the_dict'] = dct print 0, id(dct) source = """if 1: print '1', id(globals()), globals() is the_dict print ' ', globals()['s'] print ' ', s def f(): print '2', id(globals()), globals() is the_dict print ' ', globals()['s'] print ' ', s print '3' f()""" exec(source, dct) Python 2.7.3 (v2.7.3:70274d53c1dd, Apr 9 2012, 20:32:06) [GCC 4.0.1 (Apple Inc. build 5493)] on darwin >>> import curiosity2 0 2459928 1 2459928 True got s got s 2 2459928 True got s Traceback (most recent call last): File "", line 1, in File "curiosity2.py", line 22, in exec(source, dct) File "", line 10, in File "", line 8, in f NameError: global name 's' is not defined >>> |
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| msg163113 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2012-06-18 18:42 |
| Yes, this is definitely a dark corner of Python, and one that it seems worth trying to illuminate a bit in the documentation. |
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