On Wed, 26 Feb 2014 14:46:39 +0100, Peter Otten wrote:
> Steven D'Aprano wrote:
>
>> I have to dynamically generate some code inside a function using exec,
>> but I'm not sure if it is working by accident or if I can rely on it.
>>
>> Here is a trivial example:
>>
>>
>> py> def spam():
>> ... exec( """x = 23""" )
>> ... return x
>> ...
>> py> spam()
>> 23
>>
>>
>> (My real example is more complex than this.)
>>
>> According to the documentation of exec, I don't think this should
>> actually work, and yet it appears to. The documentation says:
>>
>> The default locals act as described for function locals() below:
>> modifications to the default locals dictionary should not be
>> attempted. Pass an explicit locals dictionary if you need to see
>> effects of the code on locals after function exec() returns.
>>
>> http://docs.python.org/3.4/library/functions.html#exec
>>
>>
>> I *think* this means that if I want to guarantee that a local variable
>> x is created by exec, I need to do this instead:
>>
>> py> def eggs():
>> ... mylocals = {}
>> ... exec( """x = 23""", globals(), mylocals) ... x =
>> mylocals['x']
>> ... return x
>> ...
>> py> eggs()
>> 23
>>
>> The fact that it works in spam() above is perhaps an accident of
>> implementation? Yes no maybe?
>
> eggs() should work in Python 2 and 3, spam() should work in Python 2,
> but not in Python 3.
Aha! That explains it -- I was reading the 3.x docs and testing in Python
2.7.
Thanks everyone for answering.
By the way, if anyone cares what my actual use-case is, I have a function
that needs to work under Python 2.4 through 3.4, and it uses a with
statement. With statements are not available in 2.4 (or 2.5, unless you
give a from __future__ import). So after messing about for a while with
circular imports and dependency injections, I eventually settled on some
code that works something like this:
def factory():
blah blah blah
try:
exec("""def inner():
with something:
return something
""", globals(), mylocals)
inner = mylocals['inner']
except SyntaxError:
def inner():
# manually operate the context manager
call context manager __enter__
try:
try:
return something
except: # Yes, a bare except. Catch EVERYTHING.
blah blah blah
finally:
call context manager __exit__
blah blah blah
return inner
(By the way, yes, I have to use a bare except, not just "except
BaseException". Python 2.4 and 2.5 still have string exceptions.)
--
Steven
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