exec and locals

Steven D'Aprano steve at pearwood.info
Thu Feb 27 01:25:45 CET 2014


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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