> <snip>
> So when you exec 'a = "B"' in globals(), locals() you might think you
> were changing the local namespace. In fact you are changing a copy of
> the local namespace:
> <snip>
Well, that explains much, but not all that I want to be explained. Why?
Because now I understand, that by invoking
exec "a = 'B'" in globals(), locals()
I can modify only *copies* of the global and local namespaces dicts,
not the dicts themselves. OK, that's why my code doesn't work as I want
it to work.
But why on Earth *the same code* will work, if I remove the assignments
from the marked lines? Why then operating on copies of the local
namespaces dicts *will work* ?
> (...) Even allowing for the
> difficulties you've already experienced, it's nearly always better in
> practical cases to use assignment to the keys of a dictionary. Then no
> exec is required, and you have direct control over your own namespace.
Well... Is this a sugestion, that instead of messing up with the exec
statements used to modify local namespaces I should use dictionaries?
Perhaps you're right. In fact, the problem, that I'm trying to solve is
as follows:
def funcA():
def funcB():
...
var1, var2, var3, ..., varN = ( None, ) * N
t = ( (regexp1, 'var1'), (regexp2, 'var2'), ..., (regexpN, 'varN')
)
for regexp, var_name in t:
match = regexp.match( some_string )
if match != None:
# now the *magic* exec statement comes...
exec var_name + ' = match.groups()[0]' in globals(), locals()
return var1, var2, var3, ..., varN
...
k1, k2, k3, ..., kN = funcB()
Of course, the code presented above doesn't work. It works, if one
change is done in the function funcB:
def funcB():
...
# no creation of any local variables called var1, var2, ..., varN
here
t = ( (regexp1, 'var1'), (regexp2, 'var2'), ..., (regexpN, 'varN')
)
for regexp, var_name in t:
match = regexp.match( some_string )
if match != None:
# now the *magic* exec statement comes...
exec var_name + ' = match.groups()[0]' in globals(), locals()
else:
# here we put the code, that will assign None to the variable
exec var_name + ' = None'
return var1, var2, var3, ..., varN
But I *still* don't understand, why this code works, if I operate on a
copy of the local namespace dict...
Of course, I can do the same thing in a different matter - by using a
dictionary. And perhaps I will. But I still want to know, how the exec
statement works.
* * *
My problem is more complicated, that the presented example. In general,
my problem is: how to create a local variable by executing the Python
code, that isn't known at the moment of writing the program? In another
words: I have to create a local variable, whose name will be known at
the runtime, in a nested function.
Is it possible, or have I to use dictionaries, instead of exec
statement used to modify local namespaces?
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