Re: [Tutor] executing dynamic code with exec?
Thanks Steve for your help (and the humor). I can see that it was a bad idea
with your explanation. (I just didn't want to type all that extra code :-))
I am going to re-write it using your dict approach - that looks a lot cleaner
Thanks!
Chris Hare
ch...@labr.net
http://www.labr.net
On Dec 1, 2011, at 11:25 PM, Steven D'Aprano wrote:
Chris Hare wrote:
What I am trying to do is create a set of variables based upon the table
names in the table variables. I have similar code which dynamically
creates check buttons and the associated grid. But I suspect those won't
work either.
What have I got wrong?
Everything! wink
Seriously though, your basic approach is the wrong approach. Don't try to
create dynamic variables like that. Suppose you succeed:
varName = 'x' # read from a file, or something
exec('%s = 1' % varName) # creates the variable x
Great. Now you have a variable x. Later on, how do you use it?
# much later on in your code...
y = x + 1
But that won't work, because you don't know that it's called x! If you knew
it was called x, you would have just written x = 1 early and not needed exec.
Working with dynamic variable names is a pain and a nightmare. Don't do it.
Even if you succeed, you are making a rod for your own back: maintaining such
code is horrible.
The right way to do this is almost always to use a data structure that maps
names to values, in other words, a dict.
varName = 'x' # read from a file, or something
data = {varName: 1}
# ...
# much later
y = data[varName] + 1
In this case, something like:
names = [Farm, Animals, AnimalTypes, Users, Roles,
Capabilities, Pedigrees, ChipMaker, Owner, Providers,
RegistryL
]
self.cbReadTable = {}
for name in names:
self.cbReadTable[name] = IntVar()
And that's it. Instead of retrieving instance.cbFarmRead, use
instance.cbReadTable['Farm'].
If you absolutely must use instance attributes, perhaps because you think
you're writing Javascript wink, then:
for name in names:
name = 'cb' + name 'Read'
setattr(self, name, IntVar())
And best of all, you avoid the code injection security vulnerability where
somebody manages to fool your code into using a list of table names like:
names = [Farm, Animals, AnimalTypes, Users, Roles,
Capabilities, Pedigrees,
ChipMaker=1;import os;os.system('echo you are pwned rm-rf haha');,
Owner, Providers, RegistryL
]
Hope your backups are really good.
http://xkcd.com/327/
--
Steven
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[Tutor] executing dynamic code with exec?
I have this code chunk: tables = [Farm, Animals, AnimalTypes,Users,Roles,Capabilities,Pedigrees,ChipMaker,Owner,Providers,RegistryL] for x in tables: cmd = self.cb + x + Read = IntVar() exec cmd in locals(), globals() When the code is executed, I get the following error: File z.py, line 4398 exec cmd in locals(), globals() SyntaxError: function 'showRoles' uses import * and bare exec, which are illegal because it contains a nested function with free variables What I am trying to do is create a set of variables based upon the table names in the table variables. I have similar code which dynamically creates check buttons and the associated grid. But I suspect those won't work either. What have I got wrong? Thanks! Chris___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] executing dynamic code with exec?
Chris Hare wrote:
What I am trying to do is create a set of variables based upon the table
names in the table variables. I have similar code which dynamically
creates check buttons and the associated grid. But I suspect those won't
work either.
What have I got wrong?
Everything! wink
Seriously though, your basic approach is the wrong approach. Don't try to
create dynamic variables like that. Suppose you succeed:
varName = 'x' # read from a file, or something
exec('%s = 1' % varName) # creates the variable x
Great. Now you have a variable x. Later on, how do you use it?
# much later on in your code...
y = x + 1
But that won't work, because you don't know that it's called x! If you knew it
was called x, you would have just written x = 1 early and not needed exec.
Working with dynamic variable names is a pain and a nightmare. Don't do it.
Even if you succeed, you are making a rod for your own back: maintaining such
code is horrible.
The right way to do this is almost always to use a data structure that maps
names to values, in other words, a dict.
varName = 'x' # read from a file, or something
data = {varName: 1}
# ...
# much later
y = data[varName] + 1
In this case, something like:
names = [Farm, Animals, AnimalTypes, Users, Roles,
Capabilities, Pedigrees, ChipMaker, Owner, Providers,
RegistryL
]
self.cbReadTable = {}
for name in names:
self.cbReadTable[name] = IntVar()
And that's it. Instead of retrieving instance.cbFarmRead, use
instance.cbReadTable['Farm'].
If you absolutely must use instance attributes, perhaps because you think
you're writing Javascript wink, then:
for name in names:
name = 'cb' + name 'Read'
setattr(self, name, IntVar())
And best of all, you avoid the code injection security vulnerability where
somebody manages to fool your code into using a list of table names like:
names = [Farm, Animals, AnimalTypes, Users, Roles,
Capabilities, Pedigrees,
ChipMaker=1;import os;os.system('echo you are pwned rm-rf haha');,
Owner, Providers, RegistryL
]
Hope your backups are really good.
http://xkcd.com/327/
--
Steven
___
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http://mail.python.org/mailman/listinfo/tutor
