dynamic class/module use? (like Java's forName)

[Alex Hunsley]

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

As a pseudocode example, I'm looking for the following ability:
classIWantToIns tantiate = "packagenameher e.classNameHere "

anInstanceOfTha tClass = makeNewClassFro mString(classIW antToInstantiat e,
parameters[])

# now anInstanceOfTha tClass refers to an instantied
# packagenamehere .classNameHere object!

thanks
alex

[Alex Hunsley]

Alex Hunsley wrote:

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

As a pseudocode example, I'm looking for the following ability:
classIWantToIns tantiate = "packagenameher e.classNameHere "

anInstanceOfTha tClass = makeNewClassFro mString(classIW antToInstantiat e,
parameters[])

# now anInstanceOfTha tClass refers to an instantied
# packagenamehere .classNameHere object!

thanks
alex

Oops.. I goggled but not hard enough obviously! I found my answer on
google groups:

http://shorterlink.com/?N2HPGU

alex

[Roy Smith]

In article <_O************ *****@fe2.news. blueyonder.co.u k>,
Alex Hunsley <la**@tardis.ed .ac.molar.uk> wrote:

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

Just out of curiosity, why do you want to do that?

The google link you provided talked about eval and exec. My experience
is that when I first started using Python (about 7 years ago), I used
eval and exec a lot. Now, I can't remember the last time I ever wanted
to use either.

Can you give us an example of what you're trying to do? My guess,
having not yet seen it, is that there's probably a better way to do it
than what you're asking.

[Christos TZOTZIOY Georgiou]

On Sat, 17 Jul 2004 18:00:26 GMT, rumours say that Alex Hunsley
<la**@tardis.ed .ac.molar.uk> might have written:

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

[snip]

Have you already considered the 'new' module? If not, give it a try.
--
TZOTZIOY, I speak England very best,
"Tssss!" --Brad Pitt as Achilles in unprecedented Ancient Greek

[Alex Hunsley]

Roy Smith wrote:

In article <_O************ *****@fe2.news. blueyonder.co.u k>,
Alex Hunsley <la**@tardis.ed .ac.molar.uk> wrote:

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

Just out of curiosity, why do you want to do that?

The google link you provided talked about eval and exec. My experience
is that when I first started using Python (about 7 years ago), I used
eval and exec a lot. Now, I can't remember the last time I ever wanted
to use either.

Can you give us an example of what you're trying to do? My guess,
having not yet seen it, is that there's probably a better way to do it
than what you're asking.

Ok, the situation is that I am parsing a file that has "actions" (or
commands) embedded in it. Different actions cause different things to
happen to the file which is being preprocessed for something. Now, since
I will have many actions, and want my python to be extensible so other
users can add their own actions, rather than hard coding each action
like so:

if (actionString == "blah"):
blahThang = blah(constructo r stuff)
blahThang.doSom ething()
else if (actionString == "blah2"):
blahThang2 = blah2(construct or stuff)
blahThang2.doSo mething()
else if (actionString == "blah3"):
blahThang3 = blah3(construct or stuff)
blahThang3.doSo mething()
else if (actionString == "blah2"):
# etc etc

.... I can have a different python class (and hence source file) for each
action, and the code for launching an action becomes generic (i.e.
creates instance of the class based on the command string and calls
agreed methods on it). I prefer this to the altyernative of hardcoding
each action with an 'if', as above, and then getting another user who
wants to extend my project have to add yet another if clause as well as
some code.
Another advantage of this generic approach is that it encourages other
users extending my code to put actions in seperate files, one per
action, rather than adding 'magic' code into the above 'if' chaos.

Basically, modular actions is the idea!

I'd be keen to hear peoples thoughts on my approach.

thanks
alex

[Alex Hunsley]

Roy Smith wrote:

In article <_O************ *****@fe2.news. blueyonder.co.u k>,
Alex Hunsley <la**@tardis.ed .ac.molar.uk> wrote:

Does python provide a way to dynamically use modules and/or classes?
I'm thinking in the vein of Java's Class.forName.

Just out of curiosity, why do you want to do that?

p.s. to my last response:

I've just read Robert Brewer's reply, and to rephrase myself as I to
him: I'm trying to do the strategy pattern in a modular fashion. (i.e.
no big string of 'ifs' and hardcoded actions.)

alex

[Terry Reedy]

"Alex Hunsley" <la**@tardis.ed .ac.molar.uk> wrote in message
news:8g******** **********@fe2. news.blueyonder .co.uk...

if (actionString == "blah"):
blahThang = blah(constructo r stuff)
blahThang.doSom ething()
else if (actionString == "blah2"):
blahThang2 = blah2(construct or stuff)
blahThang2.doSo mething()
else if (actionString == "blah3"):
blahThang3 = blah3(construct or stuff)
blahThang3.doSo mething()
else if (actionString == "blah2"):
# etc etc

The Python idiom for something like the above is to use a dict instead of
if/else.
It is especially useful when you want to add alternatives at runtime.
Something like

actions = { 'blah':blah, 'blah2', blah2, 'blah3':blah3, ...}
# easily extended at runtime
....
actions[actionString](constructor_st uff).doSomethin g

Terry J. Reedy

[Paul McGuire]

"Alex Hunsley" <la**@tardis.ed .ac.molar.uk> wrote in message
news:8g******** **********@fe2. news.blueyonder .co.uk...
<snip>

Ok, the situation is that I am parsing a file that has "actions" (or
commands) embedded in it. Different actions cause different things to
happen to the file which is being preprocessed for something. Now, since
I will have many actions, and want my python to be extensible so other
users can add their own actions, rather than hard coding each action
like so:

if (actionString == "blah"):
blahThang = blah(constructo r stuff)
blahThang.doSom ething()
else if (actionString == "blah2"):
blahThang2 = blah2(construct or stuff)
blahThang2.doSo mething()
else if (actionString == "blah3"):
blahThang3 = blah3(construct or stuff)
blahThang3.doSo mething()
else if (actionString == "blah2"):
# etc etc

<snip>

Alex,

Have a look at how pyparsing (http://pyparsing.sourceforge.net) supports the
attachment of actions to parse expressions within a grammar.

-- Paul

[510046470588-0001]

Roy Smith <ro*@panix.co m> writes:

Just out of curiosity, why do you want to do that?

in order to be more flexible,
allowing code to be integrated that is available
only at run time of the program

Klaus Schilling

[Alex Hunsley]

Terry Reedy wrote:

"Alex Hunsley" <la**@tardis.ed .ac.molar.uk> wrote in message
news:8g******** **********@fe2. news.blueyonder .co.uk...

if (actionString == "blah"):
blahThang = blah(constructo r stuff)
blahThang.doSom ething()
else if (actionString == "blah2"):
blahThang2 = blah2(construct or stuff)
blahThang2.doSo mething()
else if (actionString == "blah3"):
blahThang3 = blah3(construct or stuff)
blahThang3.doSo mething()
else if (actionString == "blah2"):
# etc etc

The Python idiom for something like the above is to use a dict instead of
if/else.
It is especially useful when you want to add alternatives at runtime.
Something like

actions = { 'blah':blah, 'blah2', blah2, 'blah3':blah3, ...}
# easily extended at runtime
...
actions[actionString](constructor_st uff).doSomethin g

Terry J. Reedy

Interesting, I hadn't thought of using a dict.
Using a dict would be better than 'if's, but would still require a
central part of the code to know about all the actions!
My way allows new classes to be presented and used without altering
existing code.
Also my method forces new actions to interface with 'central' code via
the available official interface, rather than having new actions
potentially put in the central code and hence have access to things
directly (which isn't a good OO design).