Can you create a class from a string name

Vivek Sawant vivek-sawant at verizon.net
Sun Mar 2 02:17:39 CET 2003


My apologies. Didn't notice the reply from Alex Martelli, which already 
answers the following question. Now, I am able to do entirely what I 
describe below using __import__ followed by class and object instantiation.

Thanks a lot to all who replied.

\vivek

Vivek Sawant wrote:

> Lenard,
>
> Thanks!! That worked. I need the first method of creating class and 
> object instances as I am not creating class definitions on the fly.
>
> Another question though... To be able to use the method suggested by 
> you (obj = eval ('<classname>'), I need to have already imported the 
> class that I am trying to instantiate. Here's what I am really trying 
> to do. I propose to support an applicaiton-specific configuration file 
> which, along with other app-specific parameters, will also contain the 
> names of the classes to be instantiated. This will enable the users of 
> this application to use alternate implementations of a class by just 
> changing the config file.
>
> Given this, the '<classname>' string will come from this config file 
> at the runtime. It would be nice, if the module that parses the config 
> file does not have to know which all classes to import when I write 
> the module.
>
> I (naively) tried eval ('import <module>'). That produced syntax error :-(
> Any suggestions?
>
> \vivek
>
> Lenard Lindstrom wrote:
>
>>"Vivek Sawant" <vivek-sawant at verizon.net> wrote in message
>>news:3E611ABE.2050102 at verizon.net...
>>  
>>
>>>Hi,
>>>
>>>is there a way to create a class object or an instance object for a
>>>class if you have the name of the class as a string at the runtime.
>>>
>>>    
>>>
>>
>>You can use Python's dynamic execution features to do what you want. To
>>retrieve a reference to an existing class object use the built-in 'eval'
>>function.
>>
>>classobj = eval('<classname>')
>>
>>Create an instance of the class by calling the returned object.
>>
>>classinstance = classobj(<constructor args>)
>>
>>To define an entirely new class at run-time use the 'exec' statement. The
>>following example creates a minimal class, but the definition can be as
>>complicated as you need.
>>
>># Class name is defined by a string.
>>classname = 'mynewclass'
>>
>># The class definition is also a string. The '%s' will be replaced by the
>>class name later.
>># When writing a compound statement as a string make sure the indentation
>>method
>># is consistent, e.g. four spaces. See section 2.4.1 in Python Language
>>Manual
>># for more on triple-quoted strings.
>>definition = """class %s:
>>    pass
>>"""
>>
>># Execute the class definition statement. Insert the class name using a
>># string formatting command '%' (section 2.2.6.2 in Python Library
>>Reference).
>>exec definition % classname
>>
>># Create an instance of the class.
>>newinstance = eval(classname)()
>>
>># This also works, but only when classname == 'mynewclass'.
>>newinstance = mynewclass()
>>
>>  
>>
>>>For example, in Java you can create a 'Class' object as:
>>>
>>>Class.forname ('<classname>')
>>>
>>>Thanks.
>>>
>>>\vivek
>>>
>>>    
>>>
>>
>>The Java Class.forName method returns a reference to an already existing
>>'Class' object. It does not create a new 'Class' object. It will not create
>>a new class at run time.
>>
>>I hope this helps.
>>
>>Lenard Lindstrom
>>
>>
>>
>>
>>
>>  
>>
>

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