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Python - quickest way to create class instance from string?


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#1 Mike_Stoddart   Members   -  Reputation: 182

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Posted 23 May 2008 - 06:07 AM

Say I have a python string containing a Python class definition. What is the easiest and quickest way to create an instance of that class, assuming you don't know that class is called but you do know it can derive from one of say four other abstract (so to speak) classes? Currently I'm just using compile and execing the string into a temporary module I create. I then search that module for a class that has one of the allowed base classes. I then eval the class' name to create an instance. Is there a quicker (performance wise) method for doing this? Thanks

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#2 Kylotan   Moderators   -  Reputation: 3324

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Posted 26 May 2008 - 11:06 PM

What about just calling eval on the original string?

#3 Vorpy   Members   -  Reputation: 869

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Posted 27 May 2008 - 03:58 PM

Once you've found the class just use the () operator on it to construct it. Python classes act as constructor functions for themselves. Something like this:

x = searchForClass()
myInstance = x()

#4 Mike_Stoddart   Members   -  Reputation: 182

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Posted 28 May 2008 - 01:24 AM

Quote:
Original post by Vorpy
Once you've found the class just use the () operator on it to construct it. Python classes act as constructor functions for themselves. Something like this:

x = searchForClass()
myInstance = x()



Vorpy: Is 'x' a string here, or the class object?

Kylotan: I'm not sure how eval would work differently from my code below.



# Compile the scriptlet.
code = compile(scriptlet, '<string>', 'exec')

# Create the new 'temp' module.
temp = imp.new_module("temp")
sys.modules["temp"] = temp

exec code in temp.__dict__

# Inspect the module.
classes = inspect.getmembers(temp, callable)
for name,value in classes:
if value.__module__ == "temp":
# Create the instance.
obj = eval("temp." + name)()

break

return obj



Thanks again.

#5 Kylotan   Moderators   -  Reputation: 3324

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Posted 28 May 2008 - 01:59 AM

I guess I didn't entirely understand your first question, that's all.

#6 Vorpy   Members   -  Reputation: 869

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Posted 28 May 2008 - 04:43 PM

In my example, "x" is the class object. In your code, I think it would be "value".

Instead of:

obj = eval("temp." + name)()

it would be:

obj = value()

Using eval is nearly always unnecessary.




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