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Using python.....I have a list that contain names. I want to use each item in the list to create instances of a class. I can't use these items in their current condition (they're strings). Does anyone know how to do this in a loop.

class trap(movevariables):
    def __init__(self):
        movevariables.__init__(self)
        if self.X==0:
            self.X=input('Move Distance(mm) ')
        if self.Vmax==0:
            self.Vmax=input('Max Velocity? (mm/s)  ')
        if self.A==0:
            percentg=input('Acceleration as decimal percent of g'  )
            self.A=percentg*9806.65
        self.Xmin=((self.Vmax**2)/(2*self.A))
        self.calc()
    def calc(self):
        if (self.X/2)>self.Xmin:
            self.ta=2*((self.Vmax)/self.A)                # to reach maximum velocity, the move is a symetrical trapezoid and the (acceleration time*2) is used
            self.halfta=self.ta/2.                               # to calculate the total amount of time consumed by acceleration and deceleration
            self.xa=.5*self.A*(self.halfta)**2
        else:                                                               # If the move is not a trap, MaxV is not reached and the acceleration time is set to zero for subsequent calculations                                                        
            self.ta=0
        if (self.X/2)<self.Xmin:
            self.tva=(self.X/self.A)**.5
            self.halftva=self.tva/2
            self.Vtriang=self.A*self.halftva
        else:
            self.tva=0
        if (self.X/2)>self.Xmin:
            self.tvc=(self.X-2*self.Xmin)/(self.Vmax)  # calculate the Constant velocity time if you DO get to it
        else:
            self.tvc=0
        self.t=(self.ta+self.tva+self.tvc)
        print self

I'm a mechanical engineer. The trap class describes a motion profile that is common throughout the design of our machinery. There are many independent axes (trap classes) in our equipment so I need to distinguish between them by creating unique instances. The trap class inherits from movevariables many getter/setter functions structured as properties. In this way I can edit the variables by using the instance names. I'm thinking that I can initialize many machine axes at once by looping through the list instead of typing each one.

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    What do you need these strings to be? Need more specifics to even try to answer this. Commented Aug 28, 2009 at 13:31
  • The strings will be the new instance(s) for a class. I'm use to creating instances like this-----new=class(). I want to be able to get the 'new' out of a list. Commented Aug 28, 2009 at 13:58
  • Can you give us an example of the list? Commented Aug 28, 2009 at 14:12
  • 1
    @Tgatman, can you edit your question add those new details? what is class here? class is a keyword in python. What are you trying to achieve? Write all the details in the question to get the best help. Commented Aug 28, 2009 at 14:19
  • 1
    @Tgatman, can you post the class "trap" source code? Just edit the question and add the code. I'm trying to help. Commented Aug 28, 2009 at 14:36

5 Answers 5

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2

You could use a dict, like:

classes = {"foo" : foo, "bar" : bar}

then you could do:

myvar = classes[somestring]()

this way you'll have to initialize and keep the dict, but will have control on which classes can be created.

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2

The getattr approach seems right, a bit more detail:

def forname(modname, classname):
    ''' Returns a class of "classname" from module "modname". '''
    module = __import__(modname)
    classobj = getattr(module, classname)
    return classobj

From a blog post by Ben Snider.

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1

If it a list of classes in a string form you can:

classes = ['foo', 'bar']
for class in classes:
    obj = eval(class)

and to create an instance you simply do this:

instance = obj(arg1, arg2, arg3)
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EDIT

If you want to create several instances of the class trap, here is what to do:

namelist=['lane1', 'lane2']
traps = dict((name, trap()) for name in namelist)

That will create a dictionary that maps each name to the instance.

Then to access each instance by name you do:

traps['lane1'].Vmax
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you're probably looking for getattr.

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