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Instantiating a class from a list

Using python ..... I have a list containing the names. I want to use every item in the list to create instances of the class. I cannot use these items in their current state (they are 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 am a mechanical engineer. The trap class describes a motion profile that is common to the entire design of our equipment. Our hardware has many independent axes (trap classes), so I need to differentiate between them by creating unique instances. The trap class inherits many getter / setter functions from movevariables, structured like properties. This way I can edit the variables using the instance names. I think I can initialize many axes of the machine at once by going through the list rather than printing each one.

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5 answers


You can use a dict like:

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

then you can do:



myvar = classes[somestring]()

this way you will have to initialize and store the dict, but will have control over which classes can be instantiated.

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Getattr's approach seems to be correct, in a little 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 Snyder .

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

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

and create an instance that you just do:

instance = obj(arg1, arg2, arg3)
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EDIT

If you want to create multiple instances of a class hook, here's what to do:

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

This will create a dictionary that maps each name to an instance.

Then to access each instance by name, you do:

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

.

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