create properties with loop

Question

I would like to create object properties using a loop, and the setattr function. The problem i run into is that all the properties created with such a loop are equal to one another.

In the following example i create a class A with a single attribute z, a complex number. i then would like to loop over functions such as numpy's real and imag to create other properties, such as z_real and z_imag. The problem then, in the context of this example, is that z_real == z_imag is True.

import numpy as np

class A(object):
    def __init__(self, z):
        self.z = z

        for func in [np.real, np.imag]:
            fget = lambda x: func(x.z)
            print('self.z_%s = %i' % (func.__name__, fget(self)))
            setattr(self.__class__, 'z_%s' % (func.__name__),
                    property(fget))


a = A(1+2j)

print ('a.z_real=%i'% a.z_real)
print ('a.z_imag=%i'% a.z_imag)

running this yields

self.z_real = 1
self.z_imag = 2
a.z_real=2    
a.z_imag=2

It is not clear to me why this is.

because i want to create many properties in this manner.

alex
– alex

2012-03-04 00:30:47 +00:00
Commented Mar 4, 2012 at 0:30 — alex
– alex, Commented Mar 4, 2012 at 0:30

alex · Accepted Answer · 2012-03-04 01:43:27Z

1

You're doing it wrong.

Don't create more then one instance of the same closure per normal function!

fget = lambda x, f=func: f(x.z)

edited Mar 4, 2012 at 1:43

alex

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answered Mar 4, 2012 at 0:18

Ignacio Vazquez-Abrams

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3 Comments

alex Over a year ago

thanks! i browsed that link, and not clear on the issue, there is actually a reasonable debate in the comments. could be characterized as a sloppy use of variable scope? because func was used in the lambda function, but not passed.

Ignacio Vazquez-Abrams Over a year ago

It is exactly an issue with scope.

Eric O. Lebigot Over a year ago

Here is are two good links on this closure issue: utcc.utoronto.ca/~cks/space/blog/python/WhatClosuresClose and utcc.utoronto.ca/~cks/space/blog/python/CPythonCellsClosures.

Vojislav Stojkovic · Accepted Answer · 2012-03-04 00:22:12Z

1

Your fget is a closure. It closed over the variable func. After the loop is finished, func remains set to np.imag.

You should do the following afer self.z = z:

def make_getter(func):
    return lambda x: func(x.z)

for func in [np.real, np.imag]:
    fget = make_getter(func)

etc.

answered Mar 4, 2012 at 0:22

Vojislav Stojkovic

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1 Comment

alex Over a year ago

yes, that has been my work around, but i didnt know why my original way dint work untill now.

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