Skip to main content

Using user classes with Python's set operations

Python has an efficient, and built in, implementation of set operations. With a tiny bit of work you can make your user defined classes work nicely with set operations (as well as dictionaries).

Say you have a class:

class A:
  def __init__(self, x, s):
    self.x, self.s = x, s

Python actually doesn't have a way of knowing if the objects are the same or not:

a1 = A(1, 'Hi')
a2 = A(1, 'Hi')
a = set([a1, a2])
-> a1, a2

and by default assumes they are different.

All you have have to do, for things to work properly, is to define a hash function (__hash__()) and an equality (__eq__()) operator for your class. The two functions need to be congruent i.e. if the hash value for two objects is the same, they should also register as equal. It is possible to define the functions differently but this will lead to funny things happening with set operations.

The hash value is an integer. If your class has several attributes that you consider important for distinguishing between objects you can hash each attribute separately and then XOR the individual hashes.

class A:
  def __init__(self, x, s):
    self.x, self.s = x, s

  def __hash__(self):
    return self.x ^ self.s.__hash__()


  def __eq__(self, other):
    return self.__hash__() == other.__hash__()

And now

a1 = A(1, 'Hi')
a2 = A(1, 'Hi')
a = set([a1, a2])
-> just a1 as expected

Comments

Popular posts from this blog

Flowing text in inkscape (Poster making)

You can flow text into arbitrary shapes in inkscape. (From a hint here).

You simply create a text box, type your text into it, create a frame with some drawing tool, select both the text box and the frame (click and shift) and then go to text->flow into frame.

UPDATE:

The omnipresent anonymous asked:
Trying to enter sentence so that text forms the number three...any ideas?
The solution:
Type '3' using the text toolConvert to path using object->pathSize as necessaryRemove fillUngroupType in actual text in new text boxSelect the text and the '3' pathFlow the text

Python: Multiprocessing: passing multiple arguments to a function

Write a wrapper function to unpack the arguments before calling the real function. Lambda won't work, for some strange un-Pythonic reason.


import multiprocessing as mp def myfun(a,b): print a + b def mf_wrap(args): return myfun(*args) p = mp.Pool(4) fl = [(a,b) for a in range(3) for b in range(2)] #mf_wrap = lambda args: myfun(*args) -> this sucker, though more pythonic and compact, won't work p.map(mf_wrap, fl)

Drawing circles using matplotlib

Use the pylab.Circle command

import pylab #Imports matplotlib and a host of other useful modules cir1 = pylab.Circle((0,0), radius=0.75, fc='y') #Creates a patch that looks like a circle (fc= face color) cir2 = pylab.Circle((.5,.5), radius=0.25, alpha =.2, fc='b') #Repeat (alpha=.2 means make it very translucent) ax = pylab.axes(aspect=1) #Creates empty axes (aspect=1 means scale things so that circles look like circles) ax.add_patch(cir1) #Grab the current axes, add the patch to it ax.add_patch(cir2) #Repeat pylab.show()