This article is a reference because this cost me about an hour to debug: class test(object): foo = [] a = test() b = test() a.foo.append("bar") print b.foo #['bar'] When...

This article is a reference because this cost me about an hour to debug:

class test(object):
foo = []

a = test()
b = test()
a.foo.append("bar")
print b.foo
#['bar']

When you define the class (test, in this case), any class properties will be instanciated on class-definition and not on object instanciation. Therefore, when you create multiple copies of a class, they will have the same actual object as class properties. You should instanciate those attributes in the init-method.

For this reason, the following code will generate output, even though it looks like it probably shouldn’t, because not a single object was created:
def f():
print "foo"

class test(object):
foo = [f()]

This will obviously not concern you, if you are – as you should – using immutable objects.

This article is going to try to shed some light on how to convert map, reduce and filter (often used with lambda expressions) in python into a syntax conforming to...

This article is going to try to shed some light on how to convert map, reduce and filter (often used with lambda expressions) in python into a syntax conforming to “the python way”. At the bottom you will find a download link to execute this stuff for yourself.

l = range(10)
print l
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
This is going to be the list (l) we’re working on.

What we’ll be looking at first, is iterating through this list and applying a function. And what simpler function could there be, than the identity?
a = map(lambda x: x, l)
b = list(x for x in l)
print a, b
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
If you don’t really know what the we did in (a) you may want to read up on lambda expressions, a topic I am not covering now. We need the list-method in (b) because map() always returns a list and the for-expression coughs up a generator object which we have to fully evaluate and convert to a list, to get the equivalent. Generator objects I will cover in the future. For now, suffice to say, it has a reason we are applying the list-function.

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