Insert list of attributes into list of objects in python

Question

How do I insert a list (or numpy array) containing attribute values into a list of objects (as shown below)?

class myClass(object):
    def __init__(self, attr):
        self.attr = attr
        self.other = None

objs = []
for i in range(10):
        objs.append(myClass(i))

attrs = [o.attr for o in objs]
print attrs
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

[o.attr for o in objs] = range(10)
#SyntaxError: can't assign to list comprehension

This is the inverse problem to Extract list of attributes from list of objects in python.

As a note, your object creation could be a list comp too: objs = [myClass(i) for i in range(10)]. — Gareth Latty
– Gareth Latty, Commented Jul 4, 2013 at 11:56

John Zwinck · Accepted Answer · 2013-07-04 10:42:06Z

3

I'd do something like this:

for i, o in enumerate(objs):
    o.attr = i

enumerate(objs) is sort of like zip(range(len(objs)), objs), so in case you actually want to take values from another sequence, you can:

for i, o in zip(sequence, objs):
    o.attr = i

For efficiency you might use itertools.izip there too.

answered Jul 4, 2013 at 10:42

John Zwinck

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2 Comments

Gareth Latty Over a year ago

+1, I was just about to suggest you might want to give a more general solution, but then it appeared. As a tiny note on style, personally, I'd make sequence and obj appear the other way around in the call to zip() (and naturally swapping i, o to o, i to keep the functionality the same) as it would then mirror the order the objects are used in the code. Not important or changing any functionality, but I'd argue it's a little more readable.

John Zwinck Over a year ago

@Lattyware: I'd swap the order in the zip too in practice, but here I kept it the same to avoid distracting from the point that the zip with range is equivalent to enumerate.

ali_m · Accepted Answer · 2013-07-04 12:14:21Z

1

You can also use obj.__setattr__() within a list comprehension:

[o.__setattr__('attr',v) for o,v in zip(objs,range(10)[::-1])]
print [o.attr for o in objs]
#[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

edited Jul 4, 2013 at 12:14

answered Jul 4, 2013 at 10:56

ali_m

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6 Comments

Gareth Latty Over a year ago

-1 - this is reimplementing the wheel (setattr() exists), and it's using list comprehensions for side effects, which is a terrible idea.

ali_m Over a year ago

Quite right, that was very sloppy of me. Here's a slightly less moronic answer that uses the built in __setattr__ method, although if it was me I'd still use John Zwinck's approach.

Gareth Latty Over a year ago

Why access the magic method directly? And this still uses a list comprehension for side effects.

ali_m Over a year ago

I know it's ugly, just pointing out that it can actually be done in a list comprehension if you don't mind courting the hatred of other Python programmers :)

akk Over a year ago

@Lattyware interesting to see the pythonic aversion towards 'comp lists for side effects', coming from Matlab I would have assumed that if a single code line is used to get the data, a single code line can be used to set the data... I guess this explains it stackoverflow.com/questions/5753597/…

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