Initialize a list of objects in Python

Question

Hi all,

I'm a looking to initialize an array/list of objects that are not empty -- the class constructor generates data. In C++ and Java I would do something like this:

Object lst = new Object[100];

I've dug around, but is there a Pythonic way to get this done? This doesn't work like I thought it would(I have 100 references to the same object):

lst = [Object()]*100

But this seems to:

lst = [Object() for i in range(100)]

List comprehension seems (intellectually) like "a lot" of work for something that's so simple in Java.

Thanks!

Answer 1

There isn't a way to implicitly call an Object() constructor for each element of an array like there is in C++ (recall that in Java, each element of a new array is initialised to null for reference types).

I would say that your list comprehension method is the most Pythonic:

lst = [Object() for i in range(100)]

If you don't want to step on the lexical variable i, then a convention in Python is to use _ for a dummy variable whose value doesn't matter:

lst = [Object() for _ in range(100)]

For an equivalent of the similar construct in Java, you can of course use *:

lst = [None] * 100

Answer 2

lst = [Object() for i in range(100)]

Since an array is it's own first class object in python I think this is the only way to get what you're looking for. * does something crazy.

Answer 3

I think the list comprehension is the simplest way, but, if you don't like it, it's obviously not the only way to obtain what you desire -- calling a given callable 100 times with no arguments to form the 100 items of a new list. For example, itertools can obviously do it:

>>> import itertools as it
>>> lst = list(it.starmap(Object, it.repeat((), 100)))

or, if you're really a traditionalist, map and apply:

>>> lst = map(apply, 100*[Object], 100*[()])

Note that this is essentially the same (tiny, both conceptually and actually;-) amount of work it would take if, instead of needing to be called without arguments, Object needed to be called with one argument -- or, say, if Object was in fact a function rather than a type.

From your surprise that it might take "as much as a list comprehension" to perform this task, you appear to think that every language should special-case the need to perform "calls to a type, without arguments" over other kinds of calls to over callables, but I fail to see what's so crucial and special about this very specific case, to warrant treating it differently from all others; and, as a consequence, I'm pretty happy, personally, that Python doesn't single this one case out for peculiar and weird treatment, but handles just as regularly and easily as any other similar use case!-)

Answer 4

You should note that Python's equvalent for Java code (creating array of 100 null references to Object):

Object arr = new Object[100];

or C++ code:

Object **arr = new Object*[100];

is:

arr = [None]*100

not:

arr = [Object() for _ in range(100)]

The second would be the same as Java's:

Object arr = new Object[100];
for (int i = 0; i < arr.lenght; i++) {
    arr[i] = new Object();
}

In fact Python's capabilities to initialize complex data structures are far better then Java's.

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Note: C++ code:

Object *arr = new Object[100];

would have to do as much work as Python's list comprehension:

• allocate continuous memory for 100 Objects

• call Object::Object() for each of this Objects

And the result would be a completely different data structure.