Python - Create a list with initial capacity


Code like this often happens:

l = []
while foo:

This is really slow if you're about to append thousands of elements to your list, as the list will have to be constantly resized to fit the new elements.

In Java, you can create an ArrayList with an initial capacity. If you have some idea how big your list will be, this will be a lot more efficient.

I understand that code like this can often be re-factored into a list comprehension. If the for/while loop is very complicated, though, this is unfeasible. Is there any equivalent for us Python programmers?

5/8/2015 4:22:32 AM

Accepted Answer

def doAppend( size=10000 ):
    result = []
    for i in range(size):
        message= "some unique object %d" % ( i, )
    return result

def doAllocate( size=10000 ):
    for i in range(size):
        message= "some unique object %d" % ( i, )
        result[i]= message
    return result

Results. (evaluate each function 144 times and average the duration)

simple append 0.0102
pre-allocate  0.0098

Conclusion. It barely matters.

Premature optimization is the root of all evil.

11/22/2008 10:02:34 PM

Python lists have no built-in pre-allocation. If you really need to make a list, and need to avoid the overhead of appending (and you should verify that you do), you can do this:

l = [None] * 1000 # Make a list of 1000 None's
for i in xrange(1000):
    # baz
    l[i] = bar
    # qux

Perhaps you could avoid the list by using a generator instead:

def my_things():
    while foo:
        yield bar

for thing in my_things():
    # do something with thing

This way, the list isn't every stored all in memory at all, merely generated as needed.

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