Create a list with initial capacity in Python

Code like this often happens:

l = []
while foo:
    # baz
    l.append(bar)
    # qux

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 refactored into a list comprehension. If the for/while loop is very complicated, though, this is unfeasible. Is there an equivalent for us Python programmers?

Solution 1:

Warning: This answer is contested. See comments.

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

def doAllocate( size=10000 ):
    result=size*[None]
    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.

Solution 2:

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:
        #baz
        yield bar
        #qux

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.