Create a list with initial capacity in Python

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.

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.