Question

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?

Answer 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.