> On Mar 15, 2019, at 6:49 PM, Chris Angelico <ros...@gmail.com> wrote:
>
> On Sat, Mar 16, 2019 at 12:40 PM Raymond Hettinger
> <raymond.hettin...@gmail.com> wrote:
>> Also, it seems like the efficiency concerns were dismissed with hand-waving.
>> But usually, coping and updating aren't the desired behavior. When teaching
>> Python, I like to talk about how the design of the language nudges you
>> towards fast, clear, correct code. The principle is that things that are
>> good for you are put within easy reach. Things that require more thought are
>> placed a little further away. That is the usual justification for copy()
>> and deepcopy() having to be imported rather than being builtins. Copying is
>> an obvious thing to do; it is also not usually good for you; so, we have you
>> do one extra step to get to it.
>>
>
> I'm not sure I understand this argument. Are you saying that d1+d2 is
> bad code because it will copy the dictionary, and therefore it
> shouldn't be done? Because the exact same considerations apply to the
> addition of two lists, which already exists in the language. Is it bad
> to add lists together instead of using extend()?
Yes, that exactly.
Consider a table in a database. Usually what people want/need/ought-to-do is an
SQL UPDATE rather than copy and update which would double the memory
requirement and be potentially many times slower. The same applies to Python
lists. Unless you actually have a requirement for three distinct lists (c = a +
b), it is almost always better to extend in place. Adding lists rather than
extending them is a recipe for poor performance (especially if it occurs in a
loop):
Raymond
---- Performant version ----
s = socket.socket()
try:
s.connect((host, port))
s.send(request)
blocks = []
while True:
block = s.recv(4096)
if not block:
break
blocks += [block] # Normally done with append()
page = b''.join(blocks)
print(page.replace(b'\r\n', b'\n').decode())
finally:
s.close()
---- Catastrophic version ----
s = socket.socket()
try:
s.connect((host, port))
s.send(request)
blocks = []
while True:
block = s.recv(4096)
if not block:
break
blocks = blocks + [block] # Not good for you.
page = b''.join(blocks)
print(page.replace(b'\r\n', b'\n').decode())
finally:
s.close()
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