On 12/01/2013 05:32 AM, Amit Saha wrote:
Hello,
I was told by someone (as a comment) that a code snippet such as this
"would make Pythonistas talk my ear off about how evil the append()"
function is:
mylist = []
mylist.append(1)
# a number of times over
I have some ideas that on an append() the list's internal size
increases (doubled?) since CPython over-allocates memory, and such.
So, the question is: what is the alternative to growing a list when I
have no idea what items or how many may be there?
Maybe you are confusing with catenating _strings_, rather than lists. Python's
concat is problematic because it is a binary operation. So, catenating n bits
makes n-1 operations, each with intermediate results, of growing sizes, all
thrown away except for the last one, the actual result. If all of the bitN are
strings:
bit1 + bit2 + bit3 + bit4 + bit5
actually constructs:
bit1+bit2
bit1+bit2+bit3
bit1+bit2+bit3+bit4
bit1+bit2+bit3+bit4+bit5
A number of unneeded string object, and a very big useless memory weight.
Example Python code showing good/bad usage below (advanced pythonistas, please
correct if needed, I don't know the latest nice Python idioms):
========================================================
# === case of a series of explicite text sections ====
# don't do this:
text = ""
intro = "intro..."
text += intro
body = "body..."
text+= '\n' + body
concl = "concl..."
text += '\n' + concl
print(text) ; print()
# also do not do:
text = intro + '\n' + body + '\n' + concl
print(text) ; print()
# do that...:
intro = "intro..."
body = "body..."
concl = "concl..."
text = '\n'.join([intro, body, concl]) # but creates a list
print(text) ; print()
# ...or that:
text = "%s\n%s\n%s" % (intro, body, concl) # no such list
print(text) ; print()
# === case of a list of sections of arbitrary size ====
# (simulation code for demo)
def make_section (n): return "%s\n" % n
section_data = [13, 5, 79, 4, 268, 0, 987654321]
# don't do this:
text = ""
for dat in section_data:
section = make_section(dat)
text += section
print(text)
# do that...:
sections = []
for dat in section_data:
section = make_section(dat)
sections.append(section)
text = ''.join(sections)
print(text)
# ... or that:
sections = (make_section(dat) for dat in section_data)
text = ''.join(sections)
print(text)
# ... or even that:
text = ''.join(make_section(dat) for dat in section_data) # no need for ((...))
print(text)
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