关于python：在主函数中包装代码使其运行更快

Wrapping code in main function makes it run much faster

本问题已经有最佳答案，请猛点这里访问。

试图理解Python中的这种现象。当我将代码包装成一个函数时，它的速度会提高30%以上！！！！到目前为止，我没有在谷歌上找到任何合理的解释。

例如：

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import sys
sys.stdin = open("dd.txt")
import cProfile
import time
t0 = time.time()

def main():
from collections import defaultdict
n, T = map(int, raw_input().split())
tree = defaultdict(lambda: set())
root = None
for _ in xrange(n - 1):
a, b = map(int, raw_input().split())
tree[a].add(b)
tree[b].add(a)
if not root:
root = a

count = 0
stack = [root]
links = dict()
links[root] = 0
mem = dict()
while stack:
node = stack.pop()
path = list()
path.append(node)
lnk = links[node]
while lnk:
if lnk not in mem:
if abs(lnk - node) <= T:
count += 1
path.append(lnk)
lnk = links[lnk]
else:
path.extend(mem[lnk])
for el in mem[lnk]:
if abs(el - node) <= T:
count += 1
break
#print node, path
plen = len(path)
mem[node] = path
for next_node in tree[node]:
if plen <= 1 or next_node != path[1]:
links[next_node] = node
stack.append(next_node)

print count
main()

print time.time() - t0

这将运行时间打印为2.5秒，但是：

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import sys
sys.stdin = open("dd.txt")
import cProfile
import time
t0 = time.time()

#def main():
from collections import defaultdict
n, T = map(int, raw_input().split())
tree = defaultdict(lambda: set())
root = None
for _ in xrange(n - 1):
a, b = map(int, raw_input().split())
tree[a].add(b)
tree[b].add(a)
if not root:
root = a

count = 0
stack = [root]
links = dict()
links[root] = 0
mem = dict()
while stack:
node = stack.pop()
path = list()
path.append(node)
lnk = links[node]
while lnk:
if lnk not in mem:
if abs(lnk - node) <= T:
count += 1
path.append(lnk)
lnk = links[lnk]
else:
path.extend(mem[lnk])
for el in mem[lnk]:
if abs(el - node) <= T:
count += 1
break
#print node, path
plen = len(path)
mem[node] = path
for next_node in tree[node]:
if plen <= 1 or next_node != path[1]:
links[next_node] = node
stack.append(next_node)

print count
#main()

print time.time() - t0

只需将代码移出main()函数，它就可以运行3.5秒而不是2.5秒。

为什么会这样？？？？