我正在尝试创建一个使用def cutRod(price, n):
val = [0 for x in range(n+1)]
val[0] = 0
output = list()
for i in range(1, n+1):
max_val = 0
cur_max_index = -1
for j in range(i):
cur_val = price[j] + val[i-j-1]
if(cur_val>max_val):
max_val = cur_val #store current max
cur_max_index = j #and index
if cur_max_index != -1:
output.append(cur_max_index) #append in output index list
val[i] = max_val
print(output) #print array
return val[n]
的上下文管理器，它本身就是一个上下文管理器。最初我有一个愚蠢的打开文件问题[为什么mmap不关闭相关文件（得到PermissionError：[WinError 32]）？] here并且答案快速解释为什么它不是按需工作。
鉴于这些信息，我尝试了两种不同的方法来纠正这个问题，但没有一种方法可以解决。
第一种方法是使用mmap的{{1}}装饰器：
contextlib但结果是：
@contextmanager在下一次尝试中，我尝试了显式创建上下文管理器类：
from contextlib import contextmanager
import os
import mmap
#contextmanager
def memory_map(filename, access=mmap.ACCESS_WRITE):
size = os.path.getsize(filename)
fd = os.open(filename, os.O_RDWR)
print('about to yield')
with mmap.mmap(fd, size, access=access) as m:
yield m
print('in finally clause')
os.close(fd) # Close the associated file descriptor.
test_filename = 'data'
# First create the test file.
size = 1000000
with open(test_filename, 'wb') as f:
f.seek(size - 1)
f.write(b'\x00')
# Read and modify mmapped file in-place.
with memory_map(test_filename) as m: # Causes AttributeError: __enter__
print(len(m))
print(m[0:10])
# Reassign a slice.
m[0:11] = b'Hello World'
# Verify that changes were made
print('reading back')
with open(test_filename, 'rb') as f:
print(f.read(11))
# Delete test file.
# Causes:
# PermissionError: [WinError 32] The process cannot access the file because it
# is being used by another process: 'data'
os.remove(test_filename)
这使得它更进一步，但是Traceback (most recent call last):
File "memory_map.py", line 27, in <module>
with memory_map(test_filename) as m: # Causes AttributeError: __enter__
AttributeError: __enter__
又回来了 - 这让我感到困惑，因为文件描述符 在该版本中已关闭，正如您在输出中看到的那样：
import os
import mmap
class MemoryMap:
def __init__(self, filename, access=mmap.ACCESS_WRITE):
print('in MemoryMap.__init__')
size = os.path.getsize(filename)
self.fd = os.open(filename, os.O_RDWR)
self.mmap = mmap.mmap(self.fd, size, access=access)
def __enter__(self):
print('in MemoryMap.__enter__')
return self.mmap
def __exit__(self, exc_type, exc_value, traceback):
print('in MemoryMap.__exit__')
os.close(self.fd) # Close the associated file descriptor.
print(' file descriptor closed')
test_filename = 'data'
# First create the test file.
size = 1000000
with open(test_filename, 'wb') as f:
f.seek(size - 1)
f.write(b'\x00')
# Read and modify mmapped file in-place.
with MemoryMap(test_filename) as m:
print(len(m))
print(m[0:10])
# Reassign a slice.
m[0:11] = b'Hello World'
# Verify that changes were made
print('reading back')
with open(test_filename, 'rb') as f:
print(f.read(11))
# Delete test file.
# Causes PermissionError: [WinError 32] The process cannot access the file
# because it is being used by another process: 'data'
os.remove(test_filename)
所以看来我再次陷入困境。关于什么是错的（以及如何修复它）的任何想法？另外，如果你有意见，他们都可以修好，哪一个更好？
解决方案
两个片段都有错误。这首先是一个简单的印刷错误。 PermissionError装饰者被注释掉了。应该是：
in MemoryMap.__init__
in MemoryMap.__enter__
1000000
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
in MemoryMap.__exit__
file descriptor closed
reading back
b'Hello World'
Traceback (most recent call last):
File "memory_map2.py", line 47, in <module>
os.remove(test_filename)
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'data'
第二个是因为contextmanger 本身未在@contextmanager # Leading "#" changed to "@".
def memory_map(filename, access=mmap.ACCESS_WRITE):
size = os.path.getsize(filename)
fd = os.open(filename, os.O_RDWR)
,,,
方法中关闭，只是关联的文件描述符。从来没有发生过这种情况，因为提出的例外情况与第一种情况相同。
mmap答案 0 :(得分：2)
如果是第二次尝试，则需要关闭内存映射文件：
def __exit__(self, exc_type, exc_value, traceback):
self.mm.close()
print('in MemoryMap.__exit__')
os.close(self.fd) # Close the associated file descriptor.
print(' file descriptor closed')