任何物件，只要正確實現了上下文管理，就可以用於with語句。
實現上下文管理是通過enter和exit這兩個方法實現的。例如，下面的class實現了這兩個方法：
class
query
(object):
def__init__
(self, name):
self.name = name
def__enter__
(self):
print('begin')
return self
def__exit__
(self, exc_type, exc_value, traceback):
if exc_type:
print('error')
else:
print('end')
defquery
(self):
print('query info about %s...' % self.name)
with query('bob') as q:
q.query()
"c:\program files\python36\python.exe"
c:/users/administrator/pycharmprojects/python全網練習/常用內建模組.py
begin
query info about bob...
endprocess finished with exit code 0
from contextlib import contextmanager
class
query
(object):
def__init__
(self, name):
self.name = name
defquery
(self):
print('query info about %s...' % self.name)
@contextmanager
defcreate_query
(name):
print('begin')
q = query(name)
yield q
print('end')
with create_query('bob') as q:
q.query()
"c:\program files\python36\python.exe"
c:/users/administrator/pycharmprojects/python全網練習/常用內建模組.py
begin
query info about bob...
endprocess finished with exit code 0
from contextlib import contextmanager
@contextmanager
deftag
(name):
print("<%s>" % name)
yield
print("" % name)
with tag("h1"):
print("hello")
print("world")
"c:\program files\python36\python.exe" c:/users/administrator/pycharmprojects/python全網練習/常用內建模組.py
process finished with
exit code 0
from contextlib import contextmanager
from contextlib import closing
from urllib.request import urlopen
@contextmanager
defclosing
(thing):
try:
yield thing
finally:
thing.close()
with closing(urlopen('')) as page:
for line in page:
print(line)
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