Pythonのwith構文

2014年6月13日

1.with…as…

mgr = (EXPR)
exit = type(mgr).__exit__  # Not calling it yet
value = type(mgr).__enter__(mgr)
exc = True
try:
try:
VAR = value  # Only if “as VAR" is present
BLOCK
except:
# The exceptional case is handled here
exc = False
if not exit(mgr, *sys.exc_info()):
raise
# The exception is swallowed if exit() returns true
finally:
# The normal and non-local-goto cases are handled here
if exc:
exit(mgr, None, None, None)

2.標準ライブラリcontextlib

from contextlib import contextmanager

 

@contextmanager
def opened(name):
f = open(name)
try:
yield f
finally:
f.close()

with opened('/tmp/a.txt’) as f:
for line in f.readlines():
print(line)