Python'with'命令

Question

这是代码

with open(myfile) as f:
    data = f.read()
    process(data)

相当于这个

try:
    f = open(myfile)
    data = f.read()
    process(f)
finally:
    f.close()

还是以下一个？

f = open(myfile)
try:
    data = f.read()
    process(f)
finally:
    f.close()

本文:http://effbot.org/zone/python-with-statement.htm建议(如果我理解正确的话)后者是真的.但是,前者对我来说更有意义.如果我错了,我错过了什么？

Answer 1

根据文件:

提出了一个新语句,其语法如下:

with EXPR as VAR:
    BLOCK

上述声明的译文如下:

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)

这是您的第二个代码段的扩展版本.初始化在try ... finaly阻止之前进行.