Python 'with' command

Question

Is this code

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

equivalent to this one

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

or the following one?

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

This article: http://effbot.org/zone/python-with-statement.htm suggests (if I understand it correctly) that the latter is true. However, the former would make more sense to me. If I am wrong, what am I missing?

It's not equivalent to either. What if process raises an error? with does not handle errors here, but the other two do. — Alex Riley
– Alex Riley, Commented Dec 20, 2015 at 14:57
@ajcr: neither does try:...finally:. The second stanza is equivalent. — Martijn Pieters
– Martijn Pieters, Commented Dec 20, 2015 at 14:59

awesoon · Accepted Answer · 2015-12-20 15:05:38Z

According to the documentation:

A new statement is proposed with the syntax:

with EXPR as VAR:
    BLOCK

The translation of the above statement is:

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)

And this is an extended version of your second code snippet. Initialization goes before try ... finaly block.

John Zwinck · Accepted Answer · 2015-12-20 14:57:03Z

3

It's equivalent to the latter one, because until open() successfully returns, f has no value, and should not be closed.

answered Dec 20, 2015 at 14:57

John Zwinck

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3 Comments

agconti Over a year ago

with will always close the file for you. How does the second example do that?

poke Over a year ago

@agconti If open() fails, the file was never opened, so it doesn’t need to be closed either.

zvone Over a year ago

@agconti If open() raises an exception, f is never even initialized. Not only that there is no reason to close f, but f.close() would raise NameError: name 'f' is not defined if you tried.

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