what happens when we exit a code block where a file is opened using the keyword with?

what happens when we exit a code block where a file is opened using the keyword with? Ne90'dan bulabilirsiniz

If you're opening a file using the 'with' statement, do you still need to close the file object?

The ‘with‘ statement clarifies code that previously would use try...finally blocks to ensure that clean-up code is executed. In this section, I’ll discuss the statement as it will commonly be used. In the next section, I’ll examine the implementation details and show how to write objects for use with this statement.

The ‘with‘ statement is a control-flow structure whose basic structure is:

with expression [as variable]: with-block

The expression is evaluated, and it should result in an object that supports the context management protocol (that is, has enter() and exit() methods).

Yazı kaynağı : stackoverflow.com

Python close() File – Open and Close Files Properly

We can see that appending to the file from f2 isn't reflected in f3.

Coding like this could also potentially corrupt a file, preventing it from being read or used again in the future. This example is simple, but if you were writing a script that appended extra information to a long file filled with valuable customer data or data that's taken months to web scrape, file corruption would become a costly problem.

To fix this example, we would need to make sure to close the file after writing before trying to read it again:

Yazı kaynağı : www.learndatasci.com

PEP 343 – The “with” Statement | peps.python.org

Specification: The ‘with’ Statement

A new statement is proposed with the syntax:

Here, ‘with’ and ‘as’ are new keywords; EXPR is an arbitrary expression (but not an expression-list) and VAR is a single assignment target. It can not be a comma-separated sequence of variables, but it can be a parenthesized comma-separated sequence of variables. (This restriction makes a future extension possible of the syntax to have multiple comma-separated resources, each with its own optional as-clause.)

The “as VAR” part is optional.

The translation of the above statement is:

Here, the lowercase variables (mgr, exit, value, exc) are internal variables and not accessible to the user; they will most likely be implemented as special registers or stack positions.

The details of the above translation are intended to prescribe the exact semantics. If either of the relevant methods are not found as expected, the interpreter will raise AttributeError, in the order that they are tried (__exit__, __enter__). Similarly, if any of the calls raises an exception, the effect is exactly as it would be in the above code. Finally, if BLOCK contains a break, continue or return statement, the __exit__() method is called with three None arguments just as if BLOCK completed normally. (I.e. these “pseudo-exceptions” are not seen as exceptions by __exit__().)

If the “as VAR” part of the syntax is omitted, the “VAR =” part of the translation is omitted (but mgr.__enter__() is still called).

The calling convention for mgr.__exit__() is as follows. If the finally-suite was reached through normal completion of BLOCK or through a non-local goto (a break, continue or return statement in BLOCK), mgr.__exit__() is called with three None arguments. If the finally-suite was reached through an exception raised in BLOCK, mgr.__exit__() is called with three arguments representing the exception type, value, and traceback.

IMPORTANT: if mgr.__exit__() returns a “true” value, the exception is “swallowed”. That is, if it returns “true”, execution continues at the next statement after the with-statement, even if an exception happened inside the with-statement. However, if the with-statement was left via a non-local goto (break, continue or return), this non-local return is resumed when mgr.__exit__() returns regardless of the return value. The motivation for this detail is to make it possible for mgr.__exit__() to swallow exceptions, without making it too easy (since the default return value, None, is false and this causes the exception to be re-raised). The main use case for swallowing exceptions is to make it possible to write the @contextmanager decorator so that a try/except block in a decorated generator behaves exactly as if the body of the generator were expanded in-line at the place of the with-statement.

The motivation for passing the exception details to __exit__(), as opposed to the argument-less __exit__() from PEP 310, was given by the transactional() use case, example 3 below. The template in that example must commit or roll back the transaction depending on whether an exception occurred or not. Rather than just having a boolean flag indicating whether an exception occurred, we pass the complete exception information, for the benefit of an exception-logging facility for example. Relying on sys.exc_info() to get at the exception information was rejected; sys.exc_info() has very complex semantics and it is perfectly possible that it returns the exception information for an exception that was caught ages ago. It was also proposed to add an additional boolean to distinguish between reaching the end of BLOCK and a non-local goto. This was rejected as too complex and unnecessary; a non-local goto should be considered unexceptional for the purposes of a database transaction roll-back decision.

To facilitate chaining of contexts in Python code that directly manipulates context managers, __exit__() methods should not re-raise the error that is passed in to them. It is always the responsibility of the caller of the __exit__() method to do any reraising in that case.

That way, if the caller needs to tell whether the __exit__() invocation failed (as opposed to successfully cleaning up before propagating the original error), it can do so.

If __exit__() returns without an error, this can then be interpreted as success of the __exit__() method itself (regardless of whether or not the original error is to be propagated or suppressed).

However, if __exit__() propagates an exception to its caller, this means that __exit__() itself has failed. Thus, __exit__() methods should avoid raising errors unless they have actually failed. (And allowing the original error to proceed isn’t a failure.)

Yazı kaynağı : peps.python.org

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