Python with clause file

For how long it can go on before using all system resources? We will use the file output. For some reason, the print statement is adding new line characters that are not present in the original file. Imagine you have to write a program that takes a single list of strings and writes each string in one of two files. And our program has to write animals to a file called animals. Here is how you would do it using two open statements in a single with expression :.

To be able to use two open statements in one with expression Python 2. Here is how we can update our previous program using two with nested statements :. To give you a full understanding of the with statement I also want to show you an alternative way to write a logic that behaves like with open…as.

We will use a try…finally statement to make sure the file object is always closed after executing the try code block. I want to write something similar to the following:. It definitely makes our code more concise. Imagine if we had to use try…finally statements for each file we open! In this simple guide we have seen how to use with open in Python to simplify the way we work with files.

How can I instruct with clause to create the file if it doesn't exist? Nice table here. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Python open or create file using with clause [duplicate] Ask Question. Asked 3 years, 1 month ago.

Active 3 years, 1 month ago. Viewed times. You can just next the with inside the try , these are two separate, composable constructs. The calling convention for mgr. 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. The motivation for this detail is to make it possible for mgr.

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. 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.

And allowing the original error to proceed isn't a failure. This will make both 'with' and 'as' keywords. Without the future statement, using 'with' or 'as' as an identifier will cause a Warning to be issued to stderr.

With PEP accepted, it is possible to write a decorator that makes it possible to use a generator that yields exactly once to control a with-statement. Here's a sketch of such a decorator:. OTOH such mistakes are easily diagnosed; for example, the generator context decorator above raises RuntimeError when a second with-statement calls f. A context manager will also be added to the decimal module to support using a local decimal arithmetic context within the body of a with statement, automatically restoring the original context when the with statement is exited.

The expression immediately following the with keyword in the statement is a "context expression" as that expression provides the main clue as to the runtime environment the context manager establishes for the duration of the statement body. The code in the body of the with statement and the variable name or names after the as keyword don't really have special terms at this point in time. The general terms "statement body" and "target list" can be used, prefixing with "with" or "with statement" if the terms would otherwise be unclear.

Given the existence of objects such as the decimal module's arithmetic context, the term "context" is unfortunately ambiguous. If necessary, it can be made more specific by using the terms "context manager" for the concrete object created by the context expression and "runtime context" or preferably "runtime environment" for the actual state modifications made by the context manager.

When simply discussing use of the with statement, the ambiguity shouldn't matter too much as the context expression fully defines the changes made to the runtime environment. The distinction is more important when discussing the mechanics of the with statement itself and how to go about actually implementing context managers. Many context managers such as files and generator-based contexts will be single-use objects.

Requiring a fresh manager object for each with statement is the easiest way to avoid problems with multi-threaded code and nested with statements trying to use the same context manager.

It isn't coincidental that all of the standard library context managers that support reuse come from the threading module - they're all already designed to deal with the problems created by threaded and nested usage. This means that in order to save a context manager with particular initialisation arguments to be used in multiple with statements, it will typically be necessary to store it in a zero-argument callable that is then called in the context expression of each statement rather than caching the context manager directly.

When this restriction does not apply, the documentation of the affected context manager should make that clear. The following issues were resolved by BDFL approval and a lack of any major objections on python-dev.

What exception should GeneratorContextManager raise when the underlying generator-iterator misbehaves? The following quote is the reason behind Guido's choice of RuntimeError for both this and for the generator close method in PEP from [8] :. So now I believe they should both raise RuntimeError. In python to read or write a file, we need first to open it and python provides a function open , which returns a file object.

Using this file object, we can read and write in the file. But in the end, we need to close the file using this same. This is a sample file.

It contains some sample string. In the above example, we opened a file sample. Then printed that and in the end closed this file using the same file object. Well, it seems highly impossible now, but in big projects, people usually do big stuff after opening files, and it includes many conditions and checks.