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IDis31
Asked: 2020-11-03 14:19:44 +0800 CST 2020-11-03 14:19:44 +0800 CST

What is the "with" keyword for and how does it work in Python?

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I have found blocks of code like this:

with a as b:
    c = b.algo()

It would look like some dynamic similar to namespaces, but the following code

a = 1
with a as b:
    print a, b

returns the following error:

AttributeError: __exit__

So what is the keyword for with, and how does it work? What is the role of __exit__?

python

1 Answers

  1. Best Answer

    The clause withdoes not have in python the function of delimiting the namespace ( "namespaces" ) as it is done in other languages ​​such as VB. It's more for determining the locale that a block of code will have, which is known as "context" .

    A "context" is basically set with an initial setup and an end to retrieve the previous values. An example would be opening a file:

    with open("fichero.txt") as f:
    print(f.read())

    We start the block by opening the file and, at the end, the file will be closed automatically, even if it has not been explicitly indicated.

    To control a context, "context managers" ( "context manager" ) are used, which are objects that have the methods __enter__and __exit__. The first to initialize the context, the second to end it.

    Objects in python implement the context managerfiles interface , so using them in contexts will ensure that files are closed correctly.

    There are many more objects in the standard library that implement the context manager interface. One of the most significant is the type Decimal, where through a context you can specify the decimal precision of the operations to be performed within the block:

    from decimal import localcontext

with localcontext() as ctx:
    ctx.prec = 42   # Precisión de 42 cifras significativas
    s = calculate_something() # cálculo de alta precisión

s = +s  # regreso a la precisión por defecto del tipo decimal

    There are also libraries, such as fabric , that use the contexts to configure the connections of the servers on which to execute a script.

    In your case, the error you get is that the variable does not have the context managera methods defined .

    For more detailed information, see the PEP-343

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