The with statement (__enter__ & _exit__ methods)

See also

https://docs.python.org/3.4/reference/compound_stmts.html#with

Many (modern) classes have an __enter__ and an __exit__ method; this pair is used by the with-statement. And are used to create a context manager.

Before executing the block of statements in the the with-statements, the __enter__() of the object named in the with-statement is called. And, when leaving that block the __exit__() is called, always! This is convenient, as it cleans up the code “within”.

Tip

• It may be convenient to think placeholder to “open” and “close” the object.

• As the with-statement is often used with a newly initiated object, part of the “before” code is often located in the __init__(); leaving only return self in the __init__.

• The __exit__ has some extra parameters, that describe “how” the block is exited. Without a raised exception, they are None; else they contain the “context” of that exception.

• For the design-analyse, they are not that important. Every design can be coded without them!

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