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Mastering Object-oriented Python

Mastering Object-oriented Python

By : Steven F. Lott, Steven F. Lott
4.2 (13)
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Mastering Object-oriented Python

Mastering Object-oriented Python

4.2 (13)
By: Steven F. Lott, Steven F. Lott

Overview of this book

Table of Contents (26 chapters)
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Mastering Object-oriented Python
Credits
About the Author
About the Reviewers
www.PacktPub.com
Preface
Some Preliminaries
1
Index

Defining the __enter__() and __exit__() methods


The defining feature of a context manager is that it has two special methods: __enter__() and __exit__(). These are used by the with statement to enter and exit the context. We'll use a simple context so that we can see how they work.

We'll often use context managers to make transient global changes. This might be a change to the database transaction status or a change to the locking status, something that we want to do and then undo when the transaction is complete.

For this example, we'll make a global change to the random number generator. We'll create a context in which the random number generator uses a fixed and known seed, providing a fixed sequence of values.

The following is the context manager class definition:

import random
class KnownSequence:
    def __init__(self, seed=0):
        self.seed= 0
    def __enter__(self):
        self.was= random.getstate()
        random.seed(self.seed, version=1)
        return self
    def __exit__...
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Mastering Object-oriented Python
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