By Jacob Zimmerman
ISBN-10: 148422504X
ISBN-13: 9781484225042
ISBN-10: 1484225058
ISBN-13: 9781484225059
This brief booklet on Python descriptors is a set of data and concepts from many assets on facing and developing descriptors. And, after dealing with the issues all descriptors have in universal, the writer explores rules that experience a number of methods of being carried out in addition to thoroughly new rules by no means obvious in different places before.
This really is a accomplished advisor to making Python descriptors. As an advantage: A pip install-able library, descriptor_tools, used to be written along this e-book and is an open resource library on GitHub.
There are not many stable assets available in the market for writing Python descriptors, and intensely few books. it is a unhappy scenario, because it makes it tough for Python builders to get a truly reliable figuring out of the way descriptors paintings and the strategies to prevent the massive gotchas linked to operating with them.
What you'll Learn
- Discover descriptor protocols
- Master characteristic entry and the way it applies to descriptors
- Make descriptors and observe why you should
- Store attributes
- Create read-only descriptors and _delete()
- Explore the descriptor classes
- Apply the opposite makes use of of descriptors and more
Who This publication Is For
Experienced Python coders, programmers and developers.
Read or Download Python Descriptors PDF
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Additional resources for Python Descriptors
Sample text
There needs to be some sort of back door. Three different techniques will be discussed: set-once, secret-set, and forced-set. Set-Once Descriptors A set-once descriptor is the most restrictive of the three read-only properties in that it most strongly restricts the number of assignments to once per instance under it. Set-once descriptors work simply by checking whether a value is already set and acting accordingly. If it's already assigned, it raises an exception; if it's not, then it sets it. For example, this is what the basic __set__() method would look like if the descriptor was using on-descriptor storage in the instance attribute, storage.
While this is a good idea regardless, it’s worth mentioning that it may fail in some cases. If you want to make a descriptor safe with __slots__ while still defaulting to using __dict__, you may want to create some sort of alternative that uses on-descriptor storage when a boolean flag is set on creation. There are plenty of ways to implement that, whether using a factory that chooses a different descriptor if the flag is set or the class within has alternate paths based on the flag value. Another, simpler alternative is to document the name that the descriptor stores its values under so that users of the 33 Chapter 7 ■ Storing the Attributes descriptor who want to use __slots__ can prepare a slot for it.
Summary We’ve seen that __delete__() is a pretty simple method to implement, but deciding whether to actually implement it or not can be a difficult decision. In the end, though, it will be used so little that implementing it can probably be put off until it's needed. The default behavior of raising an exception due to lack of implementation should get you by until then. © Jacob Zimmerman 2016 J. 1007/978-1-4842-2505-9_9 47 CHAPTER 10 Descriptors are Classes Too It’s time for some more advanced stuff with descriptors.