[Python-Dev] I/O ABCs (original) (raw)

Daniel Stutzbach stutzbach at google.com
Wed Dec 1 02:11:30 CET 2010

The documentation for the collections Abstract Base Classes (ABCs) 1 contains a table listing all of the collections ABCs, their parent classes, their abstract methods, and the methods they provide. This table makes it very easy to figure out which methods I must override when I derive from one of the ABCs, as well as which methods will be provided for me.

I'm working on a similar table for the I/O ABCs ( http://bugs.python.org/issue10589). The existing documentation 2 describes the methods of each class but doesn't describe which methods provide a meaningful implementation and which methods a user should override. If I want to inherit from one of the I/O ABCs, I have to go poking into Lib/_pyio.py to figure out which methods I need to override.

While starting to examine the I/O ABCs, I discovered that there are some inconsistencies. For example, RawIOBase provides .read() if the subclass provides .readinto(). BufferedIOBase does the opposite; it provides .readinto() if the subclass provides .read() [3].

I would like to fix some of these inconsistencies. However, this will be a backwards-incompatible change. A Google Code Search suggests that the ABCs are currently only used within the standard library [4]. Just to be clear, the changes would NOT impact code that merely uses I/O objects; they would only impact code that implements I/O by subclassing one of the I/O ABCs and depending on features that are currently undocumented.

Does anyone have any categorical objections?

[3]: Possibly hurting performance by forcing .readinto() to perform the extra allocations, copies, and deallocations required by .read(). [4]: http://www.google.com/codesearch?hl=en&sa=N&q=BufferedIOBase++lang:python&ct=rr&cs_r=lang:python

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