如何制作仅在基础文件的一部分上运行的文件对象？

Question

I have a binary file consisting of an N-byte header, followed by a data block. I have input describing the locations of various bits of data, as offsets from the start of the data block; ie not the start of the file itself.

I want to open the file in such a way that seek() calls (and anything similar) seek within the data block, not the whole file.

Some options I've considered:

1. mmap . Close, but its offset argument is limited to multiples of pagesize.
2. Subclass file objects and override seek() to add the header length. Google suggests this may be annoying to do right.
3. Slurp the whole file minus the header, then make a bytesio. Problematic if the file is huge.
4. Patch seek() on the opened file object. I think this might work but monkeypatching makes me nervous.

I think what I really want is something that looks and acts like a file object but only "sees" a portion of the underlying file. Is there a good way to do that?

Answer 1

Rather than inheritance and monkey-patching, you forgot a simpler option: implement a file-like class which seeks the way you want. Then you'll have full control and non-leaky encapsulation.

Answer 2

mmap整个文件，然后使用一个memoryview的数据块你有兴趣，然后偏移到存储器视图将是相对于所述数据块。