io-wrapper

name	io-wrapper
description	Wrap file-like objects with read/write counters. Use when implementing IO wrappers, tracking file access, or counting bytes read/written through a proxy.
topic	File-Like Wrapper + Counters
token_cost	120
related	["recursion-backtracking"]
keywords	["io wrapper","wrap file","read counter","write counter","nreads","nwrites","context manager","__enter__","__exit__","passthrough","delegate","paasio","MetaRead","MetaWrite"]

When to use

To wrap a file-like object and count reads/writes.

Rules

• Store the wrapped object as self._wrapped
• Implement read(size=-1) by delegating to self._wrapped.read(size)
• Increment counters by the length of the RETURNED bytes, NOT the requested size
• For write: increment nwrites by the RETURN VALUE, or by len(data) if the wrapped write returns None
• Expose read_bytes/nreads and write_bytes/nwrites as properties or attributes
• enter returns self; exit calls self._wrapped.exit (or close()) and forwards the exception info
• ALWAYS implement close() as a plain method for non-context-manager use
• NEVER count requested bytes — only count what was actually returned/written

Edge cases

Thread safety: if the test uses threads, wrap counter updates in a threading.Lock.

Example

class MetaRead:
    def __init__(self, wrapped): self._wrapped = wrapped; self.nreads = 0
    def read(self, size=-1):
        data = self._wrapped.read(size)
        self.nreads += len(data)  # count returned bytes, not requested
        return data