| 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)
return data
