Context Managers and the with Statement
Try it: Context Managers and the with Statement
What with really does: it calls __enter__ before the block and __exit__ after it — even when an exception leaves the block — so a file opened with with is always closed before the exception continues; try/finally gives the same guarantee by hand, a bare close() is skipped by the exception, and a context manager whose __exit__ returns True suppresses the exception.
How it works
- with EXPR as f: evaluates EXPR (e.g. open(...)), calls its __enter__() and binds the returned value to f (a file's __enter__ returns the file itself).
- The block runs. If a statement raises (here int() on text that is not a number), the rest of the block is skipped.
- Whether the block finished or an exception is leaving it, Python calls __exit__(exc_type, exc, tb) — for a file this closes it. This happens before any outer except sees the exception.
- If __exit__ returns a true value the exception is suppressed and execution continues after the block; a file's __exit__ returns None, so the exception propagates to the nearest matching except (or stops the program).
- try: … finally: f.close() gives the same cleanup by hand (start with f = None and check it); open() … f.close() without either leaves the file open when an exception jumps past close().
Default run (10 steps): The program opens data.txt, which contains '4x\n'. Watch when the file is closed — and what happens if int() raises ValueError. … The program finished. The file was closed.
Simplified: Fixed program templates reading one line of data.txt (text you type, up to 12 ASCII characters) and converting it with int(). A small interpreter written for these statements runs them — nothing is executed — and it reproduces CPython 3.12's sys.settrace line/call/exception events, printed output, final error line and file state for every combination. File objects' own __enter__/__exit__ are C code, so they appear as protocol steps rather than traced lines.
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