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gh-119786: add links to code in exception handling doc (#120077)

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Irit Katriel 2024-06-05 16:28:47 +01:00 committed by GitHub
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1 changed files with 16 additions and 33 deletions
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InternalDocs/exception_handling.md
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@ -67,14 +67,18 @@ handler located at label `L1`.
Handling Exceptions Handling Exceptions
------------------- -------------------
At runtime, when an exception occurs, the interpreter looks up At runtime, when an exception occurs, the interpreter calls
the offset of the current instruction in the exception table. If ``get_exception_handler()`` in
it finds a handler, control flow transfers to it. Otherwise, the [Python/ceval.c](https://github.com/python/cpython/blob/main/Python/ceval.c)
to look up the offset of the current instruction in the exception
table. If it finds a handler, control flow transfers to it. Otherwise, the
exception bubbles up to the caller, and the caller's frame is exception bubbles up to the caller, and the caller's frame is
checked for a handler covering the `CALL` instruction. This checked for a handler covering the `CALL` instruction. This
repeats until a handler is found or the topmost frame is reached. repeats until a handler is found or the topmost frame is reached.
If no handler is found, the program terminates. During unwinding, If no handler is found, the program terminates. During unwinding,
the traceback is constructed as each frame is added to it. the traceback is constructed as each frame is added to it by
``PyTraceBack_Here()``, which is in
[Python/traceback.c](https://github.com/python/cpython/blob/main/Python/traceback.c).
Along with the location of an exception handler, each entry of the Along with the location of an exception handler, each entry of the
exception table also contains the stack depth of the `try` instruction exception table also contains the stack depth of the `try` instruction
@ -169,33 +173,12 @@ which is then encoded as:
for a total of five bytes. for a total of five bytes.
The code to construct the exception table is in ``assemble_exception_table()``
in [Python/assemble.c](https://github.com/python/cpython/blob/main/Python/assemble.c).
Script to parse the exception table The interpreter's function to lookup the table by instruction offset is
----------------------------------- ``get_exception_handler()`` in
[Python/ceval.c](https://github.com/python/cpython/blob/main/Python/ceval.c).
``` The Python function ``_parse_exception_table()`` in
def parse_varint(iterator): [Lib/dis.py](https://github.com/python/cpython/blob/main/Lib/dis.py)
b = next(iterator) returns the exception table content as a list of namedtuple instances.
val = b & 63
while b&64:
val <<= 6
b = next(iterator)
val |= b&63
return val
```
```
def parse_exception_table(code):
iterator = iter(code.co_exceptiontable)
try:
while True:
start = parse_varint(iterator)*2
length = parse_varint(iterator)*2
end = start + length - 2 # Present as inclusive, not exclusive
target = parse_varint(iterator)*2
dl = parse_varint(iterator)
depth = dl >> 1
lasti = bool(dl&1)
yield start, end, target, depth, lasti
except StopIteration:
return
```