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

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Jun 5, 2024
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gh-119786: add links to code in exception handling doc #120077

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49 changes: 16 additions & 33 deletions InternalDocs/exception_handling.md
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Original file line number Diff line number Diff line change
Expand Up @@ -67,14 +67,18 @@ handler located at label `L1`.
Handling Exceptions
-------------------

At runtime, when an exception occurs, the interpreter looks up
the offset of the current instruction in the exception table. If
it finds a handler, control flow transfers to it. Otherwise, the
At runtime, when an exception occurs, the interpreter calls
``get_exception_handler()`` in
[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
checked for a handler covering the `CALL` instruction. This
repeats until a handler is found or the topmost frame is reached.
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
exception table also contains the stack depth of the `try` instruction
Expand Down Expand Up @@ -169,33 +173,12 @@ which is then encoded as:

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
-----------------------------------

```
def parse_varint(iterator):
b = next(iterator)
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
```
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
[Lib/dis.py](https://github.com/python/cpython/blob/main/Lib/dis.py)
returns the exception table content as a list of namedtuple instances.
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