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Merged
iritkatriel merged 10 commits into from
Jun 3, 2024
Merged

gh-119786: move exception handling doc to InternalDocs #119815

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81a0e13
rename index->README and move exception handling doc to new folder
iritkatriel May 30, 2024
cd7725f
add link to exception handling file from index
iritkatriel May 30, 2024
7719234
edit exception handling
iritkatriel May 30, 2024
023ad9d
whitespace
iritkatriel May 30, 2024
fc5d217
formatting
iritkatriel May 30, 2024
3436cdf
formatting
iritkatriel May 31, 2024
49af193
add explanation of reraise and lasti. fix typos
iritkatriel May 31, 2024
d299a3e
address review - clarify pseudo-instructions/intermediate code
iritkatriel May 31, 2024
e93a253
fix markup
iritkatriel May 31, 2024
a3da517
typo
iritkatriel Jun 3, 2024
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address review - clarify pseudo-instructions/intermediate code
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@iritkatriel
iritkatriel committed May 31, 2024
commit d299a3ef2a251af90bb5615fc79d5f2005689856
32 changes: 17 additions & 15 deletions InternalDocs/exception_handling.md
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Expand Up @@ -16,7 +16,7 @@ except:

```

compiles into pseudo-code like the following:
compiles into intermediate code like the following:

```
RESUME 0
Expand All @@ -38,29 +38,31 @@ compiles into pseudo-code like the following:
STORE_NAME 1 (res)
```

The `SETUP_FINALLY` instruction specifies that henceforth, exceptions
`SETUP_FINALLY` and `POP_BLOCK` are pseudo-instruction. This means
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that they can appear in intermediate code but they are not bytecode
instructions. `SETUP_FINALLY` specifies that henceforth, exceptions
are handled by the code at label L1. The `POP_BLOCK` instruction
reverses the effect of the last `SETUP` instruction, so that the
active exception handler reverts to what it was before.

Note that the `SETUP_FINALLY` and `POP_BLOCK` instructions have no effect
when no exceptions are raised. The idea of zero-cost exception handling
is to replace these instructions by metadata which is stored alongside
the bytecode, and which is inspected only when an exception occurs.
`SETUP_FINALLY` and `POP_BLOCK` have no effect when no exceptions
are raised. The idea of zero-cost exception handling is to replace
these pseudo-instructions by metadata which is stored alongside the
bytecode, and which is inspected only when an exception occurs.
This metadata is the exception table, and it is stored in the code
object's `co_exceptiontable` field.

When the pseudo-instructions are translated into bytecode, the
`SETUP_FINALLY` and `POP_BLOCK` instructions are removed, and the
exception table is constructed, mapping each instruction to the
exception handler that covers it, if any. Instructions which
are not covered by any exception handler within the same code
object's bytecode, do not appear in the exception table at all.
When the pseudo-instructions are translated into bytecode,
`SETUP_FINALLY` and `POP_BLOCK` are removed, and the exception
table is constructed, mapping each instruction to the exception
handler that covers it, if any. Instructions which are not
covered by any exception handler within the same code object's
bytecode, do not appear in the exception table at all.

For the code object in our example above, the table has a single
entry specifying that all instructions between the `SETUP_FINALLY`
and the `POP_BLOCK` are covered by the exception handler located
at label `L1`.
entry specifying that all instructions that were between the
`SETUP_FINALLY` and the `POP_BLOCK` are covered by the exception
handler located at label `L1`.

Handling Exceptions
-------------------
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