PEP 3110 – Catching Exceptions in Python 3000

PEP: 3110
Title: Catching Exceptions in Python 3000
Author: Collin Winter <collinwinter at google.com>
Status: Final
Type: Standards Track
Created: 16-Jan-2006
Python-Version: 3.0
Post-History

Abstract
Rationale
Grammar Changes
Semantic Changes
Compatibility Issues
- 2.6 - 3.0 Compatibility
Open Issues
- Replacing or Dropping “sys.exc_info()”
Implementation
References
Copyright

Abstract

This PEP introduces changes intended to help eliminate ambiguities in Python’s grammar, simplify exception classes, simplify garbage collection for exceptions and reduce the size of the language in Python 3.0.

Rationale

except clauses in Python 2.x present a syntactic ambiguity where the parser cannot differentiate whether
```
except <expression>, <expression>:
```
should be interpreted as
```
except <type>, <type>:
```
or
```
except <type>, <name>:
```
Python 2 opts for the latter semantic, at the cost of requiring the former to be parenthesized, like so
```
except (<type>, <type>):
```
As specified in PEP 352 1, the ability to treat exceptions as tuples will be removed, meaning this code will no longer work
```
except os.error, (errno, errstr):
```
Because the automatic unpacking will no longer be possible, it is desirable to remove the ability to use tuples as except targets.
As specified in PEP 344 5, exception instances in Python 3 will possess a __traceback__ attribute. The Open Issues section of that PEP includes a paragraph on garbage collection difficulties caused by this attribute, namely a “exception -> traceback -> stack frame -> exception” reference cycle, whereby all locals are kept in scope until the next GC run. This PEP intends to resolve this issue by adding a cleanup semantic to except clauses in Python 3 whereby the target name is deleted at the end of the except suite.
In the spirit of “there should be one – and preferably only one – obvious way to do it” 2, it is desirable to consolidate duplicate functionality. To this end, the exc_value, exc_type and exc_traceback attributes of the sys module 3 will be removed in favor of sys.exc_info(), which provides the same information. These attributes are already listed in PEP 3100 4 as targeted for removal.

Grammar Changes

In Python 3, the grammar for except statements will change from 8

except_clause: 'except' [test [',' test]]

except_clause: 'except' [test ['as' NAME]]

The use of as in place of the comma token means that

except (AttributeError, os.error):

can be clearly understood as a tuple of exception classes. This new syntax was first proposed by Greg Ewing 6 and endorsed (6, 7) by the BDFL.

Further, the restriction of the token following as from test to NAME means that only valid identifiers can be used as except targets.

Note that the grammar above always requires parenthesized tuples as exception classes. That way, the ambiguous

except A, B:

which would mean different things in Python 2.x and 3.x – leading to hard-to-catch bugs – cannot legally occur in 3.x code.

Semantic Changes

In order to resolve the garbage collection issue related to PEP 344, except statements in Python 3 will generate additional bytecode to delete the target, thus eliminating the reference cycle. The source-to-source translation, as suggested by Phillip J. Eby 9, is

try:
    try_body
except E as N:
    except_body
...

gets translated to (in Python 2.5 terms)

try:
    try_body
except E, N:
    try:
        except_body
    finally:
        N = None
        del N
...

An implementation has already been checked into the py3k (formerly “p3yk”) branch 10.

Compatibility Issues

Nearly all except clauses will need to be changed. except clauses with identifier targets will be converted from

except E, N:

except E as N:

except clauses with non-tuple, non-identifier targets (e.g., a.b.c[d]) will need to be converted from

except E, T:

except E as t:
    T = t

Both of these cases can be handled by Guido van Rossum’s 2to3 utility 11 using the except fixer 12.

except clauses with tuple targets will need to be converted manually, on a case-by-case basis. These changes will usually need to be accompanied by changes to the exception classes themselves. While these changes generally cannot be automated, the 2to3 utility is able to point out cases where the target of an except clause is a tuple, simplifying conversion.

Situations where it is necessary to keep an exception instance around past the end of the except suite can be easily translated like so

try:
    ...
except E as N:
    ...
...

becomes

try:
    ...
except E as N:
    n = N
    ...
...

This way, when N is deleted at the end of the block, n will persist and can be used as normal.

Lastly, all uses of the sys module’s exc_type, exc_value and exc_traceback attributes will need to be removed. They can be replaced with sys.exc_info()[0], sys.exc_info()[1] and sys.exc_info()[2] respectively, a transformation that can be performed by 2to3’s sysexcattrs fixer.

2.6 - 3.0 Compatibility

In order to facilitate forwards compatibility between Python 2.6 and 3.0, the except ... as ...: syntax will be backported to the 2.x series. The grammar will thus change from:

except_clause: 'except' [test [',' test]]

to:

except_clause: 'except' [test [('as' | ',') test]]

The end-of-suite cleanup semantic for except statements will not be included in the 2.x series of releases.

Open Issues

Replacing or Dropping “sys.exc_info()”

The idea of dropping sys.exc_info() or replacing it with a sys.exception attribute or a sys.get_exception() function has been raised several times on python-3000 (13, 14) and mentioned in PEP 344’s “Open Issues” section.

While a 2to3 fixer to replace calls to sys.exc_info() and some attribute accesses would be trivial, it would be far more difficult for static analysis to find and fix functions that expect the values from sys.exc_info() as arguments. Similarly, this does not address the need to rewrite the documentation for all APIs that are defined in terms of sys.exc_info().