Same MemoryError object gets thrown from different places.

[SylvainDe]

BPO	24529
Nosy	@vstinner, @bitdancer, @SylvainDe

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-06-29.16:11:11.306>
created_at = <Date 2015-06-29.14:42:55.956>
labels = ['interpreter-core', 'invalid']
title = 'Same MemoryError object gets thrown from different places.'
updated_at = <Date 2015-06-29.16:12:18.888>
user = 'https://github.com/SylvainDe'

bugs.python.org fields:

activity = <Date 2015-06-29.16:12:18.888>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2015-06-29.16:11:11.306>
closer = 'r.david.murray'
components = ['Interpreter Core']
creation = <Date 2015-06-29.14:42:55.956>
creator = 'SylvainDe'
dependencies = []
files = []
hgrepos = []
issue_num = 24529
keywords = []
message_count = 2.0
messages = ['245940', '245948']
nosy_count = 3.0
nosy_names = ['vstinner', 'r.david.murray', 'SylvainDe']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue24529'
versions = ['Python 2.7']

[SylvainDe]

Disclaimer: This is very minor, impacts only Python 2 and is maybe not even a bug.

Context: I was "randomly" changing attribute of Exception objects when I discovered something I did not expect : sometimes when an exception occurs, a new object is raised (which is what I expected), sometimes a previously thrown exception is re-thrown (which surprised me).

More specifically, MemoryError objects seem to be reused while most of the other exceptions I've played with where newly created.

One can easily see this happening in the following piece of code :

• first a NameError is caught and updated
• then, we run the same code and print the re-caught exception : it is a new one
• then, a MemoryError is caught and updated
• then, we run the same code and print the re-caught exception : it is the same as previously.

<pre>
from __future__ import print_function

old_e = None
try:
    foobar
except NameError as old_e:
    print("old_e:", old_e)
    old_e.args = tuple(['NEW VALUE'])
    print("old_e:", old_e)
try:
    foobar
except NameError as new_e:
    print("new_e:", new_e)
    print(old_e is new_e)

old_e = None
try:
    l = [0] * 999999999999999999
except MemoryError as old_e:
    print("old_e:", old_e)
    old_e.args = tuple(['NEW VALUE'])
    print("old_e:", old_e)
try:
    l = [0] * 999999999999999999
except MemoryError as new_e:
    print("new_e:", new_e)
    print(old_e is new_e)
</pre>

For the record, I am quite aware that this shouldn't impact anyone using Python normally. I was just messing around as part of a toy project where I try to enhance exception messages for a better user experience ( https://github.com/SylvainDe/DidYouMean-Python ).

[bitdancer]

If the same memory exception object were not used, we'd have to allocate memory for the new one...