bz2 would use a dedicated exception for data error

[mgorny]

BPO	41725
Nosy	@terryjreedy, @mgorny, @serhiy-storchaka

^{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 = None
created_at = <Date 2020-09-05.10:26:14.525>
labels = ['extension-modules', 'type-feature', '3.10']
title = 'bz2 would use a dedicated exception for data error'
updated_at = <Date 2020-09-12.03:19:17.385>
user = 'https://github.com/mgorny'

bugs.python.org fields:

activity = <Date 2020-09-12.03:19:17.385>
actor = 'mgorny'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2020-09-05.10:26:14.525>
creator = 'mgorny'
dependencies = []
files = []
hgrepos = []
issue_num = 41725
keywords = []
message_count = 3.0
messages = ['376427', '376744', '376769']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'mgorny', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41725'
versions = ['Python 3.10']

[mgorny]

Currently, the bz2 extension functions raise a generic OSError() on BZ_DATA_ERROR and BZ_DATA_ERROR_MAGIC. When working on files, this makes it unnecessarily hard to catch compressor errors (i.e. invalid bz2 input file) without catching system I/O errors. Could you please consider adding a new exception type for bz2 errors, akin to gzip.BadGzipFile?

[terryjreedy]

An OSError for a file the OS can read seems a bit off. What is the accompanying message? Would something like ValueError("bz2 compressor failed with BZ_DATA_ERROR") be better?

Michał, are there precedents or models for this in the other file compression modules?

[mgorny]

An OSError for a file the OS can read seems a bit off. What is the accompanying message?

For example:

OSError: Invalid data stream

(I've just put random string into a file, and opened it as bzip2)

Would something like ValueError("bz2 compressor failed with BZ_DATA_ERROR") be better?

It would be a partial improvement. However, it would also break backwards compatibility with code already catching OSError. It might be better to go with custom exception type derived from OSError to keep existing code working.

Michał, are there precedents or models for this in the other file compression modules?

Python 3.8 has introduced BadGzipFile in gzip module, to replace their OSError.

lzma has used LZMAError from day one.