Provide a way to skip magic-number search in ZipFile(mode='a')

[BartRobinson]

BPO	42594

^{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 2020-12-07.22:40:05.720>
labels = ['type-feature', 'library', '3.10']
title = "Provide a way to skip magic-number search in ZipFile(mode='a')"
updated_at = <Date 2020-12-07.22:40:05.720>
user = 'https://bugs.python.org/BartRobinson'

bugs.python.org fields:

activity = <Date 2020-12-07.22:40:05.720>
actor = 'Bart Robinson'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-12-07.22:40:05.720>
creator = 'Bart Robinson'
dependencies = []
files = []
hgrepos = []
issue_num = 42594
keywords = []
message_count = 1.0
messages = ['382691']
nosy_count = 1.0
nosy_names = ['Bart Robinson']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42594'
versions = ['Python 3.10']

[BartRobinson]

When a ZipFile object is created with mode='a', the existing file contents are checked for the magic number b"PK\005\006" near the end of the file. If a non-zipfile just happens to contain this magic number, it can confuse the library into assuming the file is a zipfile when it is not. It would be great if ZipFile.__init__() provided a way to skip the magic-number check and force a new central directory to be appended to the file.

This could take the form of an additional named argument like ZipFile.__init__(force_append=True), or an additional character in the mode string, like 'a+'. Either of these options should be backward-compatible with existing code.

Currently, my company has code that uses monkey-patching to work around the lack of this feature in the standard library. We use mode='a' to append metadata to files in existing formats that can contain arbitrary binary data and so occasionally include the magic number.

[thatch]

I suspect #134250 will change this behavior, but without a repro of some sort don't know if it will fix things for you.