[issue30213] ZipFile from 'a'ppend-mode file generates invalid zip

[BoppreH]

New submission from BoppreH:

I may be misunderstanding file modes or the `zipfile` library, but

from zipfile import ZipFile
ZipFile(open('a.zip', 'ab'), 'a').writestr('f.txt', 'z')

unexpectedly creates an invalid zip file. 7zip is able to open and show the 
file list, but files inside look empty, and Windows simply says it's invalid. 

Changing the file mode from `ab` to `wb+` fixes the problem, but truncates the 
file, and `rb+` doesn't create the file. Calling `close` on both the `open` and 
`ZipFile` doesn't help either. Using `ZipFile(...).open` instead of `writestr` 
has the same problem.

I could only reproduce this on [Windows 10, Python 3.6.1, 64 bit]. The zip file 
was proper on [Windows 10, Python 3.3.5, 32 bit], [Windows 10 Bash, Python 
3.4.3, 64 bit], and [FreeBSD, Python 3.5.3, 64 bit].

This is my first bug report, so forgive me if I made any mistakes.

--
components: Library (Lib), Windows
messages: 292616
nosy: BoppreH, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: ZipFile from 'a'ppend-mode file generates invalid zip
type: behavior
versions: Python 3.6

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[issue30213] ZipFile from 'a'ppend-mode file generates invalid zip

[BoppreH]

BoppreH  added the comment:

I'm not sure this is a duplicate of issue29094. That issue includes random data 
at the start of the file, while this issue uses the 'ab' mode solely for a 
creating the file if it doesn't exist (and thus is either empty or already a 
valid zip file). It's not clear to me why 'wb' should work but not 'ab' if the 
file was empty/missing to begin with.

[Windows 10, Python 3.6.3, 64 bit] still has the same problem.

Here's a more complete test case, starting with no existing files:

from zipfile import ZipFile

# Append mode: v
with open('file.zip', 'ab') as f:
with ZipFile(f, 'a') as zip:
zip.writestr('file.txt', 'contents')
with open('file.zip', 'rb') as f:
with ZipFile(f, 'r') as zip:
print(zip.read('file.txt'))
# Fails with "zipfile.BadZipFile: Bad magic number for file 
header"

# Write mode:  v
with open('file.zip', 'wb') as f:
with ZipFile(f, 'a') as zip:
zip.writestr('file.txt', 'contents')
with open('file.zip', 'rb') as f:
with ZipFile(f, 'r') as zip:
print(zip.read('file.txt'))
# Works.

--

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