[New-bugs-announce] [issue27164] zlib can't decompress DEFLATE using shared dictionary

[Vladimir Mihailenco]

New submission from Vladimir Mihailenco:

Consider following code:

```
import zlib


hello = b'hello'

# Compress data using deflate and shared/predefined dict.
co = zlib.compressobj(wbits=-zlib.MAX_WBITS, zdict=hello)
data = co.compress(hello) + co.flush()

# Decompress deflate by providing same dict.
do = zlib.decompressobj(wbits=-zlib.MAX_WBITS, zdict=hello)
data = do.decompress(data)

print(data)
```

Decompression fails with "zlib.error: Error -3 while decompressing data: invalid distance too far back".

My original task was to decompress data I get from Golang library - https://golang.org/pkg/compress/flate/#NewWriterDict

----------
components: Library (Lib)
messages: 266747
nosy: Vladimir Mihailenco
priority: normal
severity: normal
status: open
title: zlib can't decompress DEFLATE using shared dictionary
type: behavior
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27164>
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