golang/python zlib difference

Debugging differences between Python's zlib and golang's zlib. Why don't the following have the same results?

compress.go:

package main

import (
    "compress/flate"
    "bytes"
    "fmt"
)


func compress(source string) []byte {
    w, _ := flate.NewWriter(nil, 7)
    buf := new(bytes.Buffer)

    w.Reset(buf)
    w.Write([]byte(source))
    w.Close()

    return buf.Bytes()
}


func main() {
    example := "foo"
    compressed := compress(example)
    fmt.Println(compressed)
}

compress.py:

from __future__ import print_function

import zlib


def compress(source):
    # golang zlib strips header + checksum
    compressor = zlib.compressobj(7, zlib.DEFLATED, -15)
    compressor.compress(source)
    # python zlib defaults to Z_FLUSH, but 
    # https://golang.org/pkg/compress/flate/#Writer.Flush
    # says "Flush is equivalent to Z_SYNC_FLUSH"
    return compressor.flush(zlib.Z_SYNC_FLUSH)


def main():
    example = u"foo"
    compressed = compress(example)
    print(list(bytearray(compressed)))


if __name__ == "__main__":
    main()

Results

$ go version
go version go1.7.3 darwin/amd64
$ go build compress.go
$ ./compress
[74 203 207 7 4 0 0 255 255]
$ python --version
$ python 2.7.12
$ python compress.py
[74, 203, 207, 7, 0, 0, 0, 255, 255]

The Python version has 0 for the fifth byte, but the golang version has 4 -- what's causing the different output?

The output from the python example isn't a "complete" stream, its just flushing the buffer after compressing the first string. You can get the same output from the Go code by replacing Close() with Flush():

https://play.golang.org/p/BMcjTln-ej

func compress(source string) []byte {
    buf := new(bytes.Buffer)
    w, _ := flate.NewWriter(buf, 7)
    w.Write([]byte(source))
    w.Flush()

    return buf.Bytes()
}

However, you are comparing output from zlib in python, which uses DEFLATE internally to produce a zlib format output, and flate in Go, which is a DEFLATE implementation. I don't know if you can get the python zlib library to output the raw, complete DEFLATE stream, but trying to get different libraries to output byte-for-byte matches of compressed data doesn't seem useful or maintainable. The output of the compression libraries is only guaranteed to be compatible, not identical.