cardinal_pythonlib.compression¶

Original code copyright (C) 2009-2022 Rudolf Cardinal (rudolf@pobox.com).

This file is part of cardinal_pythonlib.

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Compression functions.

cardinal_pythonlib.compression.gunzip_string(zipped: bytes, encoding: str = 'utf-8') → str[source]¶

Reverses gzip_string(), i.e. un-gzip-compresses it, then decodes it into a string.

Parameters:	zipped – zipped data encoding – encoding that was used for the string prior to compression
Returns:	text
Return type:	str
Raises:	`OsError` if the data wasn’t gzipped `UnicodeDecodeError` if the decompressed data won’t decode

cardinal_pythonlib.compression.gzip_string(text: str, encoding: str = 'utf-8') → bytes[source]¶

Encodes a string, then compresses it with gzip.

When you send data over HTTP and wish to compress it, what should you do?

Use HTTP Content-Encoding; https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding.
This is defined in HTTP/1.1; see https://www.ietf.org/rfc/rfc2616.txt.
The gzip format is the most broadly supported, according to https://en.wikipedia.org/wiki/HTTP_compression.
This format is defined in https://www.ietf.org/rfc/rfc1952.txt.
The gzip format has a header; see above and https://en.wikipedia.org/wiki/Gzip.
Python’s gzip.compress() writes to a memory file internally and writes the header.
So the work in the most popular answer here is unnecessary: https://stackoverflow.com/questions/8506897/how-do-i-gzip-compress-a-string-in-python
All we need is conversion of the string to bytes (via the appropriate encoding) and then gzip.compress().
The requestor should also set HTTP Accept-Encoding if it wants compressed data back. See RFC2616 again (as above).

Parameters:	text – a string to compress encoding – encoding to use when converting string to bytes prior to compression
Returns:	gzip-compressed data
Return type:	bytes

Test code:

import io
import gzip

teststring = "Testing; one, two, three."
testbytes = teststring.encode("utf-8")

gz1 = gzip.compress(testbytes)

out1 = io.BytesIO()
with gzip.GzipFile(fileobj=out1, mode="w") as gzfile1:
    gzfile1.write(testbytes)
gz2 = out1.getvalue()

print(len(gz1) == len(gz2))  # False
print(gz1 == gz2)  # False
# ... but the difference is probably in the timestamp bytes!