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The default pickle protocol is 4 now. But shelve still uses the pickle protocol 3. Shouldn't it be bumped? Shouldn't shelve use pickle.DEFAULT_PROTOCOL by default?
Disadvantages:
1. This will make shelve files incompatible with Python 3.3 by default.
2. Protocol 4 adds 9 bytes of overhead in comparison with protocol 3. This can be too large for the shelve containing a lot of small objects. Maybe strip redundant frame header for small pickles?

I wrote a short script to see the impact of file size depending on the protocol:
---
import shelve
import os.path
print("== Short value ==")
for proto in (0, 1, 2, 3, 4, 5):
 filename = 'shelve-picklev%s' % proto
 with shelve.open(filename, protocol=proto) as db:
 assert db._protocol == proto
 for x in range(1000):
 db[str(x)] = str(x)
 print(f'Protocol {proto}: {os.path.getsize(filename)} bytes')
 os.unlink(filename)
print()
print("== Large value ==")
large_value = [str(x) for x in range(1000)]
for proto in (0, 1, 2, 3, 4, 5):
 filename = 'shelve-picklev%s' % proto
 with shelve.open(filename, protocol=proto) as db:
 assert db._protocol == proto
 for x in range(10):
 db[str(x)] = large_value
 print(f'Protocol {proto}: {os.path.getsize(filename)} bytes')
 os.unlink(filename)
---
Output with Python 3.9.0b1 (on Fedora 32):
---
== Short value ==
Protocol 0: 90112 bytes
Protocol 1: 94208 bytes
Protocol 2: 94208 bytes
Protocol 3: 94208 bytes
Protocol 4: 94208 bytes
Protocol 5: 94208 bytes
== Large value ==
Protocol 0: 139264 bytes
Protocol 1: 139264 bytes
Protocol 2: 139264 bytes
Protocol 3: 139264 bytes
Protocol 4: 98304 bytes
Protocol 5: 98304 bytes
---
For short string values, protocol 0 produces smaller files than protocol 1 and higher.
For large value, protocol 4 and higher produce smaller files than protocol 3 and lower.

I've opened https://github.com/python/cpython/pull/22751 to fix this, I know there was already a PR, but it seems to have been abandoned.

It has not been abandoned.

New changeset df59273c7a384ea8c890fa8e9b80c92825df841c by Zackery Spytz in branch 'master':
bpo-34204: Use pickle.DEFAULT_PROTOCOL in shelve (GH-19639)
https://github.com/python/cpython/commit/df59273c7a384ea8c890fa8e9b80c92825df841c