[issue38781] Clear buffer in MemoryHandler flush

[Daniel Andersson]

Daniel Andersson  added the comment:

The suggested change has been merged. I'm closing this issue. Thank you Vinay 
Sajip for reviewing.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue38781] Clear buffer in MemoryHandler flush

[Daniel Andersson]

Change by Daniel Andersson :


--
keywords: +patch
pull_requests: +16642
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17132

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[issue38781] Clear buffer in MemoryHandler flush

[Daniel Andersson]

New submission from Daniel Andersson :

The `logging.handlers.MemoryHandler` has a method `flush` which clears the 
buffer by assigning an empty list literal:

self.buffer = []

This forces the buffer to be a list instance.
My suggestion is to clear the buffer like this instead:

self.buffer.clear()

In this way it would be possible to implement a custom buffer or use the 
`collections.deque` when subclassing `MemoryHandler`. At the moment you must 
copy-past the `flush` method and modify it accordingly in the subclass:

```
def flush(self):
# (Docstring skipped)
self.acquire()
try:
if self.target:
for record in self.buffer:
self.target.handle(record)
self.buffer = []  # <-- change to `self.buffer.clear()`
finally:
self.release()
```

Example where this change is useful
===
This example creates a DequeMemoryHandler which uses the `collections.deque` as 
a buffer. Only the latest `capacity` number of records will stored in the 
buffer. The buffer is then flushed if and only if a record has a level greater 
or equal to `logging.ERROR`.

```
import collections
import logging
from logging.handlers import MemoryHandler


class DequeMemoryHandler(MemoryHandler):

def __init__(self, capacity, *args, **kwargs):
super().__init__(capacity, *args, **kwargs)
self.buffer = collections.deque(maxlen=capacity)

def shouldFlush(self, record):
return record.levelno >= self.flushLevel


handler = DequeMemoryHandler(capacity=5, target=logging.StreamHandler())
logging.basicConfig(level=logging.INFO, handlers=[handler])

for i in range(1, 21):
logging.info('Spam %d', i)
if i % 10 == 0:
logging.error(f'---> Eggs {i}')
```


Desired output (with the change):
-
Spam 7
Spam 8
Spam 9
Spam 10
---> Eggs 10
Spam 17
Spam 18
Spam 19
Spam 20
---> Eggs 20


Actual output (without the change):
---
Spam 7
Spam 8
Spam 9
Spam 10
---> Eggs 10
Spam 11
Spam 12
Spam 13
Spam 14
Spam 15
Spam 16
Spam 17
Spam 18
Spam 19
Spam 20
---> Eggs 20

--
components: Library (Lib)
messages: 356489
nosy: penlect, vinay.sajip
priority: normal
severity: normal
status: open
title: Clear buffer in MemoryHandler flush
type: enhancement
versions: Python 3.9

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