[New-bugs-announce] [issue45936] collections.Counter drops key if value is 0 and updating using += operator

[Richard Decal]

New submission from Richard Decal <richard.decal at gmail.com>:

In brief:

```
from collections import Counter
x = Counter({'a': 0, 'b': 1})
x.update(x)  # works: Counter({'a': 0, 'b': 2})
x += x  # expected: Counter({'a': 0, 'b': 3}) actual: Counter({'b': 3})
```

I expect `+=` and `.update()` to be synonymous. However, the += operator is deleting keys if the source Counter has a zero count to begin with:

```
x = Counter({'a': 1})
x += Counter({'a': 0})  # ok: Counter({'a': 1})

y = Counter({'a': 0})
y += y  # expected: Counter({'a': 0}) actual: Counter()
```

----------
messages: 407348
nosy: crypdick
priority: normal
severity: normal
status: open
title: collections.Counter drops key if value is 0 and updating using += operator
type: behavior
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45936>
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