[issue39694] Incorrect dictionary unpacking when calling str.format

[Akos Kiss]

Akos Kiss <akosthekiss at gmail.com> added the comment:

I've come up with some extra examples (use cases?):

```py
import collections

class str2(str):
    def format(self, *args, **kwargs):
        return super().format(*args, **kwargs)

def unpacknonekey(s):
    print('input:', type(s), s)
    try:
        print('str key:', s.format(**{'bar': 'qux'}))
        print('none key:', s.format(**{'bar': 'qux', None: ''}))
    except TypeError as e:
        print('error:', e)

template = 'foo {bar} baz'
unpacknonekey(template)
unpacknonekey(str2(template))
unpacknonekey(collections.UserString(template))
```

The above script gives the following output:

```
input: <class 'str'> foo {bar} baz
str key: foo qux baz
none key: foo qux baz
input: <class '__main__.str2'> foo {bar} baz
str key: foo qux baz
error: format() keywords must be strings
input: <class 'collections.UserString'> foo {bar} baz
str key: foo qux baz
error: format() keywords must be strings
```

This shows inconsistency between `format` of `str` and subclasses of `str` or the standard library's `UserString`.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39694>
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