How does parameter `default_style` work?

[YangHanlin]

I am using PyYAML 5.4.1 and I wonder how the optional paramter default_style in the function yaml.dump works. From other issues and part of the documentation, I guessed this parameter is to control the format of strings in dumpped document, and the possible values are '"', '|', '>' and None (as default) etc.

But I am getting unexpected formats of dumpped documents when using this parameter. For example, if I am to dump a simple dictionary:

>>> data = {'cases': [{'output': '0 10\n1 4', 'timeout': 1.0}]}
>>> print(yaml.dump(data, default_style='|'))
"cases":
- "output": |-
    0 10
    1 4
  "timeout": !!float |-
    1.0

Why are keys quoted in "'s? And why the value of timeout is represented as string with extra !!float tag? Is there a way to achieve the expected result:

cases:
- output: |-
    0 10
    1 4
  timeout: 1.0

Thanks in advance, and apologies if GitHub issues are not the place for this kind of questions.