Fix parsing of fixed point numbers

[sfinkens]

This PR fixes the following problems with the parsing of fixed point numbers:

• Parsed value is not a float but a string
• Parsing fails if the format specifier includes width or precision (Parsing zero padded floats #27)

As suggested by @djhoese I added a new regular expression that is used if the format specifier includes width or precision. Finally, I updated the converter method to return floats.

---

Examples:

1. "Naive" case without width and precision, matches any float:

>>> trollsift.parse('{foo:f}', '123.456')
{'foo': 123.456}

2. Including width and precision:

>>> trollsift.parse('{foo:5.2f}', '12.34')
{'foo': 12.34}

Width is considered minimum width, so that longer strings will also be matched (as long as the decimals match). This is to facilitate roundtrips with compose.

>>> trollsift.parse('{foo:5.2f}', '1234.56')
{'foo': 1234.56}
>>> trollsift.compose('{foo:5.2f}', {'foo': 1234.56})
'1234.56'

The following cases will not be matched because the string is too short or the number of decimals is incorrect:

>>> trollsift.parse('{foo:5.2f}', '1.23')
ValueError: String does not match pattern.
>>> trollsift.parse('{foo:5.2f}', '1.234')
ValueError: String does not match pattern.

3. Zero/whitespace padding is also supported

>>> trollsift.parse('{foo:05.2f}', '01.23')
{'foo': 1.23}

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[coveralls]

Coverage increased (+0.5%) to 95.489% when pulling 3b60acb on sfinkens:fix-parsing-fixed-point into 144b2be on pytroll:master.

[djhoese]

Parsed value is not a float but a string

Do you have an example of this bug?

[djhoese]

I think this makes sense to me. I was worried it was going to get complicated and this definitely leans that way, but still not quite as bad as I thought it'd be. Nice job.

For your tests, what do you think about using pytest.mark.parameterize? You'd have to move the test function(s) outside the unittest class as regular functions or create a separate test class (ex. class TestParser:) that isn't based on unittest.TestCase.

[sfinkens]

Do you have an example of this bug?

Sure, this is with the current master:

>>> trollsift.parse('{foo:f}', '12.34'}
{'foo': '12.34'}

For your tests, what do you think about using pytest.mark.parameterize?

Good idea! I'll update the tests.

[sfinkens]

I also refactored the _convert method to reduce complexity (CodeFactor was complaining)

[djhoese]

If tests pass then LGTM

[pnuu]

LGTM