Space as a thousands separator in the format specification mini-language

Ideas
9

I developed a pypi package called sciform to address this and other scientific formatting requirements. Separators for upper digits (digits in decimal places above the decimal symbol), separators for the lower digits (digits in decimal places below the decimal symbol) and the decimal separator can be controlled using the upper_separator, lower_separator and decimal_separator options respectively. Here’s code demonstrating one way to access these options:

from sciform import GlobalOptionsContext, SciNum

num = SciNum(123456.654321)

print("Only decimal separator:")
print(f"{num}")

print("Space separators:")
with GlobalOptionsContext(upper_separator=" ", lower_separator=" "):
    print(f"{num}")

print("Point decimal separator:")
with GlobalOptionsContext(upper_separator=",", lower_separator="_"):
    print(f"{num}")

print("Comma decimal separator:")
with GlobalOptionsContext(upper_separator=".", decimal_separator=",", lower_separator=" "):
    print(f"{num}")

gives results:

>>> Only decimal separator:
>>> 123456.654321
>>> Space separators:
>>> 123 456.654 321
>>> Point decimal separator:
>>> 123,456.654_321
>>> Point decimal separator:
>>> 123.456,654 321

The SciNum object exposes a format specification mini-language (FSML) which is loosely an extension of the python built-in FSML*. In previous versions of sciform I considered doing as you suggest and including control for the various separators into the FSML. However, there got to be way to many options and the FSML had to become very complex to ensure an unambiguous reading of all specification strings (see the comments above about how the space character " " already plays a role in the FSML. In the end my conclusion was that the FSML was already very complicated and it wouldn’t be worth it to further complicate the FSML with these options. Instead, it would be better to configure these options at some higher level of configuration the the FSML.

Here’s a github issue where I discussed this in some further detail.

sciform was in part motivated by the discussion here on Python discourse: New format specifiers for string formatting of floats with SI and IEC prefixes. Near the end of that thread the conversation was moving in the direction: “This is a good idea for a pypi package, it doesn’t need to be built into python.” After having spent time thinking about this, I think that is how I lean on this idea as well.**

*It is not strictly an extension since there are some features in the python FSML which are deliberately not available in the sciform FSML.

**I do want to emphasize that I still think some of the suggestions in the other thread (most importantly better controlled sig fig rounding) would be worth including in python. Just trying to say that this specific feature would make the FSML too hairy for what is gained.

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