fvalues

This is a Python library for keeping track of the combination of components in a string. In particular it lets you separate out the formatted values in an f-string. Here's an example:

from fvalues import F, FValue

x = 1.2345
f = F(f"twice x is {x * 2:.2f}")
assert f == "twice x is 2.47"
assert f.parts == ("twice x is ", FValue(source="x * 2", value=2.469, formatted="2.47"))

Key facts:

• F is a subclass of str so it can generally be used like any other string.
• Calls to the constructor F() are magically detected using the executing library, and f-strings within are parsed to extract their components.
• These are saved in the attribute F.parts. Each part is either a str representing a constant section or an FValue representing a dynamic expression.
• FValue.source contains the source code between the braces ({}) but before the colon (:) and format spec (.2f). In some cases it may not be the exact original source code, but equivalent code produced by ast.unparse.
• FValue.value and FValue.formatted are calculated using eval(), so be careful of expressions that you wouldn't want to evaluate twice due to performance or side effects.

Concatenation

The F class also has special support for concatenation with the + operator:

f += "!"
assert f == "twice x is 2.47!"
assert f.parts == (
    FValue(
        source="f",
        value="twice x is 2.47",
        formatted="twice x is 2.47",
    ),
    "!",
)

Similar to deconstructing f-strings, you can see how the parts distinguish between the dynamic expression f on the left of +=, representing it as an FValue, and the static "!" on the right.

Flattening

In the assertion above above, FValue.value is shown as a plain string, but remember that it's actually also an F object itself. The assertion works because F is a subclass of str so they can be used interchangeably. But it still has the same parts that we saw earlier. Sometimes keeping the tree of parts in its original form can be useful, other times you may want to bring everything to the surface to make things easier. You can produce an equivalent F object with a flat list of parts using F.flatten:

assert f.flatten().parts == (
    "twice x is ",
    FValue(
        source="x * 2",
        value=2.469,
        formatted="2.47",
    ),
    "!",
)

Other string methods

Another method that's specially implemented for F is .strip() and its cousins lstrip and rstrip. It does the same thing as the usual str.strip as far as the whole string is concerned, but also strips the internal parts in the way you'd probably expect. See the docstring for more details.

All other methods are directly inherited from str, which means that methods such as .lower() will return a plain str rather than another F object. So be careful with those methods if you don't want to lose information about the parts! More specialised implementations may be added in the future.