Keep track of components in strings, especially formatted values in f-strings.

pip install fvalues==0.0.4



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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.

Usage in ICE

This library was built to enhance the Interactive Composition Explorer (ICE). In the screenshot below, the prompt under 'Inputs' on the right is an F object, and the colored text corresponds to dynamic FValues.

ICE screenshot


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

f += "!"
assert f == "twice x is 2.47!"
assert f.parts == (
        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.


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 ",
        source="x * 2",

Other string methods

Most F methods (e.g. .lower()) are directly inherited from str, which means that they 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! The methods below have specialised implementations to avoid this. More may be added in the future.


F.strip 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. The related methods lstrip and rstrip strip the left/right sides as expected.

Make sure to write F(f"...").strip() rather than F(f"...".strip()) or the f-string magic won't work.


separator.join(strings) will return an F object only if separator is an F object. If separator is a plain str, then the result will be a plain str, even if strings is a list of F objects. In practice, this typically means you should write e.g. F(" ").join(strings) rather than " ".join(strings).