Why don't f-strings change when variables they reference change?

Question

While playing with new f-strings in the recent Python 3.6 release, I\'ve noticed the following:

1. We create a foo variable with value bar

Answer 1

The f-string has already been evaluated when you executed:

>>> baz = f'Hanging on in {foo}'

Specifically, it looked up the value for the name foo and replaced it with 'bar', the value that was found for it. baz then contains the string after it has been formatted.

f-strings aren't constant; meaning, they don't have a replacement field inside them waiting for evaluation after being evaluated. They evaluate when you execute them, after that, the assigned value is just a normal string:

>>> type(f'hanging on in {foo}')

For reference, see the section on Formatted String Literals:

[..] While other string literals always have a constant value, formatted strings are really expressions evaluated at run time. [..]

After the expression (the look-up for the replacement field and its consequent formatting) is performed, there's nothing special about them, the expression has been evaluated to a string and assigned to baz.