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:

   >>> foo = 'bar'
   

2. Then, we declare a new variable, which is our f-string, and it should take foo to be formatted:

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

3. Ok, all going fine and then we call baz to check its value:

   >>> baz
   'Hanging on in bar'
   

4. Let's try to change the value of foo and call baz again:

   >>> foo = 'spam'
   >>> baz
   'Hanging on in bar'
   

Shouldn't it be dynamic? Why does this happen? I thought the f-string would update if the value of foo changed, but this didn't happened. I don't understand how this works.

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}')
<class 'str'>

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.