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pythonpython-3.xpython-3.6f-string

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

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.

Solution

  • 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.