Question: How do I transform a classic string to an f-string?

```
variable = 42
user_input = "The answer is {variable}"
print(user_input)

```

Output: `The answer is {variable}`

```
f_user_input = # Here the operation to go from a string to an f-string
print(f_user_input)

```

Desired output: `The answer is 42`

Answer: 
An f-string is **syntax**, not an object type. You can't convert an arbitrary string to that syntax, the syntax creates a string object, not the other way around.

I'm assuming you want to use `user_input` as a template, so just use the [`str.format()` method](https://docs.python.org/3/library/stdtypes.html#str.format) on the `user_input` object:

```
variable = 42
user_input = "The answer is {variable}"
formatted = user_input.format(variable=variable)

```

If you wanted to provide a configurable templating service, create a namespace dictionary with all fields that can be interpolated, and use `str.format()` with the `**kwargs` call syntax to apply the namespace:

```
namespace = {'foo': 42, 'bar': 'spam, spam, spam, ham and eggs'}
formatted = user_input.format(**namespace)

```

The user can then use any of the keys in the namespace in `{...}` fields (or none, unused fields are ignored).
Just to add one more similar way how to do the same.
But **str.format()** option is **much preferable** to use.

```
variable = 42
user_input = "The answer is {variable}"
print(eval(f"f'{user_input}'"))

```

A safer way to achieve the same as Martijn Pieters mentioned above:

```
def dynamic_string(my_str, **kwargs):
    return my_str.format(**kwargs)

variable = 42
user_input = "The answer is {variable}"
print('1: ', dynamic_string(my_str=user_input, variable=variable))
print('2: ', dynamic_string(user_input, variable=42))

```

```
1:  The answer is 42
2:  The answer is 42

```