How can I use f-string with a variable, not with a string literal?

Question

I want to use f-string with my string variable, not with string defined with a string literal, "...".

Here is my code:

name=["deep","mahesh","nirbhay"] user_input = r"certi_{element}" # this string I ask from user    for element in name:     print(f"{user_input}") 

This code gives output:

certi_{element} certi_{element} certi_{element} 

But I want:

certi_{deep} certi_{mahesh} certi_{nirbhay} 

How can I do this?

Answer 1

f"..." strings are great when interpolating expression results into a literal, but you don't have a literal, you have a template string in a separate variable.

You can use str.format() to apply values to that template:

name=["deep","mahesh","nirbhay"] user_input = "certi_{element}" # this string i ask from user    for value in name:     print(user_input.format(element=value)) 

String formatting placeholders that use names (such as {element}) are not variables. You assign a value for each name in the keyword arguments of the str.format() call instead. In the above example, element=value passes in the value of the value variable to fill in the placeholder with the element.

Unlike f-strings, the {...} placeholders are not expressions and you can't use arbitrary Python expressions in the template. This is a good thing, you wouldn't want end-users to be able to execute arbitrary Python code in your program. See the Format String Syntax documenation for details.

You can pass in any number of names; the string template doesn't have to use any of them. If you combine str.format() with the **mapping call convention, you can use any dictionary as the source of values:

template_values = {     'name': 'Ford Prefect',     'number': 42,     'company': 'Sirius Cybernetics Corporation',     'element': 'Improbability Drive', }  print(user_input.format(**template_values) 

The above would let a user use any of the names in template_values in their template, any number of times they like.

While you can use locals() and globals() to produce dictionaries mapping variable names to values, I'd not recommend that approach. Use a dedicated namespace like the above to limit what names are available, and document those names for your end-users.

Answer 2

If you define:

def fstr(template):     return eval(f"f'{template}'") 

Then you can do:

name=["deep","mahesh","nirbhay"] user_input = r"certi_{element}" # this string i ask from user    for element in name:     print(fstr(user_input)) 

Which gives as output:

certi_deep certi_mahesh certi_nirbhay 

But be aware that users can use expressions in the template, like e.g.:

import os  # assume you have used os somewhere user_input = r"certi_{os.environ}"  for element in name:     print(fstr(user_input)) 

You definitely don't want this!

Therefore, a much safer option is to define:

def fstr(template, **kwargs):     return eval(f"f'{template}'", kwargs) 

Arbitrary code is no longer possible, but users can still use string expressions like:

user_input = r"certi_{element.upper()*2}"  for element in name:     print(fstr(user_input, element=element)) 

Gives as output:

certi_DEEPDEEP certi_MAHESHMAHESH certi_NIRBHAYNIRBHAY 

Which may be desired in some cases.