Is it possible to delay the evaluation of an expression that is part of a function call?

Question

In my program I have numerous occurrences of this pattern:

if some_bool:
   print(f"some {difficult()} string")

I thought about creating a function for this:

def print_cond(a_bool, a_string):
   if a_bool:
      print(a_string)

print_cond(some_bool, f"some {difficult()} string")

But the consequence of this is that the second parameter is always evaluated, even if some_bool == False. Is there a way to delay the evaluation of the f-string up to the point that it actually gets printed?

Answer 1

You can delay the evaluation of f-string by putting it inside lambda.

For example:

def difficult():
    return "Hello World!"

def print_cond(a_bool, a_string):
    if a_bool:
        print("String is:")
        print(a_string())  # <-- note the ()


print_cond(True, lambda: f"some {difficult()} string")

Prints:

String is:
some Hello World! string