How Can You Evaluate F-Strings on Demand in Python?

Evaluating F-Strings on Demand

F-strings provide a convenient way to format strings in Python. However, when working with dynamic templates or files, the need arises to postpone or defer the evaluation of f-strings. This poses a challenge as static strings with formatting tags cannot be directly interpreted by the interpreter.

Using a Custom Function

A robust solution to this problem involves using a custom function that evaluates a string as an f-string. The following function serves this purpose:

<code class="python">def fstr(template):
    return eval(f'f"""{template}"""')</code>

Application

With the fstr function, you can postpone f-string evaluation as follows:

<code class="python">template_a = "The current name is {name}"
names = ["foo", "bar"]
for name in names:
    print(fstr(template_a))
# Output: The current name is foo
#        The current name is bar</code>

Notice that the fstr function correctly evaluates expressions within the string, such as name.upper() * 2:

<code class="python">template_b = "The current name is {name.upper() * 2}"
for name in names:
    print(fstr(template_b))
# Output: The current name is FOOFOO
#        The current name is BARBAR</code>

This approach provides a concise and convenient way to handle f-string evaluation when necessary, allowing for dynamic string formatting within your codebase.

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