How can I call an object's method from within a format string? (Python)

Question

I want to call a string's lower method from within the template string. This would ideally look something like

string = "Lowercase: {value.lower()}"

string.format(value=...)

However, this gives me an error saying AttributeError: 'str' object has no attribute 'lower()' (I understand why I'm getting this error).

I'm wondering if there's a way to achieve this. I've looked at using conversion (like '{value!r} for calling repr) but that didn't fix my problem. Could I create a custom conversion specifier?

(For the record, string.format(value=value.lower()) will not necessarily work in my case.)

Did you mean to use an f-string? Note the "f" prefix: f"Lowercase: {value.lower()}" — Hitobat
– Hitobat, Commented Oct 12, 2019 at 21:10
No, value is going to be a keyword passed into string.format(...), not necessarily a variable on its own. — benrussell80
– benrussell80, Commented Oct 12, 2019 at 21:19

Kevin · Accepted Answer · 2019-10-12 21:27:15Z

IIRC, the format specification mini-language can access attributes of objects, but it can't call methods. It assumes you're looking for an attribute with the name "lower()", parentheses included.

Does value have to be a string? Perhaps you could create a string subclass with a @property that returns a lowercased version of itself. For example:

class StringEx(str):
    @property
    def lowercase(self):
        return self.lower()

x = StringEx("Hello, World!")
s = "Lowercase: {value.lowercase}"
print(s.format(value=x))

Result:

Lowercase: hello, world!

If you expect to have a lot of different format strings and don't want to implement a property for each string method, you can cover all the zero-argument methods in one go by overriding __getattr__:

class StringEx(str):
    def __getattr__(self, name):
        if name.endswith("()"):
            return getattr(self, name[:-2])()
        else:
            raise AttributeError

x = StringEx("HeLlO, WoRlD!")
format_strings = [
    "Regular: {value}",
    "Lowercase: {value.lower()}",
    "Uppercase: {value.upper()}",
    "Title: {value.title()}"
]

for s in format_strings:
    print(s.format(value=x))

Result:

Regular: HeLlO, WoRlD!
Lowercase: hello, world!
Uppercase: HELLO, WORLD!
Title: Hello, World!

kingkong · Accepted Answer · 2023-05-14 03:50:57Z

0

Just use f-str, this will help.

value = "HELLO world"
f"Lowercase: {value.lower()}"

answered May 14, 2023 at 3:50

kingkong

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