Named Placeholders in String Formatting

Named placeholders in string formatting

Thanks for all your help! Using all your clues, I've written routine to do exactly what I want -- python-like string formatting using dictionary. Since I'm Java newbie, any hints are appreciated.

public static String dictFormat(String format, Hashtable<String, Object> values) {
    StringBuilder convFormat = new StringBuilder(format);
    Enumeration<String> keys = values.keys();
    ArrayList valueList = new ArrayList();
    int currentPos = 1;
    while (keys.hasMoreElements()) {
        String key = keys.nextElement(),
        formatKey = "%(" + key + ")",
        formatPos = "%" + Integer.toString(currentPos) + "$";
        int index = -1;
        while ((index = convFormat.indexOf(formatKey, index)) != -1) {
            convFormat.replace(index, index + formatKey.length(), formatPos);
            index += formatPos.length();
        }
        valueList.add(values.get(key));
        ++currentPos;
    }
    return String.format(convFormat.toString(), valueList.toArray());
}

Is there a String.Format that can accept named input parameters instead of index placeholders?

In C# 6 you can use string interpolation:

string name = "Lisa";
int age = 20;
string str = $"Her name is {name} and she's {age} years old";

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As Doug Clutter mentioned in his comment, string interpolation also supports format string. It's possible to change the format by specifying it after a colon. The following example will output a number with a comma and 2 decimal places:

var str = $"Your account balance is {balance:N2}"

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As Bas mentioned in his answer, string interpolation doesn't support template string. Actually, it has no built in support for that. Fortunately it exists in some great libraries.

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SmartFormat.NET for example has support for named placeholders:

Smart.Format("{Name} from {Address.City}, {Address.State}", user)

// The user object should at least be like that 

public class User
{
    public string Name { get; set; }
    public Address Address { get; set; }
}

public class Address
{
    public string City { get; set; }
    public string State { get; set; }
}

It is available on NuGet and has excellent documentation.

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Mustache is also a great solution. Bas has described its pros well in his answer.

Python string formatting - combine named placeholder and float formatter for the same argument?

'{value:.2f}'.format(value=pi)

String formatting named parameters?

Solution in Python 3.6+

Python 3.6 introduces literal string formatting, so that you can format the named parameters without any repeating any of your named parameters outside the string:

print(f'<a href="{my_url:s}">{my_url:s}</a>')

This will evaluate my_url, so if it's not defined you will get a NameError. In fact, instead of my_url, you can write an arbitrary Python expression, as long as it evaluates to a string (because of the :s formatting code). If you want a string representation for the result of an expression that might not be a string, replace :s by !s, just like with regular, pre-literal string formatting.

For details on literal string formatting, see PEP 498, where it was first introduced.

Java string templatizer / formatter with named arguments

You might also try org.apache.commons.lang3.text.StrSubstitutor if Java 7 is not an option. It does exactly what you want it to do. Whether it’s light-weight might depend on whether you use something else of commons-lang as well.

How to dynamically inspect a format string with named placeholders like {foo} {bar} {baz}

Yes, you can use the built-in parser:

>>> var = '''{foo} {bar} {baz}'''
>>> import string
>>> formatter = string.Formatter()
>>> formatter.parse(var)
<formatteriterator object at 0x109174750>
>>> list(formatter.parse(var))
[('', 'foo', '', None), (' ', 'bar', '', None), (' ', 'baz', '', None)]

Java generating Strings with placeholders

See String.format method.

String s = "hello %s!";
s = String.format(s, "world");
assertEquals(s, "hello world!"); // should be true

Named string formatting in C#

There is no built-in method for handling this.

Here's one method

string myString = "{foo} is {bar} and {yadi} is {yada}".Inject(o);

Here's another

Status.Text = "{UserName} last logged in at {LastLoginDate}".FormatWith(user);

A third improved method partially based on the two above, from Phil Haack

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