String Interpolation¶

Until now, we have created strings with variable content using the + operator to concatenate partial strings together. That works, but it’s very hard for people to read or debug a code line that includes variable names and strings and complex expressions. Consider the following:

Or perhaps more realistically:

In this section, you will learn to write that in a more readable way:

.format() is a special string method that allows you to interpolate values into strings – basically, insert values into strings and automatically make them work, as you see above.

There are a few ways to use the .format method.

The first, and most common, way, is to insert {} curly braces into strings at the location where you want to interpolate (stick in) values into the string. Then call the .format method on the string, and pass to the format method the items you want to put into the string, separated by commas.

It is important to pass arguments to the format method in the correct order, because they are matched positionally into the {} places for interpolation where there is more than one.

It is also important that you give format the same amount of arguments as there are {} waiting for interpolation in the string. If you have {} in a string that you do not pass arguments for, you may not get an error, but you will see a weird undefined value you probably did not intend suddenly inserted into your string. You can see an example below.

For example,

Another option is to specifically refer to keywords (think back to keyword arguments for functions!) for interpolation values, like below.

Sometimes, you may want to use the .format method to insert the same value into a string multiple times. You can do this by simply passing the same string into the format method, assuming you have included {} s in the string everywhere you want to interpolate them. But you can also use positional passing references to do this! The order in which you pass arguments into the format method matters: the first one is argument 0, the second is argument 1, and so on.

For example,

You can imagine some ways in which this method for string interpolation is very useful for complex programs and programs where you want to compile data together and print it out, or write it to a file. A set of strings might all be the same except for one varying piece of data, so for instance, you can use code like some you see in this section to generate all of those strings with one for loop that’s neat and easy to read!

Overall, using .format for string interpolation is much neater and easier to edit later on than just using string concatenation.