Joining, repeating and f-strings

What you will be able to do

After this lesson you can

• join and repeat strings,
• put variables and sums inside a sentence with an f-string,
• show a number with a fixed number of decimal places, and
• line up text in columns.

Joining and repeating

You met these in the earlier lessons. Now we look at them carefully.

A string and a number cannot be joined with +. "Age: " + 12 is a TypeError. You must write "Age: " + str(12). This is easy to forget, and it is the reason why we need a better tool.

Each coloured bar is one copy of the piece. + would put two texts side by side in the same way.

f-strings: values inside text

An f-string puts the values of variables and sums right inside the text. You write the letter f before the opening quotation mark. Inside the text, a name or a sum in curly brackets { } is replaced by its value.

You do not need str(). An f-string changes the numbers into text by itself. So it is shorter and easier to read than joining with +.

Showing a number the way you want

After a colon inside the curly brackets, you can say how to show the value. The three most useful forms:

The last lines of the program show the three alignments, inside square brackets so that you can see the spaces. The f-string with .2f is the best way to show money: 49.5 becomes 49.50.

A table of values

With alignment, you can make a neat table. Imagine three lines such as f"{name:<10}{marks:>5}". The names start in the same column, and the marks end in the same column. Programs that print reports use this all the time.

Common mistakes

• Forgetting the f. Without it, "Hello {name}" shows the curly brackets as they are.
• Using the wrong quotation marks inside. In f"... {d["a"]} ..." the inner double marks end the string too soon. Use single marks inside: {d['a']}.
• Joining a number with + and no str(). Use an f-string, or a comma in print().
• Forgetting the space. "Hello" + name has no space. In an f-string, write the space: f"Hello {name}".
• Writing .2f without the colon. It is {price:.2f}, with a colon.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Build a sentence. Make one sentence from three variables with an f-string.
2. Build. Open the problem A sentence with values, and the problem A price with two decimals.
3. Stretch. Open the problem Join and repeat. Then print a small three-line table of your three best friends and their ages, aligned in columns.

Remember these

• + joins strings exactly, with no space. * repeats a string.
• An f-string, f"... {name} ...", puts values and sums inside text, with no str().
• {x:.2f} gives two decimal places. {x:>8}, {x:<8} and {x:^8} align text.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is "ab" * 3 + "c"?

2. Write an f-string that shows Asha is 12 with the variables name and age.

3. How do you show 7.5 as 7.50?

4. Why is "Age: " + 12 an error?