Cut and join: split, join and strip

What you will be able to do

After this lesson you can

• cut a text into words or fields with split, and glue a list with join,
• remove the spaces at the ends with strip, and change pieces with replace,
• ask yes or no questions about a text, and
• remember that a string method gives a new string.

Cut a text into pieces

A sentence is a string, but we often want its words. A line of a file is a string, but we often want its fields: the name, the age, the subject. The method split() cuts a string into a list of pieces.

• "a b c".split() cuts at every space and gives ["a", "b", "c"].
• "a,b,c".split(",") cuts at every comma and gives the same list.

Predict first. The sentence has spaces at the start and at the end. How many words does split() find? What does "-".join(words) give? Then run it.

• split() with no argument throws away the extra spaces, and finds 4 words: the, quick, brown and fox. It treats any amount of white space as one gap.
• "-".join(words) glues the words with a hyphen between each pair: the-quick-brown-fox.
• split(",") with a separator is stricter. In "a,b,,c", there is nothing between the two commas, so one piece is empty: ["a", "b", "", "c"]. The same happens with split(" ") and two spaces. This is a common surprise. When you split at spaces, use split() with no argument.
• "2026-10-19".split("-") is a good way to cut a date into three pieces.

The four tools

Notice that join is written the other way round from what many learners expect: the separator comes first, and the list goes inside the brackets. Think of it as "put this glue between those pieces".

split() and join() undo each other: split a text at commas, join the pieces with commas, and you have the text again.

Yes or no questions

Three tools answer True or False: startswith(), endswith() and the word in.

A string does not change

Look at the last part of the program. word.strip() on its own line does nothing that you can see: the line makes a new, stripped string, and throws it away. The variable word still holds the old string with spaces. Only word = word.strip() keeps the result.

Every one of these methods gives you a new string. The old one stays the same, because a string cannot be changed. To keep the result, put it in a variable: tidy = raw.strip().

Common mistakes

• Calling a method and not keeping the result. text.strip() alone changes nothing.
• split(" ") for words. Two spaces in a row make an empty word. Use split().
• Writing list.join("-"). The separator comes first: "-".join(list).
• Joining a list of numbers. "-".join([1, 2]) is a TypeError. Change them first: "-".join(str(n) for n in numbers).
• Expecting replace to change only the first match.
• A strip that is too small. It removes spaces only. For other marks, use replace, or strip(".,!") with the marks inside the brackets.

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 problem Words in a sentence and the problem Glue a date.
2. Build. Open the lab Cut and glue. You cut a line and join the pieces, and the checker confirms each result.
3. Stretch. Open the problem Tidy a name and write the steps for a name such as " aSHA rao ".

Remember these

• split() cuts a string into a list. join() glues a list into a string, with the separator first.
• strip() removes the spaces at the two ends.
• replace() changes pieces. startswith(), endswith() and in answer True or False.
• A string method gives a new string. The old string stays the same.

Check yourself

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

1. What does "red,green,blue".split(",") give?

2. What does " ".join(["to", "be"]) give?

3. What does "a,,b".split(",") give?

4. Why does word.strip() on its own line not change word?